Debugging with GDB

This file documents the GNU debugger GDB.

This is the Tenth Edition, of Debugging with GDB: the GNU Source-Level Debugger for GDB (GDB) Version 7.8.50.20141022-cvs.

Copyright © 1988-2014 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “Free Software” and “Free Software Needs Free Documentation”, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below.

(a) The FSF’s Back-Cover Text is: “You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom.”

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Debugging with GDB

This file describes GDB, the GNU symbolic debugger.

This is the Tenth Edition, for GDB (GDB) Version 7.8.50.20141022-cvs.

Copyright (C) 1988-2014 Free Software Foundation, Inc.

This edition of the GDB manual is dedicated to the memory of Fred Fish. Fred was a long-standing contributor to GDB and to Free software in general. We will miss him.

Table of Contents


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Summary of GDB

The purpose of a debugger such as GDB is to allow you to see what is going on “inside” another program while it executes—or what another program was doing at the moment it crashed.

GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:

You can use GDB to debug programs written in C and C++. For more information, see Supported Languages. For more information, see C and C++.

Support for D is partial. For information on D, see D.

Support for Modula-2 is partial. For information on Modula-2, see Modula-2.

Support for OpenCL C is partial. For information on OpenCL C, see OpenCL C.

Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. GDB does not support entering expressions, printing values, or similar features using Pascal syntax.

GDB can be used to debug programs written in Fortran, although it may be necessary to refer to some variables with a trailing underscore.

GDB can be used to debug programs written in Objective-C, using either the Apple/NeXT or the GNU Objective-C runtime.


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Free Software

GDB is free software, protected by the GNU General Public License (GPL). The GPL gives you the freedom to copy or adapt a licensed program—but every person getting a copy also gets with it the freedom to modify that copy (which means that they must get access to the source code), and the freedom to distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve these freedoms.

Fundamentally, the General Public License is a license which says that you have these freedoms and that you cannot take these freedoms away from anyone else.


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Free Software Needs Free Documentation

The biggest deficiency in the free software community today is not in the software—it is the lack of good free documentation that we can include with the free software. Many of our most important programs do not come with free reference manuals and free introductory texts. Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap. We have many such gaps today.

Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this come about? Because the authors of those manuals published them with restrictive terms—no copying, no modification, source files not available—which exclude them from the free software world.

That wasn’t the first time this sort of thing happened, and it was far from the last. Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free.

Free documentation, like free software, is a matter of freedom, not price. The problem with the non-free manual is not that publishers charge a price for printed copies—that in itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use of the manual. Free manuals are available in source code form, and give you permission to copy and modify. Non-free manuals do not allow this.

The criteria of freedom for a free manual are roughly the same as for free software. Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper.

Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too—so they can provide accurate and clear documentation for the modified program. A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community.

Some kinds of limits on the way modification is handled are acceptable. For example, requirements to preserve the original author’s copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified. Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of restrictions are acceptable because they don’t obstruct the community’s normal use of the manual.

However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it.

Please spread the word about this issue. Our community continues to lose manuals to proprietary publishing. If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community.

If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license. Remember that this decision requires your approval—you don’t have to let the publisher decide. Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want. If the publisher you are dealing with refuses, please try other publishers. If you’re not sure whether a proposed license is free, write to licensing@gnu.org.

You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try to reward the publishers that have paid or pay the authors to work on it.

The Free Software Foundation maintains a list of free documentation published by other publishers, at http://www.fsf.org/doc/other-free-books.html.


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Contributors to GDB

Richard Stallman was the original author of GDB, and of many other GNU programs. Many others have contributed to its development. This section attempts to credit major contributors. One of the virtues of free software is that everyone is free to contribute to it; with regret, we cannot actually acknowledge everyone here. The file ChangeLog in the GDB distribution approximates a blow-by-blow account.

Changes much prior to version 2.0 are lost in the mists of time.

Plea: Additions to this section are particularly welcome. If you or your friends (or enemies, to be evenhanded) have been unfairly omitted from this list, we would like to add your names!

So that they may not regard their many labors as thankless, we particularly thank those who shepherded GDB through major releases: Andrew Cagney (releases 6.3, 6.2, 6.1, 6.0, 5.3, 5.2, 5.1 and 5.0); Jim Blandy (release 4.18); Jason Molenda (release 4.17); Stan Shebs (release 4.14); Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10, and 4.9); Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4); John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2, 3.1, and 3.0).

Richard Stallman, assisted at various times by Peter TerMaat, Chris Hanson, and Richard Mlynarik, handled releases through 2.8.

Michael Tiemann is the author of most of the GNU C++ support in GDB, with significant additional contributions from Per Bothner and Daniel Berlin. James Clark wrote the GNU C++ demangler. Early work on C++ was by Peter TerMaat (who also did much general update work leading to release 3.0).

GDB uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project of David V. Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.

David Johnson wrote the original COFF support; Pace Willison did the original support for encapsulated COFF.

Brent Benson of Harris Computer Systems contributed DWARF 2 support.

Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS support. Jean-Daniel Fekete contributed Sun 386i support. Chris Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki Hasei contributed Sony/News OS 3 support. David Johnson contributed Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support. Jeff Law contributed HP PA and SOM support. Keith Packard contributed NS32K support. Doug Rabson contributed Acorn Risc Machine support. Bob Rusk contributed Harris Nighthawk CX-UX support. Chris Smith contributed Convex support (and Fortran debugging). Jonathan Stone contributed Pyramid support. Michael Tiemann contributed SPARC support. Tim Tucker contributed support for the Gould NP1 and Gould Powernode. Pace Willison contributed Intel 386 support. Jay Vosburgh contributed Symmetry support. Marko Mlinar contributed OpenRISC 1000 support.

Andreas Schwab contributed M68K GNU/Linux support.

Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.

Jay Fenlason and Roland McGrath ensured that GDB and GAS agree about several machine instruction sets.

Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote debugging. Intel Corporation, Wind River Systems, AMD, and ARM contributed remote debugging modules for the i960, VxWorks, A29K UDI, and RDI targets, respectively.

Brian Fox is the author of the readline libraries providing command-line editing and command history.

Andrew Beers of SUNY Buffalo wrote the language-switching code, the Modula-2 support, and contributed the Languages chapter of this manual.

Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the command-completion support to cover C++ overloaded symbols.

Hitachi America (now Renesas America), Ltd. sponsored the support for H8/300, H8/500, and Super-H processors.

NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx processors.

Mitsubishi (now Renesas) sponsored the support for D10V, D30V, and M32R/D processors.

Toshiba sponsored the support for the TX39 Mips processor.

Matsushita sponsored the support for the MN10200 and MN10300 processors.

Fujitsu sponsored the support for SPARClite and FR30 processors.

Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware watchpoints.

Michael Snyder added support for tracepoints.

Stu Grossman wrote gdbserver.

Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly innumerable bug fixes and cleanups throughout GDB.

The following people at the Hewlett-Packard Company contributed support for the PA-RISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0 (narrow mode), HP’s implementation of kernel threads, HP’s aC++ compiler, and the Text User Interface (nee Terminal User Interface): Ben Krepp, Richard Title, John Bishop, Susan Macchia, Kathy Mann, Satish Pai, India Paul, Steve Rehrauer, and Elena Zannoni. Kim Haase provided HP-specific information in this manual.

DJ Delorie ported GDB to MS-DOS, for the DJGPP project. Robert Hoehne made significant contributions to the DJGPP port.

Cygnus Solutions has sponsored GDB maintenance and much of its development since 1991. Cygnus engineers who have worked on GDB fulltime include Mark Alexander, Jim Blandy, Per Bothner, Kevin Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin Hunt, Jim Ingham, John Gilmore, Stu Grossman, Kung Hsu, Jim Kingdon, John Metzler, Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich Pixley, Zdenek Radouch, Keith Seitz, Stan Shebs, David Taylor, and Elena Zannoni. In addition, Dave Brolley, Ian Carmichael, Steve Chamberlain, Nick Clifton, JT Conklin, Stan Cox, DJ Delorie, Ulrich Drepper, Frank Eigler, Doug Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff Holcomb, Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner, Jason Merrill, Catherine Moore, Drew Moseley, Ken Raeburn, Gavin Romig-Koch, Rob Savoye, Jamie Smith, Mike Stump, Ian Taylor, Angela Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim Wilson, and David Zuhn have made contributions both large and small.

Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for Cygnus Solutions, implemented the original GDB/MI interface.

Jim Blandy added support for preprocessor macros, while working for Red Hat.

Andrew Cagney designed GDB’s architecture vector. Many people including Andrew Cagney, Stephane Carrez, Randolph Chung, Nick Duffek, Richard Henderson, Mark Kettenis, Grace Sainsbury, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Andreas Schwab, Jason Thorpe, Corinna Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the migration of old architectures to this new framework.

Andrew Cagney completely re-designed and re-implemented GDB’s unwinder framework, this consisting of a fresh new design featuring frame IDs, independent frame sniffers, and the sentinel frame. Mark Kettenis implemented the DWARF 2 unwinder, Jeff Johnston the libunwind unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad unwinders. The architecture-specific changes, each involving a complete rewrite of the architecture’s frame code, were carried out by Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew Cagney, Stephane Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel Jacobowitz, Jeff Johnston, Mark Kettenis, Theodore A. Roth, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Corinna Vinschen, and Ulrich Weigand.

Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from Tensilica, Inc. contributed support for Xtensa processors. Others who have worked on the Xtensa port of GDB in the past include Steve Tjiang, John Newlin, and Scott Foehner.

Michael Eager and staff of Xilinx, Inc., contributed support for the Xilinx MicroBlaze architecture.


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1 A Sample GDB Session

You can use this manual at your leisure to read all about GDB. However, a handful of commands are enough to get started using the debugger. This chapter illustrates those commands.

One of the preliminary versions of GNU m4 (a generic macro processor) exhibits the following bug: sometimes, when we change its quote strings from the default, the commands used to capture one macro definition within another stop working. In the following short m4 session, we define a macro foo which expands to 0000; we then use the m4 built-in defn to define bar as the same thing. However, when we change the open quote string to <QUOTE> and the close quote string to <UNQUOTE>, the same procedure fails to define a new synonym baz:

$ cd gnu/m4
$ ./m4
define(foo,0000)

foo
0000
define(bar,defn(‘foo’))

bar
0000
changequote(<QUOTE>,<UNQUOTE>)

define(baz,defn(<QUOTE>foo<UNQUOTE>))
baz
Ctrl-d
m4: End of input: 0: fatal error: EOF in string

Let us use GDB to try to see what is going on.

$ gdb m4
GDB is free software and you are welcome to distribute copies
 of it under certain conditions; type "show copying" to see
 the conditions.
There is absolutely no warranty for GDB; type "show warranty"
 for details.

GDB 7.8.50.20141022-cvs, Copyright 1999 Free Software Foundation, Inc...
(gdb)

GDB reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly. We now tell GDB to use a narrower display width than usual, so that examples fit in this manual.

(gdb) set width 70

We need to see how the m4 built-in changequote works. Having looked at the source, we know the relevant subroutine is m4_changequote, so we set a breakpoint there with the GDB break command.

(gdb) break m4_changequote
Breakpoint 1 at 0x62f4: file builtin.c, line 879.

Using the run command, we start m4 running under GDB control; as long as control does not reach the m4_changequote subroutine, the program runs as usual:

(gdb) run
Starting program: /work/Editorial/gdb/gnu/m4/m4
define(foo,0000)

foo
0000

To trigger the breakpoint, we call changequote. GDB suspends execution of m4, displaying information about the context where it stops.

changequote(<QUOTE>,<UNQUOTE>)

Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
    at builtin.c:879
879         if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))

Now we use the command n (next) to advance execution to the next line of the current function.

(gdb) n
882         set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
 : nil,

set_quotes looks like a promising subroutine. We can go into it by using the command s (step) instead of next. step goes to the next line to be executed in any subroutine, so it steps into set_quotes.

(gdb) s
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
    at input.c:530
530         if (lquote != def_lquote)

The display that shows the subroutine where m4 is now suspended (and its arguments) is called a stack frame display. It shows a summary of the stack. We can use the backtrace command (which can also be spelled bt), to see where we are in the stack as a whole: the backtrace command displays a stack frame for each active subroutine.

(gdb) bt
#0  set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
    at input.c:530
#1  0x6344 in m4_changequote (argc=3, argv=0x33c70)
    at builtin.c:882
#2  0x8174 in expand_macro (sym=0x33320) at macro.c:242
#3  0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
    at macro.c:71
#4  0x79dc in expand_input () at macro.c:40
#5  0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195

We step through a few more lines to see what happens. The first two times, we can use ‘s’; the next two times we use n to avoid falling into the xstrdup subroutine.

(gdb) s
0x3b5c  532         if (rquote != def_rquote)
(gdb) s
0x3b80  535         lquote = (lq == nil || *lq == '\0') ?  \
def_lquote : xstrdup(lq);
(gdb) n
536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
 : xstrdup(rq);
(gdb) n
538         len_lquote = strlen(rquote);

The last line displayed looks a little odd; we can examine the variables lquote and rquote to see if they are in fact the new left and right quotes we specified. We use the command p (print) to see their values.

(gdb) p lquote
$1 = 0x35d40 "<QUOTE>"
(gdb) p rquote
$2 = 0x35d50 "<UNQUOTE>"

lquote and rquote are indeed the new left and right quotes. To look at some context, we can display ten lines of source surrounding the current line with the l (list) command.

(gdb) l
533             xfree(rquote);
534
535         lquote = (lq == nil || *lq == '\0') ? def_lquote\
 : xstrdup (lq);
536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
 : xstrdup (rq);
537
538         len_lquote = strlen(rquote);
539         len_rquote = strlen(lquote);
540     }
541
542     void

Let us step past the two lines that set len_lquote and len_rquote, and then examine the values of those variables.

(gdb) n
539         len_rquote = strlen(lquote);
(gdb) n
540     }
(gdb) p len_lquote
$3 = 9
(gdb) p len_rquote
$4 = 7

That certainly looks wrong, assuming len_lquote and len_rquote are meant to be the lengths of lquote and rquote respectively. We can set them to better values using the p command, since it can print the value of any expression—and that expression can include subroutine calls and assignments.

(gdb) p len_lquote=strlen(lquote)
$5 = 7
(gdb) p len_rquote=strlen(rquote)
$6 = 9

Is that enough to fix the problem of using the new quotes with the m4 built-in defn? We can allow m4 to continue executing with the c (continue) command, and then try the example that caused trouble initially:

(gdb) c
Continuing.

define(baz,defn(<QUOTE>foo<UNQUOTE>))

baz
0000

Success! The new quotes now work just as well as the default ones. The problem seems to have been just the two typos defining the wrong lengths. We allow m4 exit by giving it an EOF as input:

Ctrl-d
Program exited normally.

The message ‘Program exited normally.’ is from GDB; it indicates m4 has finished executing. We can end our GDB session with the GDB quit command.

(gdb) quit

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2 Getting In and Out of GDB

This chapter discusses how to start GDB, and how to get out of it. The essentials are:


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2.1 Invoking GDB

Invoke GDB by running the program gdb. Once started, GDB reads commands from the terminal until you tell it to exit.

You can also run gdb with a variety of arguments and options, to specify more of your debugging environment at the outset.

The command-line options described here are designed to cover a variety of situations; in some environments, some of these options may effectively be unavailable.

The most usual way to start GDB is with one argument, specifying an executable program:

gdb program

You can also start with both an executable program and a core file specified:

gdb program core

You can, instead, specify a process ID as a second argument, if you want to debug a running process:

gdb program 1234

would attach GDB to process 1234 (unless you also have a file named 1234; GDB does check for a core file first).

Taking advantage of the second command-line argument requires a fairly complete operating system; when you use GDB as a remote debugger attached to a bare board, there may not be any notion of “process”, and there is often no way to get a core dump. GDB will warn you if it is unable to attach or to read core dumps.

You can optionally have gdb pass any arguments after the executable file to the inferior using --args. This option stops option processing.

gdb --args gcc -O2 -c foo.c

This will cause gdb to debug gcc, and to set gcc’s command-line arguments (see Arguments) to ‘-O2 -c foo.c’.

You can run gdb without printing the front material, which describes GDB’s non-warranty, by specifying --silent (or -q/--quiet):

gdb --silent

You can further control how GDB starts up by using command-line options. GDB itself can remind you of the options available.

Type

gdb -help

to display all available options and briefly describe their use (‘gdb -h’ is a shorter equivalent).

All options and command line arguments you give are processed in sequential order. The order makes a difference when the ‘-x’ option is used.


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2.1.1 Choosing Files

When GDB starts, it reads any arguments other than options as specifying an executable file and core file (or process ID). This is the same as if the arguments were specified by the ‘-se’ and ‘-c’ (or ‘-p’) options respectively. (GDB reads the first argument that does not have an associated option flag as equivalent to the ‘-se’ option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the ‘-c’/‘-p’ option followed by that argument.) If the second argument begins with a decimal digit, GDB will first attempt to attach to it as a process, and if that fails, attempt to open it as a corefile. If you have a corefile whose name begins with a digit, you can prevent GDB from treating it as a pid by prefixing it with ./, e.g. ./12345.

If GDB has not been configured to included core file support, such as for most embedded targets, then it will complain about a second argument and ignore it.

Many options have both long and short forms; both are shown in the following list. GDB also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with ‘--’ rather than ‘-’, though we illustrate the more usual convention.)

-symbols file
-s file

Read symbol table from file file.

-exec file
-e file

Use file file as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.

-se file

Read symbol table from file file and use it as the executable file.

-core file
-c file

Use file file as a core dump to examine.

-pid number
-p number

Connect to process ID number, as with the attach command.

-command file
-x file

Execute commands from file file. The contents of this file is evaluated exactly as the source command would. See Command files.

-eval-command command
-ex command

Execute a single GDB command.

This option may be used multiple times to call multiple commands. It may also be interleaved with ‘-command’ as required.

gdb -ex 'target sim' -ex 'load' \
   -x setbreakpoints -ex 'run' a.out
-init-command file
-ix file

Execute commands from file file before loading the inferior (but after loading gdbinit files). See Startup.

-init-eval-command command
-iex command

Execute a single GDB command before loading the inferior (but after loading gdbinit files). See Startup.

-directory directory
-d directory

Add directory to the path to search for source and script files.

-r
-readnow

Read each symbol file’s entire symbol table immediately, rather than the default, which is to read it incrementally as it is needed. This makes startup slower, but makes future operations faster.


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2.1.2 Choosing Modes

You can run GDB in various alternative modes—for example, in batch mode or quiet mode.

-nx
-n

Do not execute commands found in any initialization file. There are three init files, loaded in the following order:

system.gdbinit

This is the system-wide init file. Its location is specified with the --with-system-gdbinit configure option (see System-wide configuration). It is loaded first when GDB starts, before command line options have been processed.

~/.gdbinit

This is the init file in your home directory. It is loaded next, after system.gdbinit, and before command options have been processed.

./.gdbinit

This is the init file in the current directory. It is loaded last, after command line options other than -x and -ex have been processed. Command line options -x and -ex are processed last, after ./.gdbinit has been loaded.

For further documentation on startup processing, See Startup. For documentation on how to write command files, See Command Files.

-nh

Do not execute commands found in ~/.gdbinit, the init file in your home directory. See Startup.

-quiet
-silent
-q

“Quiet”. Do not print the introductory and copyright messages. These messages are also suppressed in batch mode.

-batch

Run in batch mode. Exit with status 0 after processing all the command files specified with ‘-x’ (and all commands from initialization files, if not inhibited with ‘-n’). Exit with nonzero status if an error occurs in executing the GDB commands in the command files. Batch mode also disables pagination, sets unlimited terminal width and height see Screen Size, and acts as if set confirm off were in effect (see Messages/Warnings).

Batch mode may be useful for running GDB as a filter, for example to download and run a program on another computer; in order to make this more useful, the message

Program exited normally.

(which is ordinarily issued whenever a program running under GDB control terminates) is not issued when running in batch mode.

-batch-silent

Run in batch mode exactly like ‘-batch’, but totally silently. All GDB output to stdout is prevented (stderr is unaffected). This is much quieter than ‘-silent’ and would be useless for an interactive session.

This is particularly useful when using targets that give ‘Loading section’ messages, for example.

Note that targets that give their output via GDB, as opposed to writing directly to stdout, will also be made silent.

-return-child-result

The return code from GDB will be the return code from the child process (the process being debugged), with the following exceptions:

This option is useful in conjunction with ‘-batch’ or ‘-batch-silent’, when GDB is being used as a remote program loader or simulator interface.

-nowindows
-nw

“No windows”. If GDB comes with a graphical user interface (GUI) built in, then this option tells GDB to only use the command-line interface. If no GUI is available, this option has no effect.

-windows
-w

If GDB includes a GUI, then this option requires it to be used if possible.

-cd directory

Run GDB using directory as its working directory, instead of the current directory.

-data-directory directory
-D directory

Run GDB using directory as its data directory. The data directory is where GDB searches for its auxiliary files. See Data Files.

-fullname
-f

GNU Emacs sets this option when it runs GDB as a subprocess. It tells GDB to output the full file name and line number in a standard, recognizable fashion each time a stack frame is displayed (which includes each time your program stops). This recognizable format looks like two ‘\032’ characters, followed by the file name, line number and character position separated by colons, and a newline. The Emacs-to-GDB interface program uses the two ‘\032’ characters as a signal to display the source code for the frame.

-annotate level

This option sets the annotation level inside GDB. Its effect is identical to using ‘set annotate level’ (see Annotations). The annotation level controls how much information GDB prints together with its prompt, values of expressions, source lines, and other types of output. Level 0 is the normal, level 1 is for use when GDB is run as a subprocess of GNU Emacs, level 3 is the maximum annotation suitable for programs that control GDB, and level 2 has been deprecated.

The annotation mechanism has largely been superseded by GDB/MI (see GDB/MI).

--args

Change interpretation of command line so that arguments following the executable file are passed as command line arguments to the inferior. This option stops option processing.

-baud bps
-b bps

Set the line speed (baud rate or bits per second) of any serial interface used by GDB for remote debugging.

-l timeout

Set the timeout (in seconds) of any communication used by GDB for remote debugging.

-tty device
-t device

Run using device for your program’s standard input and output.

-tui

Activate the Text User Interface when starting. The Text User Interface manages several text windows on the terminal, showing source, assembly, registers and GDB command outputs (see GDB Text User Interface). Do not use this option if you run GDB from Emacs (see Using GDB under GNU Emacs).

-interpreter interp

Use the interpreter interp for interface with the controlling program or device. This option is meant to be set by programs which communicate with GDB using it as a back end. See Command Interpreters.

--interpreter=mi’ (or ‘--interpreter=mi2’) causes GDB to use the GDB/MI interface (see The GDB/MI Interface) included since GDB version 6.0. The previous GDB/MI interface, included in GDB version 5.3 and selected with ‘--interpreter=mi1’, is deprecated. Earlier GDB/MI interfaces are no longer supported.

-write

Open the executable and core files for both reading and writing. This is equivalent to the ‘set write on’ command inside GDB (see Patching).

-statistics

This option causes GDB to print statistics about time and memory usage after it completes each command and returns to the prompt.

-version

This option causes GDB to print its version number and no-warranty blurb, and exit.

-configuration

This option causes GDB to print details about its build-time configuration parameters, and then exit. These details can be important when reporting GDB bugs (see GDB Bugs).


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2.1.3 What GDB Does During Startup

Here’s the description of what GDB does during session startup:

  1. Sets up the command interpreter as specified by the command line (see interpreter).
  2. Reads the system-wide init file (if --with-system-gdbinit was used when building GDB; see System-wide configuration and settings) and executes all the commands in that file.
  3. Reads the init file (if any) in your home directory1 and executes all the commands in that file.
  4. Executes commands and command files specified by the ‘-iex’ and ‘-ix’ options in their specified order. Usually you should use the ‘-ex’ and ‘-x’ options instead, but this way you can apply settings before GDB init files get executed and before inferior gets loaded.
  5. Processes command line options and operands.
  6. Reads and executes the commands from init file (if any) in the current working directory as long as ‘set auto-load local-gdbinit’ is set to ‘on’ (see Init File in the Current Directory). This is only done if the current directory is different from your home directory. Thus, you can have more than one init file, one generic in your home directory, and another, specific to the program you are debugging, in the directory where you invoke GDB.
  7. If the command line specified a program to debug, or a process to attach to, or a core file, GDB loads any auto-loaded scripts provided for the program or for its loaded shared libraries. See Auto-loading.

    If you wish to disable the auto-loading during startup, you must do something like the following:

    $ gdb -iex "set auto-load python-scripts off" myprogram
    

    Option ‘-ex’ does not work because the auto-loading is then turned off too late.

  8. Executes commands and command files specified by the ‘-ex’ and ‘-x’ options in their specified order. See Command Files, for more details about GDB command files.
  9. Reads the command history recorded in the history file. See Command History, for more details about the command history and the files where GDB records it.

Init files use the same syntax as command files (see Command Files) and are processed by GDB in the same way. The init file in your home directory can set options (such as ‘set complaints’) that affect subsequent processing of command line options and operands. Init files are not executed if you use the ‘-nx’ option (see Choosing Modes).

To display the list of init files loaded by gdb at startup, you can use gdb --help.

The GDB init files are normally called .gdbinit. The DJGPP port of GDB uses the name gdb.ini, due to the limitations of file names imposed by DOS filesystems. The Windows port of GDB uses the standard name, but if it finds a gdb.ini file in your home directory, it warns you about that and suggests to rename the file to the standard name.


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2.2 Quitting GDB

quit [expression]
q

To exit GDB, use the quit command (abbreviated q), or type an end-of-file character (usually Ctrl-d). If you do not supply expression, GDB will terminate normally; otherwise it will terminate using the result of expression as the error code.

An interrupt (often Ctrl-c) does not exit from GDB, but rather terminates the action of any GDB command that is in progress and returns to GDB command level. It is safe to type the interrupt character at any time because GDB does not allow it to take effect until a time when it is safe.

If you have been using GDB to control an attached process or device, you can release it with the detach command (see Debugging an Already-running Process).


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2.3 Shell Commands

If you need to execute occasional shell commands during your debugging session, there is no need to leave or suspend GDB; you can just use the shell command.

shell command-string
!command-string

Invoke a standard shell to execute command-string. Note that no space is needed between ! and command-string. If it exists, the environment variable SHELL determines which shell to run. Otherwise GDB uses the default shell (/bin/sh on Unix systems, COMMAND.COM on MS-DOS, etc.).

The utility make is often needed in development environments. You do not have to use the shell command for this purpose in GDB:

make make-args

Execute the make program with the specified arguments. This is equivalent to ‘shell make make-args’.


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2.4 Logging Output

You may want to save the output of GDB commands to a file. There are several commands to control GDB’s logging.

set logging on

Enable logging.

set logging off

Disable logging.

set logging file file

Change the name of the current logfile. The default logfile is gdb.txt.

set logging overwrite [on|off]

By default, GDB will append to the logfile. Set overwrite if you want set logging on to overwrite the logfile instead.

set logging redirect [on|off]

By default, GDB output will go to both the terminal and the logfile. Set redirect if you want output to go only to the log file.

show logging

Show the current values of the logging settings.


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3 GDB Commands

You can abbreviate a GDB command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain GDB commands by typing just RET. You can also use the TAB key to get GDB to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility).


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3.1 Command Syntax

A GDB command is a single line of input. There is no limit on how long it can be. It starts with a command name, which is followed by arguments whose meaning depends on the command name. For example, the command step accepts an argument which is the number of times to step, as in ‘step 5’. You can also use the step command with no arguments. Some commands do not allow any arguments.

GDB command names may always be truncated if that abbreviation is unambiguous. Other possible command abbreviations are listed in the documentation for individual commands. In some cases, even ambiguous abbreviations are allowed; for example, s is specially defined as equivalent to step even though there are other commands whose names start with s. You can test abbreviations by using them as arguments to the help command.

A blank line as input to GDB (typing just RET) means to repeat the previous command. Certain commands (for example, run) will not repeat this way; these are commands whose unintentional repetition might cause trouble and which you are unlikely to want to repeat. User-defined commands can disable this feature; see dont-repeat.

The list and x commands, when you repeat them with RET, construct new arguments rather than repeating exactly as typed. This permits easy scanning of source or memory.

GDB can also use RET in another way: to partition lengthy output, in a way similar to the common utility more (see Screen Size). Since it is easy to press one RET too many in this situation, GDB disables command repetition after any command that generates this sort of display.

Any text from a # to the end of the line is a comment; it does nothing. This is useful mainly in command files (see Command Files).

The Ctrl-o binding is useful for repeating a complex sequence of commands. This command accepts the current line, like RET, and then fetches the next line relative to the current line from the history for editing.


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3.2 Command Completion

GDB can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you what the valid possibilities are for the next word in a command, at any time. This works for GDB commands, GDB subcommands, and the names of symbols in your program.

Press the TAB key whenever you want GDB to fill out the rest of a word. If there is only one possibility, GDB fills in the word, and waits for you to finish the command (or press RET to enter it). For example, if you type

(gdb) info bre TAB

GDB fills in the rest of the word ‘breakpoints’, since that is the only info subcommand beginning with ‘bre’:

(gdb) info breakpoints

You can either press RET at this point, to run the info breakpoints command, or backspace and enter something else, if ‘breakpoints’ does not look like the command you expected. (If you were sure you wanted info breakpoints in the first place, you might as well just type RET immediately after ‘info bre’, to exploit command abbreviations rather than command completion).

If there is more than one possibility for the next word when you press TAB, GDB sounds a bell. You can either supply more characters and try again, or just press TAB a second time; GDB displays all the possible completions for that word. For example, you might want to set a breakpoint on a subroutine whose name begins with ‘make_’, but when you type b make_TAB GDB just sounds the bell. Typing TAB again displays all the function names in your program that begin with those characters, for example:

(gdb) b make_ TAB
GDB sounds bell; press TAB again, to see:
make_a_section_from_file     make_environ
make_abs_section             make_function_type
make_blockvector             make_pointer_type
make_cleanup                 make_reference_type
make_command                 make_symbol_completion_list
(gdb) b make_

After displaying the available possibilities, GDB copies your partial input (‘b make_’ in the example) so you can finish the command.

If you just want to see the list of alternatives in the first place, you can press M-? rather than pressing TAB twice. M-? means META ?. You can type this either by holding down a key designated as the META shift on your keyboard (if there is one) while typing ?, or as ESC followed by ?.

Sometimes the string you need, while logically a “word”, may contain parentheses or other characters that GDB normally excludes from its notion of a word. To permit word completion to work in this situation, you may enclose words in ' (single quote marks) in GDB commands.

The most likely situation where you might need this is in typing the name of a C++ function. This is because C++ allows function overloading (multiple definitions of the same function, distinguished by argument type). For example, when you want to set a breakpoint you may need to distinguish whether you mean the version of name that takes an int parameter, name(int), or the version that takes a float parameter, name(float). To use the word-completion facilities in this situation, type a single quote ' at the beginning of the function name. This alerts GDB that it may need to consider more information than usual when you press TAB or M-? to request word completion:

(gdb) b 'bubble( M-?
bubble(double,double)    bubble(int,int)
(gdb) b 'bubble(

In some cases, GDB can tell that completing a name requires using quotes. When this happens, GDB inserts the quote for you (while completing as much as it can) if you do not type the quote in the first place:

(gdb) b bub TAB
GDB alters your input line to the following, and rings a bell:
(gdb) b 'bubble(

In general, GDB can tell that a quote is needed (and inserts it) if you have not yet started typing the argument list when you ask for completion on an overloaded symbol.

For more information about overloaded functions, see C++ Expressions. You can use the command set overload-resolution off to disable overload resolution; see GDB Features for C++.

When completing in an expression which looks up a field in a structure, GDB also tries2 to limit completions to the field names available in the type of the left-hand-side:

(gdb) p gdb_stdout.M-?
magic                to_fputs             to_rewind
to_data              to_isatty            to_write
to_delete            to_put               to_write_async_safe
to_flush             to_read

This is because the gdb_stdout is a variable of the type struct ui_file that is defined in GDB sources as follows:

struct ui_file
{
   int *magic;
   ui_file_flush_ftype *to_flush;
   ui_file_write_ftype *to_write;
   ui_file_write_async_safe_ftype *to_write_async_safe;
   ui_file_fputs_ftype *to_fputs;
   ui_file_read_ftype *to_read;
   ui_file_delete_ftype *to_delete;
   ui_file_isatty_ftype *to_isatty;
   ui_file_rewind_ftype *to_rewind;
   ui_file_put_ftype *to_put;
   void *to_data;
}

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3.3 Getting Help

You can always ask GDB itself for information on its commands, using the command help.

help
h

You can use help (abbreviated h) with no arguments to display a short list of named classes of commands:

(gdb) help
List of classes of commands:

aliases -- Aliases of other commands
breakpoints -- Making program stop at certain points
data -- Examining data
files -- Specifying and examining files
internals -- Maintenance commands
obscure -- Obscure features
running -- Running the program
stack -- Examining the stack
status -- Status inquiries
support -- Support facilities
tracepoints -- Tracing of program execution without
               stopping the program
user-defined -- User-defined commands

Type "help" followed by a class name for a list of
commands in that class.
Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)
help class

Using one of the general help classes as an argument, you can get a list of the individual commands in that class. For example, here is the help display for the class status:

(gdb) help status
Status inquiries.

List of commands:

info -- Generic command for showing things
        about the program being debugged
show -- Generic command for showing things
        about the debugger

Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)
help command

With a command name as help argument, GDB displays a short paragraph on how to use that command.

apropos args

The apropos command searches through all of the GDB commands, and their documentation, for the regular expression specified in args. It prints out all matches found. For example:

apropos alias

results in:

alias -- Define a new command that is an alias of an existing command
aliases -- Aliases of other commands
d -- Delete some breakpoints or auto-display expressions
del -- Delete some breakpoints or auto-display expressions
delete -- Delete some breakpoints or auto-display expressions
complete args

The complete args command lists all the possible completions for the beginning of a command. Use args to specify the beginning of the command you want completed. For example:

complete i

results in:

if
ignore
info
inspect

This is intended for use by GNU Emacs.

In addition to help, you can use the GDB commands info and show to inquire about the state of your program, or the state of GDB itself. Each command supports many topics of inquiry; this manual introduces each of them in the appropriate context. The listings under info and under show in the Command, Variable, and Function Index point to all the sub-commands. See Command and Variable Index.

info

This command (abbreviated i) is for describing the state of your program. For example, you can show the arguments passed to a function with info args, list the registers currently in use with info registers, or list the breakpoints you have set with info breakpoints. You can get a complete list of the info sub-commands with help info.

set

You can assign the result of an expression to an environment variable with set. For example, you can set the GDB prompt to a $-sign with set prompt $.

show

In contrast to info, show is for describing the state of GDB itself. You can change most of the things you can show, by using the related command set; for example, you can control what number system is used for displays with set radix, or simply inquire which is currently in use with show radix.

To display all the settable parameters and their current values, you can use show with no arguments; you may also use info set. Both commands produce the same display.

Here are several miscellaneous show subcommands, all of which are exceptional in lacking corresponding set commands:

show version

Show what version of GDB is running. You should include this information in GDB bug-reports. If multiple versions of GDB are in use at your site, you may need to determine which version of GDB you are running; as GDB evolves, new commands are introduced, and old ones may wither away. Also, many system vendors ship variant versions of GDB, and there are variant versions of GDB in GNU/Linux distributions as well. The version number is the same as the one announced when you start GDB.

show copying
info copying

Display information about permission for copying GDB.

show warranty
info warranty

Display the GNU “NO WARRANTY” statement, or a warranty, if your version of GDB comes with one.

show configuration

Display detailed information about the way GDB was configured when it was built. This displays the optional arguments passed to the configure script and also configuration parameters detected automatically by configure. When reporting a GDB bug (see GDB Bugs), it is important to include this information in your report.


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4 Running Programs Under GDB

When you run a program under GDB, you must first generate debugging information when you compile it.

You may start GDB with its arguments, if any, in an environment of your choice. If you are doing native debugging, you may redirect your program’s input and output, debug an already running process, or kill a child process.


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4.1 Compiling for Debugging

In order to debug a program effectively, you need to generate debugging information when you compile it. This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code.

To request debugging information, specify the ‘-g’ option when you run the compiler.

Programs that are to be shipped to your customers are compiled with optimizations, using the ‘-O’ compiler option. However, some compilers are unable to handle the ‘-g’ and ‘-O’ options together. Using those compilers, you cannot generate optimized executables containing debugging information.

GCC, the GNU C/C++ compiler, supports ‘-g’ with or without ‘-O’, making it possible to debug optimized code. We recommend that you always use ‘-g’ whenever you compile a program. You may think your program is correct, but there is no sense in pushing your luck. For more information, see Optimized Code.

Older versions of the GNU C compiler permitted a variant option ‘-gg for debugging information. GDB no longer supports this format; if your GNU C compiler has this option, do not use it.

GDB knows about preprocessor macros and can show you their expansion (see Macros). Most compilers do not include information about preprocessor macros in the debugging information if you specify the -g flag alone. Version 3.1 and later of GCC, the GNU C compiler, provides macro information if you are using the DWARF debugging format, and specify the option -g3.

See Options for Debugging Your Program or GCC in Using the GNU Compiler Collection (GCC), for more information on GCC options affecting debug information.

You will have the best debugging experience if you use the latest version of the DWARF debugging format that your compiler supports. DWARF is currently the most expressive and best supported debugging format in GDB.


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4.2 Starting your Program

run
r

Use the run command to start your program under GDB. You must first specify the program name with an argument to GDB (see Getting In and Out of GDB), or by using the file or exec-file command (see Commands to Specify Files).

If you are running your program in an execution environment that supports processes, run creates an inferior process and makes that process run your program. In some environments without processes, run jumps to the start of your program. Other targets, like ‘remote’, are always running. If you get an error message like this one:

The "remote" target does not support "run".
Try "help target" or "continue".

then use continue to run your program. You may need load first (see load).

The execution of a program is affected by certain information it receives from its superior. GDB provides ways to specify this information, which you must do before starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into four categories:

The arguments.

Specify the arguments to give your program as the arguments of the run command. If a shell is available on your target, the shell is used to pass the arguments, so that you may use normal conventions (such as wildcard expansion or variable substitution) in describing the arguments. In Unix systems, you can control which shell is used with the SHELL environment variable. If you do not define SHELL, GDB uses the default shell (/bin/sh). You can disable use of any shell with the set startup-with-shell command (see below for details).

The environment.

Your program normally inherits its environment from GDB, but you can use the GDB commands set environment and unset environment to change parts of the environment that affect your program. See Your Program’s Environment.

The working directory.

Your program inherits its working directory from GDB. You can set the GDB working directory with the cd command in GDB. See Your Program’s Working Directory.

The standard input and output.

Your program normally uses the same device for standard input and standard output as GDB is using. You can redirect input and output in the run command line, or you can use the tty command to set a different device for your program. See Your Program’s Input and Output.

Warning: While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program; if you attempt this, GDB is likely to wind up debugging the wrong program.

When you issue the run command, your program begins to execute immediately. See Stopping and Continuing, for discussion of how to arrange for your program to stop. Once your program has stopped, you may call functions in your program, using the print or call commands. See Examining Data.

If the modification time of your symbol file has changed since the last time GDB read its symbols, GDB discards its symbol table, and reads it again. When it does this, GDB tries to retain your current breakpoints.

start

The name of the main procedure can vary from language to language. With C or C++, the main procedure name is always main, but other languages such as Ada do not require a specific name for their main procedure. The debugger provides a convenient way to start the execution of the program and to stop at the beginning of the main procedure, depending on the language used.

The ‘start’ command does the equivalent of setting a temporary breakpoint at the beginning of the main procedure and then invoking the ‘run’ command.

Some programs contain an elaboration phase where some startup code is executed before the main procedure is called. This depends on the languages used to write your program. In C++, for instance, constructors for static and global objects are executed before main is called. It is therefore possible that the debugger stops before reaching the main procedure. However, the temporary breakpoint will remain to halt execution.

Specify the arguments to give to your program as arguments to the ‘start’ command. These arguments will be given verbatim to the underlying ‘run’ command. Note that the same arguments will be reused if no argument is provided during subsequent calls to ‘start’ or ‘run’.

It is sometimes necessary to debug the program during elaboration. In these cases, using the start command would stop the execution of your program too late, as the program would have already completed the elaboration phase. Under these circumstances, insert breakpoints in your elaboration code before running your program.

set exec-wrapper wrapper
show exec-wrapper
unset exec-wrapper

When ‘exec-wrapper’ is set, the specified wrapper is used to launch programs for debugging. GDB starts your program with a shell command of the form exec wrapper program. Quoting is added to program and its arguments, but not to wrapper, so you should add quotes if appropriate for your shell. The wrapper runs until it executes your program, and then GDB takes control.

You can use any program that eventually calls execve with its arguments as a wrapper. Several standard Unix utilities do this, e.g. env and nohup. Any Unix shell script ending with exec "$@" will also work.

For example, you can use env to pass an environment variable to the debugged program, without setting the variable in your shell’s environment:

(gdb) set exec-wrapper env 'LD_PRELOAD=libtest.so'
(gdb) run

This command is available when debugging locally on most targets, excluding DJGPP, Cygwin, MS Windows, and QNX Neutrino.

set startup-with-shell
set startup-with-shell on
set startup-with-shell off
show set startup-with-shell

On Unix systems, by default, if a shell is available on your target, GDB) uses it to start your program. Arguments of the run command are passed to the shell, which does variable substitution, expands wildcard characters and performs redirection of I/O. In some circumstances, it may be useful to disable such use of a shell, for example, when debugging the shell itself or diagnosing startup failures such as:

(gdb) run
Starting program: ./a.out
During startup program terminated with signal SIGSEGV, Segmentation fault.

which indicates the shell or the wrapper specified with ‘exec-wrapper’ crashed, not your program. Most often, this is caused by something odd in your shell’s non-interactive mode initialization file—such as .cshrc for C-shell, $.zshenv for the Z shell, or the file specified in the ‘BASH_ENV’ environment variable for BASH.

set auto-connect-native-target
set auto-connect-native-target on
set auto-connect-native-target off
show auto-connect-native-target

By default, if not connected to any target yet (e.g., with target remote), the run command starts your program as a native process under GDB, on your local machine. If you’re sure you don’t want to debug programs on your local machine, you can tell GDB to not connect to the native target automatically with the set auto-connect-native-target off command.

If on, which is the default, and if GDB is not connected to a target already, the run command automaticaly connects to the native target, if one is available.

If off, and if GDB is not connected to a target already, the run command fails with an error:

(gdb) run
Don't know how to run.  Try "help target".

If GDB is already connected to a target, GDB always uses it with the run command.

In any case, you can explicitly connect to the native target with the target native command. For example,

(gdb) set auto-connect-native-target off
(gdb) run
Don't know how to run.  Try "help target".
(gdb) target native
(gdb) run
Starting program: ./a.out
[Inferior 1 (process 10421) exited normally]

In case you connected explicitly to the native target, GDB remains connected even if all inferiors exit, ready for the next run command. Use the disconnect command to disconnect.

Examples of other commands that likewise respect the auto-connect-native-target setting: attach, info proc, info os.

set disable-randomization
set disable-randomization on

This option (enabled by default in GDB) will turn off the native randomization of the virtual address space of the started program. This option is useful for multiple debugging sessions to make the execution better reproducible and memory addresses reusable across debugging sessions.

This feature is implemented only on certain targets, including GNU/Linux. On GNU/Linux you can get the same behavior using

(gdb) set exec-wrapper setarch `uname -m` -R
set disable-randomization off

Leave the behavior of the started executable unchanged. Some bugs rear their ugly heads only when the program is loaded at certain addresses. If your bug disappears when you run the program under GDB, that might be because GDB by default disables the address randomization on platforms, such as GNU/Linux, which do that for stand-alone programs. Use set disable-randomization off to try to reproduce such elusive bugs.

On targets where it is available, virtual address space randomization protects the programs against certain kinds of security attacks. In these cases the attacker needs to know the exact location of a concrete executable code. Randomizing its location makes it impossible to inject jumps misusing a code at its expected addresses.

Prelinking shared libraries provides a startup performance advantage but it makes addresses in these libraries predictable for privileged processes by having just unprivileged access at the target system. Reading the shared library binary gives enough information for assembling the malicious code misusing it. Still even a prelinked shared library can get loaded at a new random address just requiring the regular relocation process during the startup. Shared libraries not already prelinked are always loaded at a randomly chosen address.

Position independent executables (PIE) contain position independent code similar to the shared libraries and therefore such executables get loaded at a randomly chosen address upon startup. PIE executables always load even already prelinked shared libraries at a random address. You can build such executable using gcc -fPIE -pie.

Heap (malloc storage), stack and custom mmap areas are always placed randomly (as long as the randomization is enabled).

show disable-randomization

Show the current setting of the explicit disable of the native randomization of the virtual address space of the started program.


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4.3 Your Program’s Arguments

The arguments to your program can be specified by the arguments of the run command. They are passed to a shell, which expands wildcard characters and performs redirection of I/O, and thence to your program. Your SHELL environment variable (if it exists) specifies what shell GDB uses. If you do not define SHELL, GDB uses the default shell (/bin/sh on Unix).

On non-Unix systems, the program is usually invoked directly by GDB, which emulates I/O redirection via the appropriate system calls, and the wildcard characters are expanded by the startup code of the program, not by the shell.

run with no arguments uses the same arguments used by the previous run, or those set by the set args command.

set args

Specify the arguments to be used the next time your program is run. If set args has no arguments, run executes your program with no arguments. Once you have run your program with arguments, using set args before the next run is the only way to run it again without arguments.

show args

Show the arguments to give your program when it is started.


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4.4 Your Program’s Environment

The environment consists of a set of environment variables and their values. Environment variables conventionally record such things as your user name, your home directory, your terminal type, and your search path for programs to run. Usually you set up environment variables with the shell and they are inherited by all the other programs you run. When debugging, it can be useful to try running your program with a modified environment without having to start GDB over again.

path directory

Add directory to the front of the PATH environment variable (the search path for executables) that will be passed to your program. The value of PATH used by GDB does not change. You may specify several directory names, separated by whitespace or by a system-dependent separator character (‘:’ on Unix, ‘;’ on MS-DOS and MS-Windows). If directory is already in the path, it is moved to the front, so it is searched sooner.

You can use the string ‘$cwd’ to refer to whatever is the current working directory at the time GDB searches the path. If you use ‘.’ instead, it refers to the directory where you executed the path command. GDB replaces ‘.’ in the directory argument (with the current path) before adding directory to the search path.

show paths

Display the list of search paths for executables (the PATH environment variable).

show environment [varname]

Print the value of environment variable varname to be given to your program when it starts. If you do not supply varname, print the names and values of all environment variables to be given to your program. You can abbreviate environment as env.

set environment varname [=value]

Set environment variable varname to value. The value changes for your program (and the shell GDB uses to launch it), not for GDB itself. The value may be any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself. The value parameter is optional; if it is eliminated, the variable is set to a null value.

For example, this command:

set env USER = foo

tells the debugged program, when subsequently run, that its user is named ‘foo’. (The spaces around ‘=’ are used for clarity here; they are not actually required.)

Note that on Unix systems, GDB runs your program via a shell, which also inherits the environment set with set environment. If necessary, you can avoid that by using the ‘env’ program as a wrapper instead of using set environment. See set exec-wrapper, for an example doing just that.

unset environment varname

Remove variable varname from the environment to be passed to your program. This is different from ‘set env varname =’; unset environment removes the variable from the environment, rather than assigning it an empty value.

Warning: On Unix systems, GDB runs your program using the shell indicated by your SHELL environment variable if it exists (or /bin/sh if not). If your SHELL variable names a shell that runs an initialization file when started non-interactively—such as .cshrc for C-shell, $.zshenv for the Z shell, or the file specified in the ‘BASH_ENV’ environment variable for BASH—any variables you set in that file affect your program. You may wish to move setting of environment variables to files that are only run when you sign on, such as .login or .profile.


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4.5 Your Program’s Working Directory

Each time you start your program with run, it inherits its working directory from the current working directory of GDB. The GDB working directory is initially whatever it inherited from its parent process (typically the shell), but you can specify a new working directory in GDB with the cd command.

The GDB working directory also serves as a default for the commands that specify files for GDB to operate on. See Commands to Specify Files.

cd [directory]

Set the GDB working directory to directory. If not given, directory uses '~'.

pwd

Print the GDB working directory.

It is generally impossible to find the current working directory of the process being debugged (since a program can change its directory during its run). If you work on a system where GDB is configured with the /proc support, you can use the info proc command (see SVR4 Process Information) to find out the current working directory of the debuggee.


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4.6 Your Program’s Input and Output

By default, the program you run under GDB does input and output to the same terminal that GDB uses. GDB switches the terminal to its own terminal modes to interact with you, but it records the terminal modes your program was using and switches back to them when you continue running your program.

info terminal

Displays information recorded by GDB about the terminal modes your program is using.

You can redirect your program’s input and/or output using shell redirection with the run command. For example,

run > outfile

starts your program, diverting its output to the file outfile.

Another way to specify where your program should do input and output is with the tty command. This command accepts a file name as argument, and causes this file to be the default for future run commands. It also resets the controlling terminal for the child process, for future run commands. For example,

tty /dev/ttyb

directs that processes started with subsequent run commands default to do input and output on the terminal /dev/ttyb and have that as their controlling terminal.

An explicit redirection in run overrides the tty command’s effect on the input/output device, but not its effect on the controlling terminal.

When you use the tty command or redirect input in the run command, only the input for your program is affected. The input for GDB still comes from your terminal. tty is an alias for set inferior-tty.

You can use the show inferior-tty command to tell GDB to display the name of the terminal that will be used for future runs of your program.

set inferior-tty /dev/ttyb

Set the tty for the program being debugged to /dev/ttyb.

show inferior-tty

Show the current tty for the program being debugged.


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4.7 Debugging an Already-running Process

attach process-id

This command attaches to a running process—one that was started outside GDB. (info files shows your active targets.) The command takes as argument a process ID. The usual way to find out the process-id of a Unix process is with the ps utility, or with the ‘jobs -l’ shell command.

attach does not repeat if you press RET a second time after executing the command.

To use attach, your program must be running in an environment which supports processes; for example, attach does not work for programs on bare-board targets that lack an operating system. You must also have permission to send the process a signal.

When you use attach, the debugger finds the program running in the process first by looking in the current working directory, then (if the program is not found) by using the source file search path (see Specifying Source Directories). You can also use the file command to load the program. See Commands to Specify Files.

The first thing GDB does after arranging to debug the specified process is to stop it. You can examine and modify an attached process with all the GDB commands that are ordinarily available when you start processes with run. You can insert breakpoints; you can step and continue; you can modify storage. If you would rather the process continue running, you may use the continue command after attaching GDB to the process.

detach

When you have finished debugging the attached process, you can use the detach command to release it from GDB control. Detaching the process continues its execution. After the detach command, that process and GDB become completely independent once more, and you are ready to attach another process or start one with run. detach does not repeat if you press RET again after executing the command.

If you exit GDB while you have an attached process, you detach that process. If you use the run command, you kill that process. By default, GDB asks for confirmation if you try to do either of these things; you can control whether or not you need to confirm by using the set confirm command (see Optional Warnings and Messages).


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4.8 Killing the Child Process

kill

Kill the child process in which your program is running under GDB.

This command is useful if you wish to debug a core dump instead of a running process. GDB ignores any core dump file while your program is running.

On some operating systems, a program cannot be executed outside GDB while you have breakpoints set on it inside GDB. You can use the kill command in this situation to permit running your program outside the debugger.

The kill command is also useful if you wish to recompile and relink your program, since on many systems it is impossible to modify an executable file while it is running in a process. In this case, when you next type run, GDB notices that the file has changed, and reads the symbol table again (while trying to preserve your current breakpoint settings).


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4.9 Debugging Multiple Inferiors and Programs

GDB lets you run and debug multiple programs in a single session. In addition, GDB on some systems may let you run several programs simultaneously (otherwise you have to exit from one before starting another). In the most general case, you can have multiple threads of execution in each of multiple processes, launched from multiple executables.

GDB represents the state of each program execution with an object called an inferior. An inferior typically corresponds to a process, but is more general and applies also to targets that do not have processes. Inferiors may be created before a process runs, and may be retained after a process exits. Inferiors have unique identifiers that are different from process ids. Usually each inferior will also have its own distinct address space, although some embedded targets may have several inferiors running in different parts of a single address space. Each inferior may in turn have multiple threads running in it.

To find out what inferiors exist at any moment, use info inferiors:

info inferiors

Print a list of all inferiors currently being managed by GDB.

GDB displays for each inferior (in this order):

  1. the inferior number assigned by GDB
  2. the target system’s inferior identifier
  3. the name of the executable the inferior is running.

An asterisk ‘*’ preceding the GDB inferior number indicates the current inferior.

For example,

(gdb) info inferiors
  Num  Description       Executable
  2    process 2307      hello
* 1    process 3401      goodbye

To switch focus between inferiors, use the inferior command:

inferior infno

Make inferior number infno the current inferior. The argument infno is the inferior number assigned by GDB, as shown in the first field of the ‘info inferiors’ display.

You can get multiple executables into a debugging session via the add-inferior and clone-inferior commands. On some systems GDB can add inferiors to the debug session automatically by following calls to fork and exec. To remove inferiors from the debugging session use the remove-inferiors command.

add-inferior [ -copies n ] [ -exec executable ]

Adds n inferiors to be run using executable as the executable; n defaults to 1. If no executable is specified, the inferiors begins empty, with no program. You can still assign or change the program assigned to the inferior at any time by using the file command with the executable name as its argument.

clone-inferior [ -copies n ] [ infno ]

Adds n inferiors ready to execute the same program as inferior infno; n defaults to 1, and infno defaults to the number of the current inferior. This is a convenient command when you want to run another instance of the inferior you are debugging.

(gdb) info inferiors
  Num  Description       Executable
* 1    process 29964     helloworld
(gdb) clone-inferior
Added inferior 2.
1 inferiors added.
(gdb) info inferiors
  Num  Description       Executable
  2    <null>            helloworld
* 1    process 29964     helloworld

You can now simply switch focus to inferior 2 and run it.

remove-inferiors infno

Removes the inferior or inferiors infno…. It is not possible to remove an inferior that is running with this command. For those, use the kill or detach command first.

To quit debugging one of the running inferiors that is not the current inferior, you can either detach from it by using the detach inferior command (allowing it to run independently), or kill it using the kill inferiors command:

detach inferior infno

Detach from the inferior or inferiors identified by GDB inferior number(s) infno…. Note that the inferior’s entry still stays on the list of inferiors shown by info inferiors, but its Description will show ‘<null>’.

kill inferiors infno

Kill the inferior or inferiors identified by GDB inferior number(s) infno…. Note that the inferior’s entry still stays on the list of inferiors shown by info inferiors, but its Description will show ‘<null>’.

After the successful completion of a command such as detach, detach inferiors, kill or kill inferiors, or after a normal process exit, the inferior is still valid and listed with info inferiors, ready to be restarted.

To be notified when inferiors are started or exit under GDB’s control use set print inferior-events:

set print inferior-events
set print inferior-events on
set print inferior-events off

The set print inferior-events command allows you to enable or disable printing of messages when GDB notices that new inferiors have started or that inferiors have exited or have been detached. By default, these messages will not be printed.

show print inferior-events

Show whether messages will be printed when GDB detects that inferiors have started, exited or have been detached.

Many commands will work the same with multiple programs as with a single program: e.g., print myglobal will simply display the value of myglobal in the current inferior.

Occasionaly, when debugging GDB itself, it may be useful to get more info about the relationship of inferiors, programs, address spaces in a debug session. You can do that with the maint info program-spaces command.

maint info program-spaces

Print a list of all program spaces currently being managed by GDB.

GDB displays for each program space (in this order):

  1. the program space number assigned by GDB
  2. the name of the executable loaded into the program space, with e.g., the file command.

An asterisk ‘*’ preceding the GDB program space number indicates the current program space.

In addition, below each program space line, GDB prints extra information that isn’t suitable to display in tabular form. For example, the list of inferiors bound to the program space.

(gdb) maint info program-spaces
  Id   Executable
  2    goodbye
        Bound inferiors: ID 1 (process 21561)
* 1    hello

Here we can see that no inferior is running the program hello, while process 21561 is running the program goodbye. On some targets, it is possible that multiple inferiors are bound to the same program space. The most common example is that of debugging both the parent and child processes of a vfork call. For example,

(gdb) maint info program-spaces
  Id   Executable
* 1    vfork-test
        Bound inferiors: ID 2 (process 18050), ID 1 (process 18045)

Here, both inferior 2 and inferior 1 are running in the same program space as a result of inferior 1 having executed a vfork call.


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4.10 Debugging Programs with Multiple Threads

In some operating systems, such as HP-UX and Solaris, a single program may have more than one thread of execution. The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes—except that they share one address space (that is, they can all examine and modify the same variables). On the other hand, each thread has its own registers and execution stack, and perhaps private memory.

GDB provides these facilities for debugging multi-thread programs:

Warning: These facilities are not yet available on every GDB configuration where the operating system supports threads. If your GDB does not support threads, these commands have no effect. For example, a system without thread support shows no output from ‘info threads’, and always rejects the thread command, like this:

(gdb) info threads
(gdb) thread 1
Thread ID 1 not known.  Use the "info threads" command to
see the IDs of currently known threads.

The GDB thread debugging facility allows you to observe all threads while your program runs—but whenever GDB takes control, one thread in particular is always the focus of debugging. This thread is called the current thread. Debugging commands show program information from the perspective of the current thread.

Whenever GDB detects a new thread in your program, it displays the target system’s identification for the thread with a message in the form ‘[New systag]’, where systag is a thread identifier whose form varies depending on the particular system. For example, on GNU/Linux, you might see

[New Thread 0x41e02940 (LWP 25582)]

when GDB notices a new thread. In contrast, on an SGI system, the systag is simply something like ‘process 368’, with no further qualifier.

For debugging purposes, GDB associates its own thread number—always a single integer—with each thread in your program.

info threads [id]

Display a summary of all threads currently in your program. Optional argument id… is one or more thread ids separated by spaces, and means to print information only about the specified thread or threads. GDB displays for each thread (in this order):

  1. the thread number assigned by GDB
  2. the target system’s thread identifier (systag)
  3. the thread’s name, if one is known. A thread can either be named by the user (see thread name, below), or, in some cases, by the program itself.
  4. the current stack frame summary for that thread

An asterisk ‘*’ to the left of the GDB thread number indicates the current thread.

For example,

(gdb) info threads
  Id   Target Id         Frame
  3    process 35 thread 27  0x34e5 in sigpause ()
  2    process 35 thread 23  0x34e5 in sigpause ()
* 1    process 35 thread 13  main (argc=1, argv=0x7ffffff8)
    at threadtest.c:68

On Solaris, you can display more information about user threads with a Solaris-specific command:

maint info sol-threads

Display info on Solaris user threads.

thread threadno

Make thread number threadno the current thread. The command argument threadno is the internal GDB thread number, as shown in the first field of the ‘info threads’ display. GDB responds by displaying the system identifier of the thread you selected, and its current stack frame summary:

(gdb) thread 2
[Switching to thread 2 (Thread 0xb7fdab70 (LWP 12747))]
#0  some_function (ignore=0x0) at example.c:8
8	    printf ("hello\n");

As with the ‘[New …]’ message, the form of the text after ‘Switching to’ depends on your system’s conventions for identifying threads.

The debugger convenience variable ‘$_thread’ contains the number of the current thread. You may find this useful in writing breakpoint conditional expressions, command scripts, and so forth. See See Convenience Variables, for general information on convenience variables.

thread apply [threadno | all] command

The thread apply command allows you to apply the named command to one or more threads. Specify the numbers of the threads that you want affected with the command argument threadno. It can be a single thread number, one of the numbers shown in the first field of the ‘info threads’ display; or it could be a range of thread numbers, as in 2-4. To apply a command to all threads, type thread apply all command.

thread name [name]

This command assigns a name to the current thread. If no argument is given, any existing user-specified name is removed. The thread name appears in the ‘info threads’ display.

On some systems, such as GNU/Linux, GDB is able to determine the name of the thread as given by the OS. On these systems, a name specified with ‘thread name’ will override the system-give name, and removing the user-specified name will cause GDB to once again display the system-specified name.

thread find [regexp]

Search for and display thread ids whose name or systag matches the supplied regular expression.

As well as being the complement to the ‘thread name’ command, this command also allows you to identify a thread by its target systag. For instance, on GNU/Linux, the target systag is the LWP id.

(GDB) thread find 26688
Thread 4 has target id 'Thread 0x41e02940 (LWP 26688)'
(GDB) info thread 4
  Id   Target Id         Frame 
  4    Thread 0x41e02940 (LWP 26688) 0x00000031ca6cd372 in select ()
set print thread-events
set print thread-events on
set print thread-events off

The set print thread-events command allows you to enable or disable printing of messages when GDB notices that new threads have started or that threads have exited. By default, these messages will be printed if detection of these events is supported by the target. Note that these messages cannot be disabled on all targets.

show print thread-events

Show whether messages will be printed when GDB detects that threads have started and exited.

See Stopping and Starting Multi-thread Programs, for more information about how GDB behaves when you stop and start programs with multiple threads.

See Setting Watchpoints, for information about watchpoints in programs with multiple threads.

set libthread-db-search-path [path]

If this variable is set, path is a colon-separated list of directories GDB will use to search for libthread_db. If you omit path, ‘libthread-db-search-path’ will be reset to its default value ($sdir:$pdir on GNU/Linux and Solaris systems). Internally, the default value comes from the LIBTHREAD_DB_SEARCH_PATH macro.

On GNU/Linux and Solaris systems, GDB uses a “helper” libthread_db library to obtain information about threads in the inferior process. GDB will use ‘libthread-db-search-path’ to find libthread_db. GDB also consults first if inferior specific thread debugging library loading is enabled by ‘set auto-load libthread-db’ (see libthread_db.so.1 file).

A special entry ‘$sdir’ for ‘libthread-db-search-path’ refers to the default system directories that are normally searched for loading shared libraries. The ‘$sdir’ entry is the only kind not needing to be enabled by ‘set auto-load libthread-db’ (see libthread_db.so.1 file).

A special entry ‘$pdir’ for ‘libthread-db-search-path’ refers to the directory from which libpthread was loaded in the inferior process.

For any libthread_db library GDB finds in above directories, GDB attempts to initialize it with the current inferior process. If this initialization fails (which could happen because of a version mismatch between libthread_db and libpthread), GDB will unload libthread_db, and continue with the next directory. If none of libthread_db libraries initialize successfully, GDB will issue a warning and thread debugging will be disabled.

Setting libthread-db-search-path is currently implemented only on some platforms.

show libthread-db-search-path

Display current libthread_db search path.

set debug libthread-db
show debug libthread-db

Turns on or off display of libthread_db-related events. Use 1 to enable, 0 to disable.


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4.11 Debugging Forks

On most systems, GDB has no special support for debugging programs which create additional processes using the fork function. When a program forks, GDB will continue to debug the parent process and the child process will run unimpeded. If you have set a breakpoint in any code which the child then executes, the child will get a SIGTRAP signal which (unless it catches the signal) will cause it to terminate.

However, if you want to debug the child process there is a workaround which isn’t too painful. Put a call to sleep in the code which the child process executes after the fork. It may be useful to sleep only if a certain environment variable is set, or a certain file exists, so that the delay need not occur when you don’t want to run GDB on the child. While the child is sleeping, use the ps program to get its process ID. Then tell GDB (a new invocation of GDB if you are also debugging the parent process) to attach to the child process (see Attach). From that point on you can debug the child process just like any other process which you attached to.

On some systems, GDB provides support for debugging programs that create additional processes using the fork or vfork functions. Currently, the only platforms with this feature are HP-UX (11.x and later only?) and GNU/Linux (kernel version 2.5.60 and later).

By default, when a program forks, GDB will continue to debug the parent process and the child process will run unimpeded.

If you want to follow the child process instead of the parent process, use the command set follow-fork-mode.

set follow-fork-mode mode

Set the debugger response to a program call of fork or vfork. A call to fork or vfork creates a new process. The mode argument can be:

parent

The original process is debugged after a fork. The child process runs unimpeded. This is the default.

child

The new process is debugged after a fork. The parent process runs unimpeded.

show follow-fork-mode

Display the current debugger response to a fork or vfork call.

On Linux, if you want to debug both the parent and child processes, use the command set detach-on-fork.

set detach-on-fork mode

Tells gdb whether to detach one of the processes after a fork, or retain debugger control over them both.

on

The child process (or parent process, depending on the value of follow-fork-mode) will be detached and allowed to run independently. This is the default.

off

Both processes will be held under the control of GDB. One process (child or parent, depending on the value of follow-fork-mode) is debugged as usual, while the other is held suspended.

show detach-on-fork

Show whether detach-on-fork mode is on/off.

If you choose to set ‘detach-on-fork’ mode off, then GDB will retain control of all forked processes (including nested forks). You can list the forked processes under the control of GDB by using the info inferiors command, and switch from one fork to another by using the inferior command (see Debugging Multiple Inferiors and Programs).

To quit debugging one of the forked processes, you can either detach from it by using the detach inferiors command (allowing it to run independently), or kill it using the kill inferiors command. See Debugging Multiple Inferiors and Programs.

If you ask to debug a child process and a vfork is followed by an exec, GDB executes the new target up to the first breakpoint in the new target. If you have a breakpoint set on main in your original program, the breakpoint will also be set on the child process’s main.

On some systems, when a child process is spawned by vfork, you cannot debug the child or parent until an exec call completes.

If you issue a run command to GDB after an exec call executes, the new target restarts. To restart the parent process, use the file command with the parent executable name as its argument. By default, after an exec call executes, GDB discards the symbols of the previous executable image. You can change this behaviour with the set follow-exec-mode command.

set follow-exec-mode mode

Set debugger response to a program call of exec. An exec call replaces the program image of a process.

follow-exec-mode can be:

new

GDB creates a new inferior and rebinds the process to this new inferior. The program the process was running before the exec call can be restarted afterwards by restarting the original inferior.

For example:

(gdb) info inferiors
(gdb) info inferior
  Id   Description   Executable
* 1    <null>        prog1
(gdb) run
process 12020 is executing new program: prog2
Program exited normally.
(gdb) info inferiors
  Id   Description   Executable
* 2    <null>        prog2
  1    <null>        prog1
same

GDB keeps the process bound to the same inferior. The new executable image replaces the previous executable loaded in the inferior. Restarting the inferior after the exec call, with e.g., the run command, restarts the executable the process was running after the exec call. This is the default mode.

For example:

(gdb) info inferiors
  Id   Description   Executable
* 1    <null>        prog1
(gdb) run
process 12020 is executing new program: prog2
Program exited normally.
(gdb) info inferiors
  Id   Description   Executable
* 1    <null>        prog2

You can use the catch command to make GDB stop whenever a fork, vfork, or exec call is made. See Setting Catchpoints.


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4.12 Setting a Bookmark to Return to Later

On certain operating systems3, GDB is able to save a snapshot of a program’s state, called a checkpoint, and come back to it later.

Returning to a checkpoint effectively undoes everything that has happened in the program since the checkpoint was saved. This includes changes in memory, registers, and even (within some limits) system state. Effectively, it is like going back in time to the moment when the checkpoint was saved.

Thus, if you’re stepping thru a program and you think you’re getting close to the point where things go wrong, you can save a checkpoint. Then, if you accidentally go too far and miss the critical statement, instead of having to restart your program from the beginning, you can just go back to the checkpoint and start again from there.

This can be especially useful if it takes a lot of time or steps to reach the point where you think the bug occurs.

To use the checkpoint/restart method of debugging:

checkpoint

Save a snapshot of the debugged program’s current execution state. The checkpoint command takes no arguments, but each checkpoint is assigned a small integer id, similar to a breakpoint id.

info checkpoints

List the checkpoints that have been saved in the current debugging session. For each checkpoint, the following information will be listed:

Checkpoint ID
Process ID
Code Address
Source line, or label
restart checkpoint-id

Restore the program state that was saved as checkpoint number checkpoint-id. All program variables, registers, stack frames etc. will be returned to the values that they had when the checkpoint was saved. In essence, gdb will “wind back the clock” to the point in time when the checkpoint was saved.

Note that breakpoints, GDB variables, command history etc. are not affected by restoring a checkpoint. In general, a checkpoint only restores things that reside in the program being debugged, not in the debugger.

delete checkpoint checkpoint-id

Delete the previously-saved checkpoint identified by checkpoint-id.

Returning to a previously saved checkpoint will restore the user state of the program being debugged, plus a significant subset of the system (OS) state, including file pointers. It won’t “un-write” data from a file, but it will rewind the file pointer to the previous location, so that the previously written data can be overwritten. For files opened in read mode, the pointer will also be restored so that the previously read data can be read again.

Of course, characters that have been sent to a printer (or other external device) cannot be “snatched back”, and characters received from eg. a serial device can be removed from internal program buffers, but they cannot be “pushed back” into the serial pipeline, ready to be received again. Similarly, the actual contents of files that have been changed cannot be restored (at this time).

However, within those constraints, you actually can “rewind” your program to a previously saved point in time, and begin debugging it again — and you can change the course of events so as to debug a different execution path this time.

Finally, there is one bit of internal program state that will be different when you return to a checkpoint — the program’s process id. Each checkpoint will have a unique process id (or pid), and each will be different from the program’s original pid. If your program has saved a local copy of its process id, this could potentially pose a problem.

4.12.1 A Non-obvious Benefit of Using Checkpoints

On some systems such as GNU/Linux, address space randomization is performed on new processes for security reasons. This makes it difficult or impossible to set a breakpoint, or watchpoint, on an absolute address if you have to restart the program, since the absolute location of a symbol will change from one execution to the next.

A checkpoint, however, is an identical copy of a process. Therefore if you create a checkpoint at (eg.) the start of main, and simply return to that checkpoint instead of restarting the process, you can avoid the effects of address randomization and your symbols will all stay in the same place.


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5 Stopping and Continuing

The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and find out why.

Inside GDB, your program may stop for any of several reasons, such as a signal, a breakpoint, or reaching a new line after a GDB command such as step. You may then examine and change variables, set new breakpoints or remove old ones, and then continue execution. Usually, the messages shown by GDB provide ample explanation of the status of your program—but you can also explicitly request this information at any time.

info program

Display information about the status of your program: whether it is running or not, what process it is, and why it stopped.


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5.1 Breakpoints, Watchpoints, and Catchpoints

A breakpoint makes your program stop whenever a certain point in the program is reached. For each breakpoint, you can add conditions to control in finer detail whether your program stops. You can set breakpoints with the break command and its variants (see Setting Breakpoints), to specify the place where your program should stop by line number, function name or exact address in the program.

On some systems, you can set breakpoints in shared libraries before the executable is run. There is a minor limitation on HP-UX systems: you must wait until the executable is run in order to set breakpoints in shared library routines that are not called directly by the program (for example, routines that are arguments in a pthread_create call).

A watchpoint is a special breakpoint that stops your program when the value of an expression changes. The expression may be a value of a variable, or it could involve values of one or more variables combined by operators, such as ‘a + b’. This is sometimes called data breakpoints. You must use a different command to set watchpoints (see Setting Watchpoints), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.

You can arrange to have values from your program displayed automatically whenever GDB stops at a breakpoint. See Automatic Display.

A catchpoint is another special breakpoint that stops your program when a certain kind of event occurs, such as the throwing of a C++ exception or the loading of a library. As with watchpoints, you use a different command to set a catchpoint (see Setting Catchpoints), but aside from that, you can manage a catchpoint like any other breakpoint. (To stop when your program receives a signal, use the handle command; see Signals.)

GDB assigns a number to each breakpoint, watchpoint, or catchpoint when you create it; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be enabled or disabled; if disabled, it has no effect on your program until you enable it again.

Some GDB commands accept a range of breakpoints on which to operate. A breakpoint range is either a single breakpoint number, like ‘5’, or two such numbers, in increasing order, separated by a hyphen, like ‘5-7’. When a breakpoint range is given to a command, all breakpoints in that range are operated on.


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5.1.1 Setting Breakpoints

Breakpoints are set with the break command (abbreviated b). The debugger convenience variable ‘$bpnum’ records the number of the breakpoint you’ve set most recently; see Convenience Variables, for a discussion of what you can do with convenience variables.

break location

Set a breakpoint at the given location, which can specify a function name, a line number, or an address of an instruction. (See Specify Location, for a list of all the possible ways to specify a location.) The breakpoint will stop your program just before it executes any of the code in the specified location.

When using source languages that permit overloading of symbols, such as C++, a function name may refer to more than one possible place to break. See Ambiguous Expressions, for a discussion of that situation.

It is also possible to insert a breakpoint that will stop the program only if a specific thread (see Thread-Specific Breakpoints) or a specific task (see Ada Tasks) hits that breakpoint.

break

When called without any arguments, break sets a breakpoint at the next instruction to be executed in the selected stack frame (see Examining the Stack). In any selected frame but the innermost, this makes your program stop as soon as control returns to that frame. This is similar to the effect of a finish command in the frame inside the selected frame—except that finish does not leave an active breakpoint. If you use break without an argument in the innermost frame, GDB stops the next time it reaches the current location; this may be useful inside loops.

GDB normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies whether or not the breakpoint already existed when your program stopped.

break … if cond

Set a breakpoint with condition cond; evaluate the expression cond each time the breakpoint is reached, and stop only if the value is nonzero—that is, if cond evaluates as true. ‘’ stands for one of the possible arguments described above (or no argument) specifying where to break. See Break Conditions, for more information on breakpoint conditions.

tbreak args

Set a breakpoint enabled only for one stop. The args are the same as for the break command, and the breakpoint is set in the same way, but the breakpoint is automatically deleted after the first time your program stops there. See Disabling Breakpoints.

hbreak args

Set a hardware-assisted breakpoint. The args are the same as for the break command and the breakpoint is set in the same way, but the breakpoint requires hardware support and some target hardware may not have this support. The main purpose of this is EPROM/ROM code debugging, so you can set a breakpoint at an instruction without changing the instruction. This can be used with the new trap-generation provided by SPARClite DSU and most x86-based targets. These targets will generate traps when a program accesses some data or instruction address that is assigned to the debug registers. However the hardware breakpoint registers can take a limited number of breakpoints. For example, on the DSU, only two data breakpoints can be set at a time, and GDB will reject this command if more than two are used. Delete or disable unused hardware breakpoints before setting new ones (see Disabling Breakpoints). See Break Conditions. For remote targets, you can restrict the number of hardware breakpoints GDB will use, see set remote hardware-breakpoint-limit.

thbreak args

Set a hardware-assisted breakpoint enabled only for one stop. The args are the same as for the hbreak command and the breakpoint is set in the same way. However, like the tbreak command, the breakpoint is automatically deleted after the first time your program stops there. Also, like the hbreak command, the breakpoint requires hardware support and some target hardware may not have this support. See Disabling Breakpoints. See also Break Conditions.

rbreak regex

Set breakpoints on all functions matching the regular expression regex. This command sets an unconditional breakpoint on all matches, printing a list of all breakpoints it set. Once these breakpoints are set, they are treated just like the breakpoints set with the break command. You can delete them, disable them, or make them conditional the same way as any other breakpoint.

The syntax of the regular expression is the standard one used with tools like grep. Note that this is different from the syntax used by shells, so for instance foo* matches all functions that include an fo followed by zero or more os. There is an implicit .* leading and trailing the regular expression you supply, so to match only functions that begin with foo, use ^foo.

When debugging C++ programs, rbreak is useful for setting breakpoints on overloaded functions that are not members of any special classes.

The rbreak command can be used to set breakpoints in all the functions in a program, like this:

(gdb) rbreak .
rbreak file:regex

If rbreak is called with a filename qualification, it limits the search for functions matching the given regular expression to the specified file. This can be used, for example, to set breakpoints on every function in a given file:

(gdb) rbreak file.c:.

The colon separating the filename qualifier from the regex may optionally be surrounded by spaces.

info breakpoints [n]
info break [n]

Print a table of all breakpoints, watchpoints, and catchpoints set and not deleted. Optional argument n means print information only about the specified breakpoint(s) (or watchpoint(s) or catchpoint(s)). For each breakpoint, following columns are printed:

Breakpoint Numbers
Type

Breakpoint, watchpoint, or catchpoint.

Disposition

Whether the breakpoint is marked to be disabled or deleted when hit.

Enabled or Disabled

Enabled breakpoints are marked with ‘y’. ‘n’ marks breakpoints that are not enabled.

Address

Where the breakpoint is in your program, as a memory address. For a pending breakpoint whose address is not yet known, this field will contain ‘<PENDING>’. Such breakpoint won’t fire until a shared library that has the symbol or line referred by breakpoint is loaded. See below for details. A breakpoint with several locations will have ‘<MULTIPLE>’ in this field—see below for details.

What

Where the breakpoint is in the source for your program, as a file and line number. For a pending breakpoint, the original string passed to the breakpoint command will be listed as it cannot be resolved until the appropriate shared library is loaded in the future.

If a breakpoint is conditional, there are two evaluation modes: “host” and “target”. If mode is “host”, breakpoint condition evaluation is done by GDB on the host’s side. If it is “target”, then the condition is evaluated by the target. The info break command shows the condition on the line following the affected breakpoint, together with its condition evaluation mode in between parentheses.

Breakpoint commands, if any, are listed after that. A pending breakpoint is allowed to have a condition specified for it. The condition is not parsed for validity until a shared library is loaded that allows the pending breakpoint to resolve to a valid location.

info break with a breakpoint number n as argument lists only that breakpoint. The convenience variable $_ and the default examining-address for the x command are set to the address of the last breakpoint listed (see Examining Memory).

info break displays a count of the number of times the breakpoint has been hit. This is especially useful in conjunction with the ignore command. You can ignore a large number of breakpoint hits, look at the breakpoint info to see how many times the breakpoint was hit, and then run again, ignoring one less than that number. This will get you quickly to the last hit of that breakpoint.

For a breakpoints with an enable count (xref) greater than 1, info break also displays that count.

GDB allows you to set any number of breakpoints at the same place in your program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (see Break Conditions).

It is possible that a breakpoint corresponds to several locations in your program. Examples of this situation are:

In all those cases, GDB will insert a breakpoint at all the relevant locations.

A breakpoint with multiple locations is displayed in the breakpoint table using several rows—one header row, followed by one row for each breakpoint location. The header row has ‘<MULTIPLE>’ in the address column. The rows for individual locations contain the actual addresses for locations, and show the functions to which those locations belong. The number column for a location is of the form breakpoint-number.location-number.

For example:

Num     Type           Disp Enb  Address    What
1       breakpoint     keep y    <MULTIPLE>
        stop only if i==1
        breakpoint already hit 1 time
1.1                         y    0x080486a2 in void foo<int>() at t.cc:8
1.2                         y    0x080486ca in void foo<double>() at t.cc:8

Each location can be individually enabled or disabled by passing breakpoint-number.location-number as argument to the enable and disable commands. Note that you cannot delete the individual locations from the list, you can only delete the entire list of locations that belong to their parent breakpoint (with the delete num command, where num is the number of the parent breakpoint, 1 in the above example). Disabling or enabling the parent breakpoint (see Disabling) affects all of the locations that belong to that breakpoint.

It’s quite common to have a breakpoint inside a shared library. Shared libraries can be loaded and unloaded explicitly, and possibly repeatedly, as the program is executed. To support this use case, GDB updates breakpoint locations whenever any shared library is loaded or unloaded. Typically, you would set a breakpoint in a shared library at the beginning of your debugging session, when the library is not loaded, and when the symbols from the library are not available. When you try to set breakpoint, GDB will ask you if you want to set a so called pending breakpoint—breakpoint whose address is not yet resolved.

After the program is run, whenever a new shared library is loaded, GDB reevaluates all the breakpoints. When a newly loaded shared library contains the symbol or line referred to by some pending breakpoint, that breakpoint is resolved and becomes an ordinary breakpoint. When a library is unloaded, all breakpoints that refer to its symbols or source lines become pending again.

This logic works for breakpoints with multiple locations, too. For example, if you have a breakpoint in a C++ template function, and a newly loaded shared library has an instantiation of that template, a new location is added to the list of locations for the breakpoint.

Except for having unresolved address, pending breakpoints do not differ from regular breakpoints. You can set conditions or commands, enable and disable them and perform other breakpoint operations.

GDB provides some additional commands for controlling what happens when the ‘break’ command cannot resolve breakpoint address specification to an address:

set breakpoint pending auto

This is the default behavior. When GDB cannot find the breakpoint location, it queries you whether a pending breakpoint should be created.

set breakpoint pending on

This indicates that an unrecognized breakpoint location should automatically result in a pending breakpoint being created.

set breakpoint pending off

This indicates that pending breakpoints are not to be created. Any unrecognized breakpoint location results in an error. This setting does not affect any pending breakpoints previously created.

show breakpoint pending

Show the current behavior setting for creating pending breakpoints.

The settings above only affect the break command and its variants. Once breakpoint is set, it will be automatically updated as shared libraries are loaded and unloaded.

For some targets, GDB can automatically decide if hardware or software breakpoints should be used, depending on whether the breakpoint address is read-only or read-write. This applies to breakpoints set with the break command as well as to internal breakpoints set by commands like next and finish. For breakpoints set with hbreak, GDB will always use hardware breakpoints.

You can control this automatic behaviour with the following commands::

set breakpoint auto-hw on

This is the default behavior. When GDB sets a breakpoint, it will try to use the target memory map to decide if software or hardware breakpoint must be used.

set breakpoint auto-hw off

This indicates GDB should not automatically select breakpoint type. If the target provides a memory map, GDB will warn when trying to set software breakpoint at a read-only address.

GDB normally implements breakpoints by replacing the program code at the breakpoint address with a special instruction, which, when executed, given control to the debugger. By default, the program code is so modified only when the program is resumed. As soon as the program stops, GDB restores the original instructions. This behaviour guards against leaving breakpoints inserted in the target should gdb abrubptly disconnect. However, with slow remote targets, inserting and removing breakpoint can reduce the performance. This behavior can be controlled with the following commands::

set breakpoint always-inserted off

All breakpoints, including newly added by the user, are inserted in the target only when the target is resumed. All breakpoints are removed from the target when it stops. This is the default mode.

set breakpoint always-inserted on

Causes all breakpoints to be inserted in the target at all times. If the user adds a new breakpoint, or changes an existing breakpoint, the breakpoints in the target are updated immediately. A breakpoint is removed from the target only when breakpoint itself is deleted.

GDB handles conditional breakpoints by evaluating these conditions when a breakpoint breaks. If the condition is true, then the process being debugged stops, otherwise the process is resumed.

If the target supports evaluating conditions on its end, GDB may download the breakpoint, together with its conditions, to it.

This feature can be controlled via the following commands:

set breakpoint condition-evaluation host

This option commands GDB to evaluate the breakpoint conditions on the host’s side. Unconditional breakpoints are sent to the target which in turn receives the triggers and reports them back to GDB for condition evaluation. This is the standard evaluation mode.

set breakpoint condition-evaluation target

This option commands GDB to download breakpoint conditions to the target at the moment of their insertion. The target is responsible for evaluating the conditional expression and reporting breakpoint stop events back to GDB whenever the condition is true. Due to limitations of target-side evaluation, some conditions cannot be evaluated there, e.g., conditions that depend on local data that is only known to the host. Examples include conditional expressions involving convenience variables, complex types that cannot be handled by the agent expression parser and expressions that are too long to be sent over to the target, specially when the target is a remote system. In these cases, the conditions will be evaluated by GDB.

set breakpoint condition-evaluation auto

This is the default mode. If the target supports evaluating breakpoint conditions on its end, GDB will download breakpoint conditions to the target (limitations mentioned previously apply). If the target does not support breakpoint condition evaluation, then GDB will fallback to evaluating all these conditions on the host’s side.

GDB itself sometimes sets breakpoints in your program for special purposes, such as proper handling of longjmp (in C programs). These internal breakpoints are assigned negative numbers, starting with -1; ‘info breakpoints’ does not display them. You can see these breakpoints with the GDB maintenance command ‘maint info breakpoints’ (see maint info breakpoints).


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5.1.2 Setting Watchpoints

You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen. (This is sometimes called a data breakpoint.) The expression may be as simple as the value of a single variable, or as complex as many variables combined by operators. Examples include:

You can set a watchpoint on an expression even if the expression can not be evaluated yet. For instance, you can set a watchpoint on ‘*global_ptr’ before ‘global_ptr’ is initialized. GDB will stop when your program sets ‘global_ptr’ and the expression produces a valid value. If the expression becomes valid in some other way than changing a variable (e.g. if the memory pointed to by ‘*global_ptr’ becomes readable as the result of a malloc call), GDB may not stop until the next time the expression changes.

Depending on your system, watchpoints may be implemented in software or hardware. GDB does software watchpointing by single-stepping your program and testing the variable’s value each time, which is hundreds of times slower than normal execution. (But this may still be worth it, to catch errors where you have no clue what part of your program is the culprit.)

On some systems, such as HP-UX, PowerPC, GNU/Linux and most other x86-based targets, GDB includes support for hardware watchpoints, which do not slow down the running of your program.

watch [-l|-location] expr [thread threadnum] [mask maskvalue]

Set a watchpoint for an expression. GDB will break when the expression expr is written into by the program and its value changes. The simplest (and the most popular) use of this command is to watch the value of a single variable:

(gdb) watch foo

If the command includes a [thread threadnum] argument, GDB breaks only when the thread identified by threadnum changes the value of expr. If any other threads change the value of expr, GDB will not break. Note that watchpoints restricted to a single thread in this way only work with Hardware Watchpoints.

Ordinarily a watchpoint respects the scope of variables in expr (see below). The -location argument tells GDB to instead watch the memory referred to by expr. In this case, GDB will evaluate expr, take the address of the result, and watch the memory at that address. The type of the result is used to determine the size of the watched memory. If the expression’s result does not have an address, then GDB will print an error.

The [mask maskvalue] argument allows creation of masked watchpoints, if the current architecture supports this feature (e.g., PowerPC Embedded architecture, see PowerPC Embedded.) A masked watchpoint specifies a mask in addition to an address to watch. The mask specifies that some bits of an address (the bits which are reset in the mask) should be ignored when matching the address accessed by the inferior against the watchpoint address. Thus, a masked watchpoint watches many addresses simultaneously—those addresses whose unmasked bits are identical to the unmasked bits in the watchpoint address. The mask argument implies -location. Examples:

(gdb) watch foo mask 0xffff00ff
(gdb) watch *0xdeadbeef mask 0xffffff00
rwatch [-l|-location] expr [thread threadnum] [mask maskvalue]

Set a watchpoint that will break when the value of expr is read by the program.

awatch [-l|-location] expr [thread threadnum] [mask maskvalue]

Set a watchpoint that will break when expr is either read from or written into by the program.

info watchpoints [n]

This command prints a list of watchpoints, using the same format as info break (see Set Breaks).

If you watch for a change in a numerically entered address you need to dereference it, as the address itself is just a constant number which will never change. GDB refuses to create a watchpoint that watches a never-changing value:

(gdb) watch 0x600850
Cannot watch constant value 0x600850.
(gdb) watch *(int *) 0x600850
Watchpoint 1: *(int *) 6293584

GDB sets a hardware watchpoint if possible. Hardware watchpoints execute very quickly, and the debugger reports a change in value at the exact instruction where the change occurs. If GDB cannot set a hardware watchpoint, it sets a software watchpoint, which executes more slowly and reports the change in value at the next statement, not the instruction, after the change occurs.

You can force GDB to use only software watchpoints with the set can-use-hw-watchpoints 0 command. With this variable set to zero, GDB will never try to use hardware watchpoints, even if the underlying system supports them. (Note that hardware-assisted watchpoints that were set before setting can-use-hw-watchpoints to zero will still use the hardware mechanism of watching expression values.)

set can-use-hw-watchpoints

Set whether or not to use hardware watchpoints.

show can-use-hw-watchpoints

Show the current mode of using hardware watchpoints.

For remote targets, you can restrict the number of hardware watchpoints GDB will use, see set remote hardware-breakpoint-limit.

When you issue the watch command, GDB reports

Hardware watchpoint num: expr

if it was able to set a hardware watchpoint.

Currently, the awatch and rwatch commands can only set hardware watchpoints, because accesses to data that don’t change the value of the watched expression cannot be detected without examining every instruction as it is being executed, and GDB does not do that currently. If GDB finds that it is unable to set a hardware breakpoint with the awatch or rwatch command, it will print a message like this:

Expression cannot be implemented with read/access watchpoint.

Sometimes, GDB cannot set a hardware watchpoint because the data type of the watched expression is wider than what a hardware watchpoint on the target machine can handle. For example, some systems can only watch regions that are up to 4 bytes wide; on such systems you cannot set hardware watchpoints for an expression that yields a double-precision floating-point number (which is typically 8 bytes wide). As a work-around, it might be possible to break the large region into a series of smaller ones and watch them with separate watchpoints.

If you set too many hardware watchpoints, GDB might be unable to insert all of them when you resume the execution of your program. Since the precise number of active watchpoints is unknown until such time as the program is about to be resumed, GDB might not be able to warn you about this when you set the watchpoints, and the warning will be printed only when the program is resumed:

Hardware watchpoint num: Could not insert watchpoint

If this happens, delete or disable some of the watchpoints.

Watching complex expressions that reference many variables can also exhaust the resources available for hardware-assisted watchpoints. That’s because GDB needs to watch every variable in the expression with separately allocated resources.

If you call a function interactively using print or call, any watchpoints you have set will be inactive until GDB reaches another kind of breakpoint or the call completes.

GDB automatically deletes watchpoints that watch local (automatic) variables, or expressions that involve such variables, when they go out of scope, that is, when the execution leaves the block in which these variables were defined. In particular, when the program being debugged terminates, all local variables go out of scope, and so only watchpoints that watch global variables remain set. If you rerun the program, you will need to set all such watchpoints again. One way of doing that would be to set a code breakpoint at the entry to the main function and when it breaks, set all the watchpoints.

In multi-threaded programs, watchpoints will detect changes to the watched expression from every thread.

Warning: In multi-threaded programs, software watchpoints have only limited usefulness. If GDB creates a software watchpoint, it can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread’s activity (and if you are also confident that no other thread can become current), then you can use software watchpoints as usual. However, GDB may not notice when a non-current thread’s activity changes the expression. (Hardware watchpoints, in contrast, watch an expression in all threads.)

See set remote hardware-watchpoint-limit.


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5.1.3 Setting Catchpoints

You can use catchpoints to cause the debugger to stop for certain kinds of program events, such as C++ exceptions or the loading of a shared library. Use the catch command to set a catchpoint.

catch event

Stop when event occurs. The event can be any of the following:

throw [regexp]
rethrow [regexp]
catch [regexp]

The throwing, re-throwing, or catching of a C++ exception.

If regexp is given, then only exceptions whose type matches the regular expression will be caught.

The convenience variable $_exception is available at an exception-related catchpoint, on some systems. This holds the exception being thrown.

There are currently some limitations to C++ exception handling in GDB:

  • The support for these commands is system-dependent. Currently, only systems using the ‘gnu-v3’ C++ ABI (see ABI) are supported.
  • The regular expression feature and the $_exception convenience variable rely on the presence of some SDT probes in libstdc++. If these probes are not present, then these features cannot be used. These probes were first available in the GCC 4.8 release, but whether or not they are available in your GCC also depends on how it was built.
  • The $_exception convenience variable is only valid at the instruction at which an exception-related catchpoint is set.
  • When an exception-related catchpoint is hit, GDB stops at a location in the system library which implements runtime exception support for C++, usually libstdc++. You can use up (see Selection) to get to your code.
  • If you call a function interactively, GDB normally returns control to you when the function has finished executing. If the call raises an exception, however, the call may bypass the mechanism that returns control to you and cause your program either to abort or to simply continue running until it hits a breakpoint, catches a signal that GDB is listening for, or exits. This is the case even if you set a catchpoint for the exception; catchpoints on exceptions are disabled within interactive calls. See Calling, for information on controlling this with set unwind-on-terminating-exception.
  • You cannot raise an exception interactively.
  • You cannot install an exception handler interactively.
exception

An Ada exception being raised. If an exception name is specified at the end of the command (eg catch exception Program_Error), the debugger will stop only when this specific exception is raised. Otherwise, the debugger stops execution when any Ada exception is raised.

When inserting an exception catchpoint on a user-defined exception whose name is identical to one of the exceptions defined by the language, the fully qualified name must be used as the exception name. Otherwise, GDB will assume that it should stop on the pre-defined exception rather than the user-defined one. For instance, assuming an exception called Constraint_Error is defined in package Pck, then the command to use to catch such exceptions is catch exception Pck.Constraint_Error.

exception unhandled

An exception that was raised but is not handled by the program.

assert

A failed Ada assertion.

exec

A call to exec. This is currently only available for HP-UX and GNU/Linux.

syscall
syscall [name | number]

A call to or return from a system call, a.k.a. syscall. A syscall is a mechanism for application programs to request a service from the operating system (OS) or one of the OS system services. GDB can catch some or all of the syscalls issued by the debuggee, and show the related information for each syscall. If no argument is specified, calls to and returns from all system calls will be caught.

name can be any system call name that is valid for the underlying OS. Just what syscalls are valid depends on the OS. On GNU and Unix systems, you can find the full list of valid syscall names on /usr/include/asm/unistd.h.

Normally, GDB knows in advance which syscalls are valid for each OS, so you can use the GDB command-line completion facilities (see command completion) to list the available choices.

You may also specify the system call numerically. A syscall’s number is the value passed to the OS’s syscall dispatcher to identify the requested service. When you specify the syscall by its name, GDB uses its database of syscalls to convert the name into the corresponding numeric code, but using the number directly may be useful if GDB’s database does not have the complete list of syscalls on your system (e.g., because GDB lags behind the OS upgrades).

The example below illustrates how this command works if you don’t provide arguments to it:

(gdb) catch syscall
Catchpoint 1 (syscall)
(gdb) r
Starting program: /tmp/catch-syscall

Catchpoint 1 (call to syscall 'close'), \
	   0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.

Catchpoint 1 (returned from syscall 'close'), \
	0xffffe424 in __kernel_vsyscall ()
(gdb)

Here is an example of catching a system call by name:

(gdb) catch syscall chroot
Catchpoint 1 (syscall 'chroot' [61])
(gdb) r
Starting program: /tmp/catch-syscall

Catchpoint 1 (call to syscall 'chroot'), \
		   0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.

Catchpoint 1 (returned from syscall 'chroot'), \
	0xffffe424 in __kernel_vsyscall ()
(gdb)

An example of specifying a system call numerically. In the case below, the syscall number has a corresponding entry in the XML file, so GDB finds its name and prints it:

(gdb) catch syscall 252
Catchpoint 1 (syscall(s) 'exit_group')
(gdb) r
Starting program: /tmp/catch-syscall

Catchpoint 1 (call to syscall 'exit_group'), \
		   0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.

Program exited normally.
(gdb)

However, there can be situations when there is no corresponding name in XML file for that syscall number. In this case, GDB prints a warning message saying that it was not able to find the syscall name, but the catchpoint will be set anyway. See the example below:

(gdb) catch syscall 764
warning: The number '764' does not represent a known syscall.
Catchpoint 2 (syscall 764)
(gdb)

If you configure GDB using the ‘--without-expat’ option, it will not be able to display syscall names. Also, if your architecture does not have an XML file describing its system calls, you will not be able to see the syscall names. It is important to notice that these two features are used for accessing the syscall name database. In either case, you will see a warning like this:

(gdb) catch syscall
warning: Could not open "syscalls/i386-linux.xml"
warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'.
GDB will not be able to display syscall names.
Catchpoint 1 (syscall)
(gdb)

Of course, the file name will change depending on your architecture and system.

Still using the example above, you can also try to catch a syscall by its number. In this case, you would see something like:

(gdb) catch syscall 252
Catchpoint 1 (syscall(s) 252)

Again, in this case GDB would not be able to display syscall’s names.

fork

A call to fork. This is currently only available for HP-UX and GNU/Linux.

vfork

A call to vfork. This is currently only available for HP-UX and GNU/Linux.

load [regexp]
unload [regexp]

The loading or unloading of a shared library. If regexp is given, then the catchpoint will stop only if the regular expression matches one of the affected libraries.

signal [signal|all]

The delivery of a signal.

With no arguments, this catchpoint will catch any signal that is not used internally by GDB, specifically, all signals except ‘SIGTRAP’ and ‘SIGINT’.

With the argument ‘all’, all signals, including those used by GDB, will be caught. This argument cannot be used with other signal names.

Otherwise, the arguments are a list of signal names as given to handle (see Signals). Only signals specified in this list will be caught.

One reason that catch signal can be more useful than handle is that you can attach commands and conditions to the catchpoint.

When a signal is caught by a catchpoint, the signal’s stop and print settings, as specified by handle, are ignored. However, whether the signal is still delivered to the inferior depends on the pass setting; this can be changed in the catchpoint’s commands.

tcatch event

Set a catchpoint that is enabled only for one stop. The catchpoint is automatically deleted after the first time the event is caught.

Use the info break command to list the current catchpoints.


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5.1.4 Deleting Breakpoints

It is often necessary to eliminate a breakpoint, watchpoint, or catchpoint once it has done its job and you no longer want your program to stop there. This is called deleting the breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten.

With the clear command you can delete breakpoints according to where they are in your program. With the delete command you can delete individual breakpoints, watchpoints, or catchpoints by specifying their breakpoint numbers.

It is not necessary to delete a breakpoint to proceed past it. GDB automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.

clear

Delete any breakpoints at the next instruction to be executed in the selected stack frame (see Selecting a Frame). When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped.

clear location

Delete any breakpoints set at the specified location. See Specify Location, for the various forms of location; the most useful ones are listed below:

clear function
clear filename:function

Delete any breakpoints set at entry to the named function.

clear linenum
clear filename:linenum

Delete any breakpoints set at or within the code of the specified linenum of the specified filename.

delete [breakpoints] [range]

Delete the breakpoints, watchpoints, or catchpoints of the breakpoint ranges specified as arguments. If no argument is specified, delete all breakpoints (GDB asks confirmation, unless you have set confirm off). You can abbreviate this command as d.


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5.1.5 Disabling Breakpoints

Rather than deleting a breakpoint, watchpoint, or catchpoint, you might prefer to disable it. This makes the breakpoint inoperative as if it had been deleted, but remembers the information on the breakpoint so that you can enable it again later.

You disable and enable breakpoints, watchpoints, and catchpoints with the enable and disable commands, optionally specifying one or more breakpoint numbers as arguments. Use info break to print a list of all breakpoints, watchpoints, and catchpoints if you do not know which numbers to use.

Disabling and enabling a breakpoint that has multiple locations affects all of its locations.

A breakpoint, watchpoint, or catchpoint can have any of several different states of enablement:

You can use the following commands to enable or disable breakpoints, watchpoints, and catchpoints:

disable [breakpoints] [range]

Disable the specified breakpoints—or all breakpoints, if none are listed. A disabled breakpoint has no effect but is not forgotten. All options such as ignore-counts, conditions and commands are remembered in case the breakpoint is enabled again later. You may abbreviate disable as dis.

enable [breakpoints] [range]

Enable the specified breakpoints (or all defined breakpoints). They become effective once again in stopping your program.

enable [breakpoints] once range

Enable the specified breakpoints temporarily. GDB disables any of these breakpoints immediately after stopping your program.

enable [breakpoints] count count range

Enable the specified breakpoints temporarily. GDB records count with each of the specified breakpoints, and decrements a breakpoint’s count when it is hit. When any count reaches 0, GDB disables that breakpoint. If a breakpoint has an ignore count (see Break Conditions), that will be decremented to 0 before count is affected.

enable [breakpoints] delete range

Enable the specified breakpoints to work once, then die. GDB deletes any of these breakpoints as soon as your program stops there. Breakpoints set by the tbreak command start out in this state.

Except for a breakpoint set with tbreak (see Setting Breakpoints), breakpoints that you set are initially enabled; subsequently, they become disabled or enabled only when you use one of the commands above. (The command until can set and delete a breakpoint of its own, but it does not change the state of your other breakpoints; see Continuing and Stepping.)


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5.1.6 Break Conditions

The simplest sort of breakpoint breaks every time your program reaches a specified place. You can also specify a condition for a breakpoint. A condition is just a Boolean expression in your programming language (see Expressions). A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.

This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated—that is, when the condition is false. In C, if you want to test an assertion expressed by the condition assert, you should set the condition ‘! assert’ on the appropriate breakpoint.

Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow—but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one.

Break conditions can have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, GDB might see the other breakpoint first and stop your program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible than break conditions for the purpose of performing side effects when a breakpoint is reached (see Breakpoint Command Lists).

Breakpoint conditions can also be evaluated on the target’s side if the target supports it. Instead of evaluating the conditions locally, GDB encodes the expression into an agent expression (see Agent Expressions) suitable for execution on the target, independently of GDB. Global variables become raw memory locations, locals become stack accesses, and so forth.

In this case, GDB will only be notified of a breakpoint trigger when its condition evaluates to true. This mechanism may provide faster response times depending on the performance characteristics of the target since it does not need to keep GDB informed about every breakpoint trigger, even those with false conditions.

Break conditions can be specified when a breakpoint is set, by using ‘if’ in the arguments to the break command. See Setting Breakpoints. They can also be changed at any time with the condition command.

You can also use the if keyword with the watch command. The catch command does not recognize the if keyword; condition is the only way to impose a further condition on a catchpoint.

condition bnum expression

Specify expression as the break condition for breakpoint, watchpoint, or catchpoint number bnum. After you set a condition, breakpoint bnum stops your program only if the value of expression is true (nonzero, in C). When you use condition, GDB checks expression immediately for syntactic correctness, and to determine whether symbols in it have referents in the context of your breakpoint. If expression uses symbols not referenced in the context of the breakpoint, GDB prints an error message:

No symbol "foo" in current context.

GDB does not actually evaluate expression at the time the condition command (or a command that sets a breakpoint with a condition, like break if …) is given, however. See Expressions.

condition bnum

Remove the condition from breakpoint number bnum. It becomes an ordinary unconditional breakpoint.

A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times. This is so useful that there is a special way to do it, using the ignore count of the breakpoint. Every breakpoint has an ignore count, which is an integer. Most of the time, the ignore count is zero, and therefore has no effect. But if your program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues. As a result, if the ignore count value is n, the breakpoint does not stop the next n times your program reaches it.

ignore bnum count

Set the ignore count of breakpoint number bnum to count. The next count times the breakpoint is reached, your program’s execution does not stop; other than to decrement the ignore count, GDB takes no action.

To make the breakpoint stop the next time it is reached, specify a count of zero.

When you use continue to resume execution of your program from a breakpoint, you can specify an ignore count directly as an argument to continue, rather than using ignore. See Continuing and Stepping.

If a breakpoint has a positive ignore count and a condition, the condition is not checked. Once the ignore count reaches zero, GDB resumes checking the condition.

You could achieve the effect of the ignore count with a condition such as ‘$foo-- <= 0 using a debugger convenience variable that is decremented each time. See Convenience Variables.

Ignore counts apply to breakpoints, watchpoints, and catchpoints.


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5.1.7 Breakpoint Command Lists

You can give any breakpoint (or watchpoint or catchpoint) a series of commands to execute when your program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints.

commands [range]
command-list
end

Specify a list of commands for the given breakpoints. The commands themselves appear on the following lines. Type a line containing just end to terminate the commands.

To remove all commands from a breakpoint, type commands and follow it immediately with end; that is, give no commands.

With no argument, commands refers to the last breakpoint, watchpoint, or catchpoint set (not to the breakpoint most recently encountered). If the most recent breakpoints were set with a single command, then the commands will apply to all the breakpoints set by that command. This applies to breakpoints set by rbreak, and also applies when a single break command creates multiple breakpoints (see Ambiguous Expressions).

Pressing RET as a means of repeating the last GDB command is disabled within a command-list.

You can use breakpoint commands to start your program up again. Simply use the continue command, or step, or any other command that resumes execution.

Any other commands in the command list, after a command that resumes execution, are ignored. This is because any time you resume execution (even with a simple next or step), you may encounter another breakpoint—which could have its own command list, leading to ambiguities about which list to execute.

If the first command you specify in a command list is silent, the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue. If none of the remaining commands print anything, you see no sign that the breakpoint was reached. silent is meaningful only at the beginning of a breakpoint command list.

The commands echo, output, and printf allow you to print precisely controlled output, and are often useful in silent breakpoints. See Commands for Controlled Output.

For example, here is how you could use breakpoint commands to print the value of x at entry to foo whenever x is positive.

break foo if x>0
commands
silent
printf "x is %d\n",x
cont
end

One application for breakpoint commands is to compensate for one bug so you can test for another. Put a breakpoint just after the erroneous line of code, give it a condition to detect the case in which something erroneous has been done, and give it commands to assign correct values to any variables that need them. End with the continue command so that your program does not stop, and start with the silent command so that no output is produced. Here is an example:

break 403
commands
silent
set x = y + 4
cont
end

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5.1.8 Dynamic Printf

The dynamic printf command dprintf combines a breakpoint with formatted printing of your program’s data to give you the effect of inserting printf calls into your program on-the-fly, without having to recompile it.

In its most basic form, the output goes to the GDB console. However, you can set the variable dprintf-style for alternate handling. For instance, you can ask to format the output by calling your program’s printf function. This has the advantage that the characters go to the program’s output device, so they can recorded in redirects to files and so forth.

If you are doing remote debugging with a stub or agent, you can also ask to have the printf handled by the remote agent. In addition to ensuring that the output goes to the remote program’s device along with any other output the program might produce, you can also ask that the dprintf remain active even after disconnecting from the remote target. Using the stub/agent is also more efficient, as it can do everything without needing to communicate with GDB.

dprintf location,template,expression[,expression…]

Whenever execution reaches location, print the values of one or more expressions under the control of the string template. To print several values, separate them with commas.

set dprintf-style style

Set the dprintf output to be handled in one of several different styles enumerated below. A change of style affects all existing dynamic printfs immediately. (If you need individual control over the print commands, simply define normal breakpoints with explicitly-supplied command lists.)

gdb

Handle the output using the GDB printf command.

call

Handle the output by calling a function in your program (normally printf).

agent

Have the remote debugging agent (such as gdbserver) handle the output itself. This style is only available for agents that support running commands on the target.

set dprintf-function function

Set the function to call if the dprintf style is call. By default its value is printf. You may set it to any expression. that GDB can evaluate to a function, as per the call command.

set dprintf-channel channel

Set a “channel” for dprintf. If set to a non-empty value, GDB will evaluate it as an expression and pass the result as a first argument to the dprintf-function, in the manner of fprintf and similar functions. Otherwise, the dprintf format string will be the first argument, in the manner of printf.

As an example, if you wanted dprintf output to go to a logfile that is a standard I/O stream assigned to the variable mylog, you could do the following:

(gdb) set dprintf-style call
(gdb) set dprintf-function fprintf
(gdb) set dprintf-channel mylog
(gdb) dprintf 25,"at line 25, glob=%d\n",glob
Dprintf 1 at 0x123456: file main.c, line 25.
(gdb) info break
1       dprintf        keep y   0x00123456 in main at main.c:25
        call (void) fprintf (mylog,"at line 25, glob=%d\n",glob)
        continue
(gdb)

Note that the info break displays the dynamic printf commands as normal breakpoint commands; you can thus easily see the effect of the variable settings.

set disconnected-dprintf on
set disconnected-dprintf off

Choose whether dprintf commands should continue to run if GDB has disconnected from the target. This only applies if the dprintf-style is agent.

show disconnected-dprintf off

Show the current choice for disconnected dprintf.

GDB does not check the validity of function and channel, relying on you to supply values that are meaningful for the contexts in which they are being used. For instance, the function and channel may be the values of local variables, but if that is the case, then all enabled dynamic prints must be at locations within the scope of those locals. If evaluation fails, GDB will report an error.


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5.1.9 How to save breakpoints to a file

To save breakpoint definitions to a file use the save breakpoints command.

save breakpoints [filename]

This command saves all current breakpoint definitions together with their commands and ignore counts, into a file filename suitable for use in a later debugging session. This includes all types of breakpoints (breakpoints, watchpoints, catchpoints, tracepoints). To read the saved breakpoint definitions, use the source command (see Command Files). Note that watchpoints with expressions involving local variables may fail to be recreated because it may not be possible to access the context where the watchpoint is valid anymore. Because the saved breakpoint definitions are simply a sequence of GDB commands that recreate the breakpoints, you can edit the file in your favorite editing program, and remove the breakpoint definitions you’re not interested in, or that can no longer be recreated.


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5.1.10 Static Probe Points

GDB supports SDT probes in the code. SDT stands for Statically Defined Tracing, and the probes are designed to have a tiny runtime code and data footprint, and no dynamic relocations. They are usable from assembly, C and C++ languages. See http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation for a good reference on how the SDT probes are implemented.

Currently, SystemTap (http://sourceware.org/systemtap/) SDT probes are supported on ELF-compatible systems. See http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps for more information on how to add SystemTap SDT probes in your applications.

Some probes have an associated semaphore variable; for instance, this happens automatically if you defined your probe using a DTrace-style .d file. If your probe has a semaphore, GDB will automatically enable it when you specify a breakpoint using the ‘-probe-stap’ notation. But, if you put a breakpoint at a probe’s location by some other method (e.g., break file:line), then GDB will not automatically set the semaphore.

You can examine the available static static probes using info probes, with optional arguments:

info probes stap [provider [name [objfile]]]

If given, provider is a regular expression used to match against provider names when selecting which probes to list. If omitted, probes by all probes from all providers are listed.

If given, name is a regular expression to match against probe names when selecting which probes to list. If omitted, probe names are not considered when deciding whether to display them.

If given, objfile is a regular expression used to select which object files (executable or shared libraries) to examine. If not given, all object files are considered.

info probes all

List the available static probes, from all types.

A probe may specify up to twelve arguments. These are available at the point at which the probe is defined—that is, when the current PC is at the probe’s location. The arguments are available using the convenience variables (see Convenience Vars) $_probe_arg0$_probe_arg11. Each probe argument is an integer of the appropriate size; types are not preserved. The convenience variable $_probe_argc holds the number of arguments at the current probe point.

These variables are always available, but attempts to access them at any location other than a probe point will cause GDB to give an error message.


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5.1.11 “Cannot insert breakpoints”

If you request too many active hardware-assisted breakpoints and watchpoints, you will see this error message:

Stopped; cannot insert breakpoints.
You may have requested too many hardware breakpoints and watchpoints.

This message is printed when you attempt to resume the program, since only then GDB knows exactly how many hardware breakpoints and watchpoints it needs to insert.

When this message is printed, you need to disable or remove some of the hardware-assisted breakpoints and watchpoints, and then continue.


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5.1.12 “Breakpoint address adjusted...”

Some processor architectures place constraints on the addresses at which breakpoints may be placed. For architectures thus constrained, GDB will attempt to adjust the breakpoint’s address to comply with the constraints dictated by the architecture.

One example of such an architecture is the Fujitsu FR-V. The FR-V is a VLIW architecture in which a number of RISC-like instructions may be bundled together for parallel execution. The FR-V architecture constrains the location of a breakpoint instruction within such a bundle to the instruction with the lowest address. GDB honors this constraint by adjusting a breakpoint’s address to the first in the bundle.

It is not uncommon for optimized code to have bundles which contain instructions from different source statements, thus it may happen that a breakpoint’s address will be adjusted from one source statement to another. Since this adjustment may significantly alter GDB’s breakpoint related behavior from what the user expects, a warning is printed when the breakpoint is first set and also when the breakpoint is hit.

A warning like the one below is printed when setting a breakpoint that’s been subject to address adjustment:

warning: Breakpoint address adjusted from 0x00010414 to 0x00010410.

Such warnings are printed both for user settable and GDB’s internal breakpoints. If you see one of these warnings, you should verify that a breakpoint set at the adjusted address will have the desired affect. If not, the breakpoint in question may be removed and other breakpoints may be set which will have the desired behavior. E.g., it may be sufficient to place the breakpoint at a later instruction. A conditional breakpoint may also be useful in some cases to prevent the breakpoint from triggering too often.

GDB will also issue a warning when stopping at one of these adjusted breakpoints:

warning: Breakpoint 1 address previously adjusted from 0x00010414
to 0x00010410.

When this warning is encountered, it may be too late to take remedial action except in cases where the breakpoint is hit earlier or more frequently than expected.


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5.2 Continuing and Stepping

Continuing means resuming program execution until your program completes normally. In contrast, stepping means executing just one more “step” of your program, where “step” may mean either one line of source code, or one machine instruction (depending on what particular command you use). Either when continuing or when stepping, your program may stop even sooner, due to a breakpoint or a signal. (If it stops due to a signal, you may want to use handle, or use ‘signal 0’ to resume execution. See Signals.)

continue [ignore-count]
c [ignore-count]
fg [ignore-count]

Resume program execution, at the address where your program last stopped; any breakpoints set at that address are bypassed. The optional argument ignore-count allows you to specify a further number of times to ignore a breakpoint at this location; its effect is like that of ignore (see Break Conditions).

The argument ignore-count is meaningful only when your program stopped due to a breakpoint. At other times, the argument to continue is ignored.

The synonyms c and fg (for foreground, as the debugged program is deemed to be the foreground program) are provided purely for convenience, and have exactly the same behavior as continue.

To resume execution at a different place, you can use return (see Returning from a Function) to go back to the calling function; or jump (see Continuing at a Different Address) to go to an arbitrary location in your program.

A typical technique for using stepping is to set a breakpoint (see Breakpoints; Watchpoints; and Catchpoints) at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.

step

Continue running your program until control reaches a different source line, then stop it and return control to GDB. This command is abbreviated s.

Warning: If you use the step command while control is within a function that was compiled without debugging information, execution proceeds until control reaches a function that does have debugging information. Likewise, it will not step into a function which is compiled without debugging information. To step through functions without debugging information, use the stepi command, described below.

The step command only stops at the first instruction of a source line. This prevents the multiple stops that could otherwise occur in switch statements, for loops, etc. step continues to stop if a function that has debugging information is called within the line. In other words, step steps inside any functions called within the line.

Also, the step command only enters a function if there is line number information for the function. Otherwise it acts like the next command. This avoids problems when using cc -gl on MIPS machines. Previously, step entered subroutines if there was any debugging information about the routine.

step count

Continue running as in step, but do so count times. If a breakpoint is reached, or a signal not related to stepping occurs before count steps, stepping stops right away.

next [count]

Continue to the next source line in the current (innermost) stack frame. This is similar to step, but function calls that appear within the line of code are executed without stopping. Execution stops when control reaches a different line of code at the original stack level that was executing when you gave the next command. This command is abbreviated n.

An argument count is a repeat count, as for step.

The next command only stops at the first instruction of a source line. This prevents multiple stops that could otherwise occur in switch statements, for loops, etc.

set step-mode
set step-mode on

The set step-mode on command causes the step command to stop at the first instruction of a function which contains no debug line information rather than stepping over it.

This is useful in cases where you may be interested in inspecting the machine instructions of a function which has no symbolic info and do not want GDB to automatically skip over this function.

set step-mode off

Causes the step command to step over any functions which contains no debug information. This is the default.

show step-mode

Show whether GDB will stop in or step over functions without source line debug information.

finish

Continue running until just after function in the selected stack frame returns. Print the returned value (if any). This command can be abbreviated as fin.

Contrast this with the return command (see Returning from a Function).

until
u

Continue running until a source line past the current line, in the current stack frame, is reached. This command is used to avoid single stepping through a loop more than once. It is like the next command, except that when until encounters a jump, it automatically continues execution until the program counter is greater than the address of the jump.

This means that when you reach the end of a loop after single stepping though it, until makes your program continue execution until it exits the loop. In contrast, a next command at the end of a loop simply steps back to the beginning of the loop, which forces you to step through the next iteration.

until always stops your program if it attempts to exit the current stack frame.

until may produce somewhat counterintuitive results if the order of machine code does not match the order of the source lines. For example, in the following excerpt from a debugging session, the f (frame) command shows that execution is stopped at line 206; yet when we use until, we get to line 195:

(gdb) f
#0  main (argc=4, argv=0xf7fffae8) at m4.c:206
206                 expand_input();
(gdb) until
195             for ( ; argc > 0; NEXTARG) {

This happened because, for execution efficiency, the compiler had generated code for the loop closure test at the end, rather than the start, of the loop—even though the test in a C for-loop is written before the body of the loop. The until command appeared to step back to the beginning of the loop when it advanced to this expression; however, it has not really gone to an earlier statement—not in terms of the actual machine code.

until with no argument works by means of single instruction stepping, and hence is slower than until with an argument.

until location
u location

Continue running your program until either the specified location is reached, or the current stack frame returns. The location is any of the forms described in Specify Location. This form of the command uses temporary breakpoints, and hence is quicker than until without an argument. The specified location is actually reached only if it is in the current frame. This implies that until can be used to skip over recursive function invocations. For instance in the code below, if the current location is line 96, issuing until 99 will execute the program up to line 99 in the same invocation of factorial, i.e., after the inner invocations have returned.

94	int factorial (int value)
95	{
96	    if (value > 1) {
97            value *= factorial (value - 1);
98	    }
99	    return (value);
100     }
advance location

Continue running the program up to the given location. An argument is required, which should be of one of the forms described in Specify Location. Execution will also stop upon exit from the current stack frame. This command is similar to until, but advance will not skip over recursive function calls, and the target location doesn’t have to be in the same frame as the current one.

stepi
stepi arg
si

Execute one machine instruction, then stop and return to the debugger.

It is often useful to do ‘display/i $pc’ when stepping by machine instructions. This makes GDB automatically display the next instruction to be executed, each time your program stops. See Automatic Display.

An argument is a repeat count, as in step.

nexti
nexti arg
ni

Execute one machine instruction, but if it is a function call, proceed until the function returns.

An argument is a repeat count, as in next.

By default, and if available, GDB makes use of target-assisted range stepping. In other words, whenever you use a stepping command (e.g., step, next), GDB tells the target to step the corresponding range of instruction addresses instead of issuing multiple single-steps. This speeds up line stepping, particularly for remote targets. Ideally, there should be no reason you would want to turn range stepping off. However, it’s possible that a bug in the debug info, a bug in the remote stub (for remote targets), or even a bug in GDB could make line stepping behave incorrectly when target-assisted range stepping is enabled. You can use the following command to turn off range stepping if necessary:

set range-stepping
show range-stepping

Control whether range stepping is enabled.

If on, and the target supports it, GDB tells the target to step a range of addresses itself, instead of issuing multiple single-steps. If off, GDB always issues single-steps, even if range stepping is supported by the target. The default is on.


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5.3 Skipping Over Functions and Files

The program you are debugging may contain some functions which are uninteresting to debug. The skip comand lets you tell GDB to skip a function or all functions in a file when stepping.

For example, consider the following C function:

101     int func()
102     {
103         foo(boring());
104         bar(boring());
105     }

Suppose you wish to step into the functions foo and bar, but you are not interested in stepping through boring. If you run step at line 103, you’ll enter boring(), but if you run next, you’ll step over both foo and boring!

One solution is to step into boring and use the finish command to immediately exit it. But this can become tedious if boring is called from many places.

A more flexible solution is to execute skip boring. This instructs GDB never to step into boring. Now when you execute step at line 103, you’ll step over boring and directly into foo.

You can also instruct GDB to skip all functions in a file, with, for example, skip file boring.c.

skip [linespec]
skip function [linespec]

After running this command, the function named by linespec or the function containing the line named by linespec will be skipped over when stepping. See Specify Location.

If you do not specify linespec, the function you’re currently debugging will be skipped.

(If you have a function called file that you want to skip, use skip function file.)

skip file [filename]

After running this command, any function whose source lives in filename will be skipped over when stepping.

If you do not specify filename, functions whose source lives in the file you’re currently debugging will be skipped.

Skips can be listed, deleted, disabled, and enabled, much like breakpoints. These are the commands for managing your list of skips:

info skip [range]

Print details about the specified skip(s). If range is not specified, print a table with details about all functions and files marked for skipping. info skip prints the following information about each skip:

Identifier

A number identifying this skip.

Type

The type of this skip, either ‘function’ or ‘file’.

Enabled or Disabled

Enabled skips are marked with ‘y’. Disabled skips are marked with ‘n’.

Address

For function skips, this column indicates the address in memory of the function being skipped. If you’ve set a function skip on a function which has not yet been loaded, this field will contain ‘<PENDING>’. Once a shared library which has the function is loaded, info skip will show the function’s address here.

What

For file skips, this field contains the filename being skipped. For functions skips, this field contains the function name and its line number in the file where it is defined.

skip delete [range]

Delete the specified skip(s). If range is not specified, delete all skips.

skip enable [range]

Enable the specified skip(s). If range is not specified, enable all skips.

skip disable [range]

Disable the specified skip(s). If range is not specified, disable all skips.


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5.4 Signals

A signal is an asynchronous event that can happen in a program. The operating system defines the possible kinds of signals, and gives each kind a name and a number. For example, in Unix SIGINT is the signal a program gets when you type an interrupt character (often Ctrl-c); SIGSEGV is the signal a program gets from referencing a place in memory far away from all the areas in use; SIGALRM occurs when the alarm clock timer goes off (which happens only if your program has requested an alarm).

Some signals, including SIGALRM, are a normal part of the functioning of your program. Others, such as SIGSEGV, indicate errors; these signals are fatal (they kill your program immediately) if the program has not specified in advance some other way to handle the signal. SIGINT does not indicate an error in your program, but it is normally fatal so it can carry out the purpose of the interrupt: to kill the program.

GDB has the ability to detect any occurrence of a signal in your program. You can tell GDB in advance what to do for each kind of signal.

Normally, GDB is set up to let the non-erroneous signals like SIGALRM be silently passed to your program (so as not to interfere with their role in the program’s functioning) but to stop your program immediately whenever an error signal happens. You can change these settings with the handle command.

info signals
info handle

Print a table of all the kinds of signals and how GDB has been told to handle each one. You can use this to see the signal numbers of all the defined types of signals.

info signals sig

Similar, but print information only about the specified signal number.

info handle is an alias for info signals.

catch signal [signal|all]

Set a catchpoint for the indicated signals. See Set Catchpoints, for details about this command.

handle signal [keywords]

Change the way GDB handles signal signal. The signal can be the number of a signal or its name (with or without the ‘SIG’ at the beginning); a list of signal numbers of the form ‘low-high’; or the word ‘all’, meaning all the known signals. Optional arguments keywords, described below, say what change to make.

The keywords allowed by the handle command can be abbreviated. Their full names are:

nostop

GDB should not stop your program when this signal happens. It may still print a message telling you that the signal has come in.

stop

GDB should stop your program when this signal happens. This implies the print keyword as well.

print

GDB should print a message when this signal happens.

noprint

GDB should not mention the occurrence of the signal at all. This implies the nostop keyword as well.

pass
noignore

GDB should allow your program to see this signal; your program can handle the signal, or else it may terminate if the signal is fatal and not handled. pass and noignore are synonyms.

nopass
ignore

GDB should not allow your program to see this signal. nopass and ignore are synonyms.

When a signal stops your program, the signal is not visible to the program until you continue. Your program sees the signal then, if pass is in effect for the signal in question at that time. In other words, after GDB reports a signal, you can use the handle command with pass or nopass to control whether your program sees that signal when you continue.

The default is set to nostop, noprint, pass for non-erroneous signals such as SIGALRM, SIGWINCH and SIGCHLD, and to stop, print, pass for the erroneous signals.

You can also use the signal command to prevent your program from seeing a signal, or cause it to see a signal it normally would not see, or to give it any signal at any time. For example, if your program stopped due to some sort of memory reference error, you might store correct values into the erroneous variables and continue, hoping to see more execution; but your program would probably terminate immediately as a result of the fatal signal once it saw the signal. To prevent this, you can continue with ‘signal 0’. See Giving your Program a Signal.

On some targets, GDB can inspect extra signal information associated with the intercepted signal, before it is actually delivered to the program being debugged. This information is exported by the convenience variable $_siginfo, and consists of data that is passed by the kernel to the signal handler at the time of the receipt of a signal. The data type of the information itself is target dependent. You can see the data type using the ptype $_siginfo command. On Unix systems, it typically corresponds to the standard siginfo_t type, as defined in the signal.h system header.

Here’s an example, on a GNU/Linux system, printing the stray referenced address that raised a segmentation fault.

(gdb) continue
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400766 in main ()
69        *(int *)p = 0;
(gdb) ptype $_siginfo
type = struct {
    int si_signo;
    int si_errno;
    int si_code;
    union {
        int _pad[28];
        struct {...} _kill;
        struct {...} _timer;
        struct {...} _rt;
        struct {...} _sigchld;
        struct {...} _sigfault;
        struct {...} _sigpoll;
    } _sifields;
}
(gdb) ptype $_siginfo._sifields._sigfault
type = struct {
    void *si_addr;
}
(gdb) p $_siginfo._sifields._sigfault.si_addr
$1 = (void *) 0x7ffff7ff7000

Depending on target support, $_siginfo may also be writable.


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5.5 Stopping and Starting Multi-thread Programs

GDB supports debugging programs with multiple threads (see Debugging Programs with Multiple Threads). There are two modes of controlling execution of your program within the debugger. In the default mode, referred to as all-stop mode, when any thread in your program stops (for example, at a breakpoint or while being stepped), all other threads in the program are also stopped by GDB. On some targets, GDB also supports non-stop mode, in which other threads can continue to run freely while you examine the stopped thread in the debugger.


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5.5.1 All-Stop Mode

In all-stop mode, whenever your program stops under GDB for any reason, all threads of execution stop, not just the current thread. This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot.

Conversely, whenever you restart the program, all threads start executing. This is true even when single-stepping with commands like step or next.

In particular, GDB cannot single-step all threads in lockstep. Since thread scheduling is up to your debugging target’s operating system (not controlled by GDB), other threads may execute more than one statement while the current thread completes a single step. Moreover, in general other threads stop in the middle of a statement, rather than at a clean statement boundary, when the program stops.

You might even find your program stopped in another thread after continuing or even single-stepping. This happens whenever some other thread runs into a breakpoint, a signal, or an exception before the first thread completes whatever you requested.

Whenever GDB stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. GDB alerts you to the context switch with a message such as ‘[Switching to Thread n]’ to identify the thread.

On some OSes, you can modify GDB’s default behavior by locking the OS scheduler to allow only a single thread to run.

set scheduler-locking mode

Set the scheduler locking mode. If it is off, then there is no locking and any thread may run at any time. If on, then only the current thread may run when the inferior is resumed. The step mode optimizes for single-stepping; it prevents other threads from preempting the current thread while you are stepping, so that the focus of debugging does not change unexpectedly. Other threads only rarely (or never) get a chance to run when you step. They are more likely to run when you ‘next’ over a function call, and they are completely free to run when you use commands like ‘continue’, ‘until’, or ‘finish’. However, unless another thread hits a breakpoint during its timeslice, GDB does not change the current thread away from the thread that you are debugging.

show scheduler-locking

Display the current scheduler locking mode.

By default, when you issue one of the execution commands such as continue, next or step, GDB allows only threads of the current inferior to run. For example, if GDB is attached to two inferiors, each with two threads, the continue command resumes only the two threads of the current inferior. This is useful, for example, when you debug a program that forks and you want to hold the parent stopped (so that, for instance, it doesn’t run to exit), while you debug the child. In other situations, you may not be interested in inspecting the current state of any of the processes GDB is attached to, and you may want to resume them all until some breakpoint is hit. In the latter case, you can instruct GDB to allow all threads of all the inferiors to run with the set schedule-multiple command.

set schedule-multiple

Set the mode for allowing threads of multiple processes to be resumed when an execution command is issued. When on, all threads of all processes are allowed to run. When off, only the threads of the current process are resumed. The default is off. The scheduler-locking mode takes precedence when set to on, or while you are stepping and set to step.

show schedule-multiple

Display the current mode for resuming the execution of threads of multiple processes.


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5.5.2 Non-Stop Mode

For some multi-threaded targets, GDB supports an optional mode of operation in which you can examine stopped program threads in the debugger while other threads continue to execute freely. This minimizes intrusion when debugging live systems, such as programs where some threads have real-time constraints or must continue to respond to external events. This is referred to as non-stop mode.

In non-stop mode, when a thread stops to report a debugging event, only that thread is stopped; GDB does not stop other threads as well, in contrast to the all-stop mode behavior. Additionally, execution commands such as continue and step apply by default only to the current thread in non-stop mode, rather than all threads as in all-stop mode. This allows you to control threads explicitly in ways that are not possible in all-stop mode — for example, stepping one thread while allowing others to run freely, stepping one thread while holding all others stopped, or stepping several threads independently and simultaneously.

To enter non-stop mode, use this sequence of commands before you run or attach to your program:

# If using the CLI, pagination breaks non-stop.
set pagination off

# Finally, turn it on!
set non-stop on

You can use these commands to manipulate the non-stop mode setting:

set non-stop on

Enable selection of non-stop mode.

set non-stop off

Disable selection of non-stop mode.

show non-stop

Show the current non-stop enablement setting.

Note these commands only reflect whether non-stop mode is enabled, not whether the currently-executing program is being run in non-stop mode. In particular, the set non-stop preference is only consulted when GDB starts or connects to the target program, and it is generally not possible to switch modes once debugging has started. Furthermore, since not all targets support non-stop mode, even when you have enabled non-stop mode, GDB may still fall back to all-stop operation by default.

In non-stop mode, all execution commands apply only to the current thread by default. That is, continue only continues one thread. To continue all threads, issue continue -a or c -a.

You can use GDB’s background execution commands (see Background Execution) to run some threads in the background while you continue to examine or step others from GDB. The MI execution commands (see GDB/MI Program Execution) are always executed asynchronously in non-stop mode.

Suspending execution is done with the interrupt command when running in the background, or Ctrl-c during foreground execution. In all-stop mode, this stops the whole process; but in non-stop mode the interrupt applies only to the current thread. To stop the whole program, use interrupt -a.

Other execution commands do not currently support the -a option.

In non-stop mode, when a thread stops, GDB doesn’t automatically make that thread current, as it does in all-stop mode. This is because the thread stop notifications are asynchronous with respect to GDB’s command interpreter, and it would be confusing if GDB unexpectedly changed to a different thread just as you entered a command to operate on the previously current thread.


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5.5.3 Background Execution

GDB’s execution commands have two variants: the normal foreground (synchronous) behavior, and a background (asynchronous) behavior. In foreground execution, GDB waits for the program to report that some thread has stopped before prompting for another command. In background execution, GDB immediately gives a command prompt so that you can issue other commands while your program runs.

If the target doesn’t support async mode, GDB issues an error message if you attempt to use the background execution commands.

To specify background execution, add a & to the command. For example, the background form of the continue command is continue&, or just c&. The execution commands that accept background execution are:

run

See Starting your Program.

attach

See Debugging an Already-running Process.

step

See step.

stepi

See stepi.

next

See next.

nexti

See nexti.

continue

See continue.

finish

See finish.

until

See until.

Background execution is especially useful in conjunction with non-stop mode for debugging programs with multiple threads; see Non-Stop Mode. However, you can also use these commands in the normal all-stop mode with the restriction that you cannot issue another execution command until the previous one finishes. Examples of commands that are valid in all-stop mode while the program is running include help and info break.

You can interrupt your program while it is running in the background by using the interrupt command.

interrupt
interrupt -a

Suspend execution of the running program. In all-stop mode, interrupt stops the whole process, but in non-stop mode, it stops only the current thread. To stop the whole program in non-stop mode, use interrupt -a.


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5.5.4 Thread-Specific Breakpoints

When your program has multiple threads (see Debugging Programs with Multiple Threads), you can choose whether to set breakpoints on all threads, or on a particular thread.

break linespec thread threadno
break linespec thread threadno if …

linespec specifies source lines; there are several ways of writing them (see Specify Location), but the effect is always to specify some source line.

Use the qualifier ‘thread threadno’ with a breakpoint command to specify that you only want GDB to stop the program when a particular thread reaches this breakpoint. The threadno specifier is one of the numeric thread identifiers assigned by GDB, shown in the first column of the ‘info threads’ display.

If you do not specify ‘thread threadno’ when you set a breakpoint, the breakpoint applies to all threads of your program.

You can use the thread qualifier on conditional breakpoints as well; in this case, place ‘thread threadno’ before or after the breakpoint condition, like this:

(gdb) break frik.c:13 thread 28 if bartab > lim

Thread-specific breakpoints are automatically deleted when GDB detects the corresponding thread is no longer in the thread list. For example:

(gdb) c
Thread-specific breakpoint 3 deleted - thread 28 no longer in the thread list.

There are several ways for a thread to disappear, such as a regular thread exit, but also when you detach from the process with the detach command (see Debugging an Already-running Process), or if GDB loses the remote connection (see Remote Debugging), etc. Note that with some targets, GDB is only able to detect a thread has exited when the user explictly asks for the thread list with the info threads command.


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5.5.5 Interrupted System Calls

There is an unfortunate side effect when using GDB to debug multi-threaded programs. If one thread stops for a breakpoint, or for some other reason, and another thread is blocked in a system call, then the system call may return prematurely. This is a consequence of the interaction between multiple threads and the signals that GDB uses to implement breakpoints and other events that stop execution.

To handle this problem, your program should check the return value of each system call and react appropriately. This is good programming style anyways.

For example, do not write code like this:

  sleep (10);

The call to sleep will return early if a different thread stops at a breakpoint or for some other reason.

Instead, write this:

  int unslept = 10;
  while (unslept > 0)
    unslept = sleep (unslept);

A system call is allowed to return early, so the system is still conforming to its specification. But GDB does cause your multi-threaded program to behave differently than it would without GDB.

Also, GDB uses internal breakpoints in the thread library to monitor certain events such as thread creation and thread destruction. When such an event happens, a system call in another thread may return prematurely, even though your program does not appear to stop.


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5.5.6 Observer Mode

If you want to build on non-stop mode and observe program behavior without any chance of disruption by GDB, you can set variables to disable all of the debugger’s attempts to modify state, whether by writing memory, inserting breakpoints, etc. These operate at a low level, intercepting operations from all commands.

When all of these are set to off, then GDB is said to be observer mode. As a convenience, the variable observer can be set to disable these, plus enable non-stop mode.

Note that GDB will not prevent you from making nonsensical combinations of these settings. For instance, if you have enabled may-insert-breakpoints but disabled may-write-memory, then breakpoints that work by writing trap instructions into the code stream will still not be able to be placed.

set observer on
set observer off

When set to on, this disables all the permission variables below (except for insert-fast-tracepoints), plus enables non-stop debugging. Setting this to off switches back to normal debugging, though remaining in non-stop mode.

show observer

Show whether observer mode is on or off.

set may-write-registers on
set may-write-registers off

This controls whether GDB will attempt to alter the values of registers, such as with assignment expressions in print, or the jump command. It defaults to on.

show may-write-registers

Show the current permission to write registers.

set may-write-memory on
set may-write-memory off

This controls whether GDB will attempt to alter the contents of memory, such as with assignment expressions in print. It defaults to on.

show may-write-memory

Show the current permission to write memory.

set may-insert-breakpoints on
set may-insert-breakpoints off

This controls whether GDB will attempt to insert breakpoints. This affects all breakpoints, including internal breakpoints defined by GDB. It defaults to on.

show may-insert-breakpoints

Show the current permission to insert breakpoints.

set may-insert-tracepoints on
set may-insert-tracepoints off

This controls whether GDB will attempt to insert (regular) tracepoints at the beginning of a tracing experiment. It affects only non-fast tracepoints, fast tracepoints being under the control of may-insert-fast-tracepoints. It defaults to on.

show may-insert-tracepoints

Show the current permission to insert tracepoints.

set may-insert-fast-tracepoints on
set may-insert-fast-tracepoints off

This controls whether GDB will attempt to insert fast tracepoints at the beginning of a tracing experiment. It affects only fast tracepoints, regular (non-fast) tracepoints being under the control of may-insert-tracepoints. It defaults to on.

show may-insert-fast-tracepoints

Show the current permission to insert fast tracepoints.

set may-interrupt on
set may-interrupt off

This controls whether GDB will attempt to interrupt or stop program execution. When this variable is off, the interrupt command will have no effect, nor will Ctrl-c. It defaults to on.

show may-interrupt

Show the current permission to interrupt or stop the program.


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6 Running programs backward

When you are debugging a program, it is not unusual to realize that you have gone too far, and some event of interest has already happened. If the target environment supports it, GDB can allow you to “rewind” the program by running it backward.

A target environment that supports reverse execution should be able to “undo” the changes in machine state that have taken place as the program was executing normally. Variables, registers etc. should revert to their previous values. Obviously this requires a great deal of sophistication on the part of the target environment; not all target environments can support reverse execution.

When a program is executed in reverse, the instructions that have most recently been executed are “un-executed”, in reverse order. The program counter runs backward, following the previous thread of execution in reverse. As each instruction is “un-executed”, the values of memory and/or registers that were changed by that instruction are reverted to their previous states. After executing a piece of source code in reverse, all side effects of that code should be “undone”, and all variables should be returned to their prior values4.

If you are debugging in a target environment that supports reverse execution, GDB provides the following commands.

reverse-continue [ignore-count]
rc [ignore-count]

Beginning at the point where your program last stopped, start executing in reverse. Reverse execution will stop for breakpoints and synchronous exceptions (signals), just like normal execution. Behavior of asynchronous signals depends on the target environment.

reverse-step [count]

Run the program backward until control reaches the start of a different source line; then stop it, and return control to GDB.

Like the step command, reverse-step will only stop at the beginning of a source line. It “un-executes” the previously executed source line. If the previous source line included calls to debuggable functions, reverse-step will step (backward) into the called function, stopping at the beginning of the last statement in the called function (typically a return statement).

Also, as with the step command, if non-debuggable functions are called, reverse-step will run thru them backward without stopping.

reverse-stepi [count]

Reverse-execute one machine instruction. Note that the instruction to be reverse-executed is not the one pointed to by the program counter, but the instruction executed prior to that one. For instance, if the last instruction was a jump, reverse-stepi will take you back from the destination of the jump to the jump instruction itself.

reverse-next [count]

Run backward to the beginning of the previous line executed in the current (innermost) stack frame. If the line contains function calls, they will be “un-executed” without stopping. Starting from the first line of a function, reverse-next will take you back to the caller of that function, before the function was called, just as the normal next command would take you from the last line of a function back to its return to its caller 5.

reverse-nexti [count]

Like nexti, reverse-nexti executes a single instruction in reverse, except that called functions are “un-executed” atomically. That is, if the previously executed instruction was a return from another function, reverse-nexti will continue to execute in reverse until the call to that function (from the current stack frame) is reached.

reverse-finish

Just as the finish command takes you to the point where the current function returns, reverse-finish takes you to the point where it was called. Instead of ending up at the end of the current function invocation, you end up at the beginning.

set exec-direction

Set the direction of target execution.

set exec-direction reverse

GDB will perform all execution commands in reverse, until the exec-direction mode is changed to “forward”. Affected commands include step, stepi, next, nexti, continue, and finish. The return command cannot be used in reverse mode.

set exec-direction forward

GDB will perform all execution commands in the normal fashion. This is the default.


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7 Recording Inferior’s Execution and Replaying It

On some platforms, GDB provides a special process record and replay target that can record a log of the process execution, and replay it later with both forward and reverse execution commands.

When this target is in use, if the execution log includes the record for the next instruction, GDB will debug in replay mode. In the replay mode, the inferior does not really execute code instructions. Instead, all the events that normally happen during code execution are taken from the execution log. While code is not really executed in replay mode, the values of registers (including the program counter register) and the memory of the inferior are still changed as they normally would. Their contents are taken from the execution log.

If the record for the next instruction is not in the execution log, GDB will debug in record mode. In this mode, the inferior executes normally, and GDB records the execution log for future replay.

The process record and replay target supports reverse execution (see Reverse Execution), even if the platform on which the inferior runs does not. However, the reverse execution is limited in this case by the range of the instructions recorded in the execution log. In other words, reverse execution on platforms that don’t support it directly can only be done in the replay mode.

When debugging in the reverse direction, GDB will work in replay mode as long as the execution log includes the record for the previous instruction; otherwise, it will work in record mode, if the platform supports reverse execution, or stop if not.

For architecture environments that support process record and replay, GDB provides the following commands:

record method

This command starts the process record and replay target. The recording method can be specified as parameter. Without a parameter the command uses the full recording method. The following recording methods are available:

full

Full record/replay recording using GDB’s software record and replay implementation. This method allows replaying and reverse execution.

btrace

Hardware-supported instruction recording. This method does not record data. Further, the data is collected in a ring buffer so old data will be overwritten when the buffer is full. It allows limited replay and reverse execution.

This recording method may not be available on all processors.

The process record and replay target can only debug a process that is already running. Therefore, you need first to start the process with the run or start commands, and then start the recording with the record method command.

Both record method and rec method are aliases of target record-method.

Displaced stepping (see displaced stepping) will be automatically disabled when process record and replay target is started. That’s because the process record and replay target doesn’t support displaced stepping.

If the inferior is in the non-stop mode (see Non-Stop Mode) or in the asynchronous execution mode (see Background Execution), not all recording methods are available. The full recording method does not support these two modes.

record stop

Stop the process record and replay target. When process record and replay target stops, the entire execution log will be deleted and the inferior will either be terminated, or will remain in its final state.

When you stop the process record and replay target in record mode (at the end of the execution log), the inferior will be stopped at the next instruction that would have been recorded. In other words, if you record for a while and then stop recording, the inferior process will be left in the same state as if the recording never happened.

On the other hand, if the process record and replay target is stopped while in replay mode (that is, not at the end of the execution log, but at some earlier point), the inferior process will become “live” at that earlier state, and it will then be possible to continue the usual “live” debugging of the process from that state.

When the inferior process exits, or GDB detaches from it, process record and replay target will automatically stop itself.

record goto

Go to a specific location in the execution log. There are several ways to specify the location to go to:

record goto begin
record goto start

Go to the beginning of the execution log.

record goto end

Go to the end of the execution log.

record goto n

Go to instruction number n in the execution log.

record save filename

Save the execution log to a file filename. Default filename is gdb_record.process_id, where process_id is the process ID of the inferior.

This command may not be available for all recording methods.

record restore filename

Restore the execution log from a file filename. File must have been created with record save.

set record full insn-number-max limit
set record full insn-number-max unlimited

Set the limit of instructions to be recorded for the full recording method. Default value is 200000.

If limit is a positive number, then GDB will start deleting instructions from the log once the number of the record instructions becomes greater than limit. For every new recorded instruction, GDB will delete the earliest recorded instruction to keep the number of recorded instructions at the limit. (Since deleting recorded instructions loses information, GDB lets you control what happens when the limit is reached, by means of the stop-at-limit option, described below.)

If limit is unlimited or zero, GDB will never delete recorded instructions from the execution log. The number of recorded instructions is limited only by the available memory.

show record full insn-number-max

Show the limit of instructions to be recorded with the full recording method.

set record full stop-at-limit

Control the behavior of the full recording method when the number of recorded instructions reaches the limit. If ON (the default), GDB will stop when the limit is reached for the first time and ask you whether you want to stop the inferior or continue running it and recording the execution log. If you decide to continue recording, each new recorded instruction will cause the oldest one to be deleted.

If this option is OFF, GDB will automatically delete the oldest record to make room for each new one, without asking.

show record full stop-at-limit

Show the current setting of stop-at-limit.

set record full memory-query

Control the behavior when GDB is unable to record memory changes caused by an instruction for the full recording method. If ON, GDB will query whether to stop the inferior in that case.

If this option is OFF (the default), GDB will automatically ignore the effect of such instructions on memory. Later, when GDB replays this execution log, it will mark the log of this instruction as not accessible, and it will not affect the replay results.

show record full memory-query

Show the current setting of memory-query.

The btrace record target does not trace data. As a convenience, when replaying, GDB reads read-only memory off the live program directly, assuming that the addresses of the read-only areas don’t change. This for example makes it possible to disassemble code while replaying, but not to print variables. In some cases, being able to inspect variables might be useful. You can use the following command for that:

set record btrace replay-memory-access

Control the behavior of the btrace recording method when accessing memory during replay. If read-only (the default), GDB will only allow accesses to read-only memory. If read-write, GDB will allow accesses to read-only and to read-write memory. Beware that the accessed memory corresponds to the live target and not necessarily to the current replay position.

show record btrace replay-memory-access

Show the current setting of replay-memory-access.

info record

Show various statistics about the recording depending on the recording method:

full

For the full recording method, it shows the state of process record and its in-memory execution log buffer, including:

  • Whether in record mode or replay mode.
  • Lowest recorded instruction number (counting from when the current execution log started recording instructions).
  • Highest recorded instruction number.
  • Current instruction about to be replayed (if in replay mode).
  • Number of instructions contained in the execution log.
  • Maximum number of instructions that may be contained in the execution log.
btrace

For the btrace recording method, it shows the number of instructions that have been recorded and the number of blocks of sequential control-flow that is formed by the recorded instructions.

record delete

When record target runs in replay mode (“in the past”), delete the subsequent execution log and begin to record a new execution log starting from the current address. This means you will abandon the previously recorded “future” and begin recording a new “future”.

record instruction-history

Disassembles instructions from the recorded execution log. By default, ten instructions are disassembled. This can be changed using the set record instruction-history-size command. Instructions are printed in execution order. There are several ways to specify what part of the execution log to disassemble:

record instruction-history insn

Disassembles ten instructions starting from instruction number insn.

record instruction-history insn, +/-n

Disassembles n instructions around instruction number insn. If n is preceded with +, disassembles n instructions after instruction number insn. If n is preceded with -, disassembles n instructions before instruction number insn.

record instruction-history

Disassembles ten more instructions after the last disassembly.

record instruction-history -

Disassembles ten more instructions before the last disassembly.

record instruction-history begin end

Disassembles instructions beginning with instruction number begin until instruction number end. The instruction number end is included.

This command may not be available for all recording methods.

set record instruction-history-size size
set record instruction-history-size unlimited

Define how many instructions to disassemble in the record instruction-history command. The default value is 10. A size of unlimited means unlimited instructions.

show record instruction-history-size

Show how many instructions to disassemble in the record instruction-history command.

record function-call-history

Prints the execution history at function granularity. It prints one line for each sequence of instructions that belong to the same function giving the name of that function, the source lines for this instruction sequence (if the /l modifier is specified), and the instructions numbers that form the sequence (if the /i modifier is specified). The function names are indented to reflect the call stack depth if the /c modifier is specified. The /l, /i, and /c modifiers can be given together.

(gdb) list 1, 10
1   void foo (void)
2   {
3   }
4
5   void bar (void)
6   {
7     ...
8     foo ();
9     ...
10  }
(gdb) record function-call-history /ilc
1  bar     inst 1,4     at foo.c:6,8
2    foo   inst 5,10    at foo.c:2,3
3  bar     inst 11,13   at foo.c:9,10

By default, ten lines are printed. This can be changed using the set record function-call-history-size command. Functions are printed in execution order. There are several ways to specify what to print:

record function-call-history func

Prints ten functions starting from function number func.

record function-call-history func, +/-n

Prints n functions around function number func. If n is preceded with +, prints n functions after function number func. If n is preceded with -, prints n functions before function number func.

record function-call-history

Prints ten more functions after the last ten-line print.

record function-call-history -

Prints ten more functions before the last ten-line print.

record function-call-history begin end

Prints functions beginning with function number begin until function number end. The function number end is included.

This command may not be available for all recording methods.

set record function-call-history-size size
set record function-call-history-size unlimited

Define how many lines to print in the record function-call-history command. The default value is 10. A size of unlimited means unlimited lines.

show record function-call-history-size

Show how many lines to print in the record function-call-history command.


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8 Examining the Stack

When your program has stopped, the first thing you need to know is where it stopped and how it got there.

Each time your program performs a function call, information about the call is generated. That information includes the location of the call in your program, the arguments of the call, and the local variables of the function being called. The information is saved in a block of data called a stack frame. The stack frames are allocated in a region of memory called the call stack.

When your program stops, the GDB commands for examining the stack allow you to see all of this information.

One of the stack frames is selected by GDB and many GDB commands refer implicitly to the selected frame. In particular, whenever you ask GDB for the value of a variable in your program, the value is found in the selected frame. There are special GDB commands to select whichever frame you are interested in. See Selecting a Frame.

When your program stops, GDB automatically selects the currently executing frame and describes it briefly, similar to the frame command (see Information about a Frame).


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8.1 Stack Frames

The call stack is divided up into contiguous pieces called stack frames, or frames for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function’s local variables, and the address at which the function is executing.

When your program is started, the stack has only one frame, that of the function main. This is called the initial frame or the outermost frame. Each time a function is called, a new frame is made. Each time a function returns, the frame for that function invocation is eliminated. If a function is recursive, there can be many frames for the same function. The frame for the function in which execution is actually occurring is called the innermost frame. This is the most recently created of all the stack frames that still exist.

Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame. Usually this address is kept in a register called the frame pointer register (see $fp) while execution is going on in that frame.

GDB assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by GDB to give you a way of designating stack frames in GDB commands.

Some compilers provide a way to compile functions so that they operate without stack frames. (For example, the GCC option

-fomit-frame-pointer

generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. GDB has limited facilities for dealing with these function invocations. If the innermost function invocation has no stack frame, GDB nevertheless regards it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain. However, GDB has no provision for frameless functions elsewhere in the stack.

frame [framespec]

The frame command allows you to move from one stack frame to another, and to print the stack frame you select. The framespec may be either the address of the frame or the stack frame number. Without an argument, frame prints the current stack frame.

select-frame

The select-frame command allows you to move from one stack frame to another without printing the frame. This is the silent version of frame.


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8.2 Backtraces

A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.

backtrace
bt

Print a backtrace of the entire stack: one line per frame for all frames in the stack.

You can stop the backtrace at any time by typing the system interrupt character, normally Ctrl-c.

backtrace n
bt n

Similar, but print only the innermost n frames.

backtrace -n
bt -n

Similar, but print only the outermost n frames.

backtrace full
bt full
bt full n
bt full -n

Print the values of the local variables also. As described above, n specifies the number of frames to print.

backtrace no-filters
bt no-filters
bt no-filters n
bt no-filters -n
bt no-filters full
bt no-filters full n
bt no-filters full -n

Do not run Python frame filters on this backtrace. See Frame Filter API, for more information. Additionally use disable frame-filter all to turn off all frame filters. This is only relevant when GDB has been configured with Python support.

The names where and info stack (abbreviated info s) are additional aliases for backtrace.

In a multi-threaded program, GDB by default shows the backtrace only for the current thread. To display the backtrace for several or all of the threads, use the command thread apply (see thread apply). For example, if you type thread apply all backtrace, GDB will display the backtrace for all the threads; this is handy when you debug a core dump of a multi-threaded program.

Each line in the backtrace shows the frame number and the function name. The program counter value is also shown—unless you use set print address off. The backtrace also shows the source file name and line number, as well as the arguments to the function. The program counter value is omitted if it is at the beginning of the code for that line number.

Here is an example of a backtrace. It was made with the command ‘bt 3’, so it shows the innermost three frames.

#0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
    at builtin.c:993
#1  0x6e38 in expand_macro (sym=0x2b600, data=...) at macro.c:242
#2  0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
    at macro.c:71
(More stack frames follow...)

The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line 993 of builtin.c.

The value of parameter data in frame 1 has been replaced by . By default, GDB prints the value of a parameter only if it is a scalar (integer, pointer, enumeration, etc). See command set print frame-arguments in Print Settings for more details on how to configure the way function parameter values are printed.

If your program was compiled with optimizations, some compilers will optimize away arguments passed to functions if those arguments are never used after the call. Such optimizations generate code that passes arguments through registers, but doesn’t store those arguments in the stack frame. GDB has no way of displaying such arguments in stack frames other than the innermost one. Here’s what such a backtrace might look like:

#0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
    at builtin.c:993
#1  0x6e38 in expand_macro (sym=<optimized out>) at macro.c:242
#2  0x6840 in expand_token (obs=0x0, t=<optimized out>, td=0xf7fffb08)
    at macro.c:71
(More stack frames follow...)

The values of arguments that were not saved in their stack frames are shown as ‘<optimized out>’.

If you need to display the values of such optimized-out arguments, either deduce that from other variables whose values depend on the one you are interested in, or recompile without optimizations.

Most programs have a standard user entry point—a place where system libraries and startup code transition into user code. For C this is main6. When GDB finds the entry function in a backtrace it will terminate the backtrace, to avoid tracing into highly system-specific (and generally uninteresting) code.

If you need to examine the startup code, or limit the number of levels in a backtrace, you can change this behavior:

set backtrace past-main
set backtrace past-main on

Backtraces will continue past the user entry point.

set backtrace past-main off

Backtraces will stop when they encounter the user entry point. This is the default.

show backtrace past-main

Display the current user entry point backtrace policy.

set backtrace past-entry
set backtrace past-entry on

Backtraces will continue past the internal entry point of an application. This entry point is encoded by the linker when the application is built, and is likely before the user entry point main (or equivalent) is called.

set backtrace past-entry off

Backtraces will stop when they encounter the internal entry point of an application. This is the default.

show backtrace past-entry

Display the current internal entry point backtrace policy.

set backtrace limit n
set backtrace limit 0
set backtrace limit unlimited

Limit the backtrace to n levels. A value of unlimited or zero means unlimited levels.

show backtrace limit

Display the current limit on backtrace levels.

You can control how file names are displayed.

set filename-display
set filename-display relative

Display file names relative to the compilation directory. This is the default.

set filename-display basename

Display only basename of a filename.

set filename-display absolute

Display an absolute filename.

show filename-display

Show the current way to display filenames.


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8.3 Management of Frame Filters.

Frame filters are Python based utilities to manage and decorate the output of frames. See Frame Filter API, for further information.

Managing frame filters is performed by several commands available within GDB, detailed here.

info frame-filter

Print a list of installed frame filters from all dictionaries, showing their name, priority and enabled status.

disable frame-filter filter-dictionary filter-name

Disable a frame filter in the dictionary matching filter-dictionary and filter-name. The filter-dictionary may be all, global, progspace, or the name of the object file where the frame filter dictionary resides. When all is specified, all frame filters across all dictionaries are disabled. The filter-name is the name of the frame filter and is used when all is not the option for filter-dictionary. A disabled frame-filter is not deleted, it may be enabled again later.

enable frame-filter filter-dictionary filter-name

Enable a frame filter in the dictionary matching filter-dictionary and filter-name. The filter-dictionary may be all, global, progspace or the name of the object file where the frame filter dictionary resides. When all is specified, all frame filters across all dictionaries are enabled. The filter-name is the name of the frame filter and is used when all is not the option for filter-dictionary.

Example:

(gdb) info frame-filter

global frame-filters:
  Priority  Enabled  Name
  1000      No       PrimaryFunctionFilter
  100       Yes      Reverse

progspace /build/test frame-filters:
  Priority  Enabled  Name
  100       Yes      ProgspaceFilter

objfile /build/test frame-filters:
  Priority  Enabled  Name
  999       Yes      BuildProgra Filter

(gdb) disable frame-filter /build/test BuildProgramFilter
(gdb) info frame-filter

global frame-filters:
  Priority  Enabled  Name
  1000      No       PrimaryFunctionFilter
  100       Yes      Reverse

progspace /build/test frame-filters:
  Priority  Enabled  Name
  100       Yes      ProgspaceFilter

objfile /build/test frame-filters:
  Priority  Enabled  Name
  999       No       BuildProgramFilter

(gdb) enable frame-filter global PrimaryFunctionFilter
(gdb) info frame-filter

global frame-filters:
  Priority  Enabled  Name
  1000      Yes      PrimaryFunctionFilter
  100       Yes      Reverse

progspace /build/test frame-filters:
  Priority  Enabled  Name
  100       Yes      ProgspaceFilter

objfile /build/test frame-filters:
  Priority  Enabled  Name
  999       No       BuildProgramFilter
set frame-filter priority filter-dictionary filter-name priority

Set the priority of a frame filter in the dictionary matching filter-dictionary, and the frame filter name matching filter-name. The filter-dictionary may be global, progspace or the name of the object file where the frame filter dictionary resides. The priority is an integer.

show frame-filter priority filter-dictionary filter-name

Show the priority of a frame filter in the dictionary matching filter-dictionary, and the frame filter name matching filter-name. The filter-dictionary may be global, progspace or the name of the object file where the frame filter dictionary resides.

Example:

(gdb) info frame-filter

global frame-filters:
  Priority  Enabled  Name
  1000      Yes      PrimaryFunctionFilter
  100       Yes      Reverse

progspace /build/test frame-filters:
  Priority  Enabled  Name
  100       Yes      ProgspaceFilter

objfile /build/test frame-filters:
  Priority  Enabled  Name
  999       No       BuildProgramFilter

(gdb) set frame-filter priority global Reverse 50
(gdb) info frame-filter

global frame-filters:
  Priority  Enabled  Name
  1000      Yes      PrimaryFunctionFilter
  50        Yes      Reverse

progspace /build/test frame-filters:
  Priority  Enabled  Name
  100       Yes      ProgspaceFilter

objfile /build/test frame-filters:
  Priority  Enabled  Name
  999       No       BuildProgramFilter

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8.4 Selecting a Frame

Most commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment. Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected.

frame n
f n

Select frame number n. Recall that frame zero is the innermost (currently executing) frame, frame one is the frame that called the innermost one, and so on. The highest-numbered frame is the one for main.

frame addr
f addr

Select the frame at address addr. This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for GDB to assign numbers properly to all frames. In addition, this can be useful when your program has multiple stacks and switches between them.

On the SPARC architecture, frame needs two addresses to select an arbitrary frame: a frame pointer and a stack pointer.

On the MIPS and Alpha architecture, it needs two addresses: a stack pointer and a program counter.

On the 29k architecture, it needs three addresses: a register stack pointer, a program counter, and a memory stack pointer.

up n

Move n frames up the stack; n defaults to 1. For positive numbers n, this advances toward the outermost frame, to higher frame numbers, to frames that have existed longer.

down n

Move n frames down the stack; n defaults to 1. For positive numbers n, this advances toward the innermost frame, to lower frame numbers, to frames that were created more recently. You may abbreviate down as do.

All of these commands end by printing two lines of output describing the frame. The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame. The second line shows the text of that source line.

For example:

(gdb) up
#1  0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)
    at env.c:10
10              read_input_file (argv[i]);

After such a printout, the list command with no arguments prints ten lines centered on the point of execution in the frame. You can also edit the program at the point of execution with your favorite editing program by typing edit. See Printing Source Lines, for details.

up-silently n
down-silently n

These two commands are variants of up and down, respectively; they differ in that they do their work silently, without causing display of the new frame. They are intended primarily for use in GDB command scripts, where the output might be unnecessary and distracting.


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8.5 Information About a Frame

There are several other commands to print information about the selected stack frame.

frame
f

When used without any argument, this command does not change which frame is selected, but prints a brief description of the currently selected stack frame. It can be abbreviated f. With an argument, this command is used to select a stack frame. See Selecting a Frame.

info frame
info f

This command prints a verbose description of the selected stack frame, including:

The verbose description is useful when something has gone wrong that has made the stack format fail to fit the usual conventions.

info frame addr
info f addr

Print a verbose description of the frame at address addr, without selecting that frame. The selected frame remains unchanged by this command. This requires the same kind of address (more than one for some architectures) that you specify in the frame command. See Selecting a Frame.

info args

Print the arguments of the selected frame, each on a separate line.

info locals

Print the local variables of the selected frame, each on a separate line. These are all variables (declared either static or automatic) accessible at the point of execution of the selected frame.


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9 Examining Source Files

GDB can print parts of your program’s source, since the debugging information recorded in the program tells GDB what source files were used to build it. When your program stops, GDB spontaneously prints the line where it stopped. Likewise, when you select a stack frame (see Selecting a Frame), GDB prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command.

If you use GDB through its GNU Emacs interface, you may prefer to use Emacs facilities to view source; see Using GDB under GNU Emacs.


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9.1 Printing Source Lines

To print lines from a source file, use the list command (abbreviated l). By default, ten lines are printed. There are several ways to specify what part of the file you want to print; see Specify Location, for the full list.

Here are the forms of the list command most commonly used:

list linenum

Print lines centered around line number linenum in the current source file.

list function

Print lines centered around the beginning of function function.

list

Print more lines. If the last lines printed were printed with a list command, this prints lines following the last lines printed; however, if the last line printed was a solitary line printed as part of displaying a stack frame (see Examining the Stack), this prints lines centered around that line.

list -

Print lines just before the lines last printed.

By default, GDB prints ten source lines with any of these forms of the list command. You can change this using set listsize:

set listsize count
set listsize unlimited

Make the list command display count source lines (unless the list argument explicitly specifies some other number). Setting count to unlimited or 0 means there’s no limit.

show listsize

Display the number of lines that list prints.

Repeating a list command with RET discards the argument, so it is equivalent to typing just list. This is more useful than listing the same lines again. An exception is made for an argument of ‘-’; that argument is preserved in repetition so that each repetition moves up in the source file.

In general, the list command expects you to supply zero, one or two linespecs. Linespecs specify source lines; there are several ways of writing them (see Specify Location), but the effect is always to specify some source line.

Here is a complete description of the possible arguments for list:

list linespec

Print lines centered around the line specified by linespec.

list first,last

Print lines from first to last. Both arguments are linespecs. When a list command has two linespecs, and the source file of the second linespec is omitted, this refers to the same source file as the first linespec.

list ,last

Print lines ending with last.

list first,

Print lines starting with first.

list +

Print lines just after the lines last printed.

list -

Print lines just before the lines last printed.

list

As described in the preceding table.


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9.2 Specifying a Location

Several GDB commands accept arguments that specify a location of your program’s code. Since GDB is a source-level debugger, a location usually specifies some line in the source code; for that reason, locations are also known as linespecs.

Here are all the different ways of specifying a code location that GDB understands:

linenum

Specifies the line number linenum of the current source file.

-offset
+offset

Specifies the line offset lines before or after the current line. For the list command, the current line is the last one printed; for the breakpoint commands, this is the line at which execution stopped in the currently selected stack frame (see Frames, for a description of stack frames.) When used as the second of the two linespecs in a list command, this specifies the line offset lines up or down from the first linespec.

filename:linenum

Specifies the line linenum in the source file filename. If filename is a relative file name, then it will match any source file name with the same trailing components. For example, if filename is ‘gcc/expr.c’, then it will match source file name of /build/trunk/gcc/expr.c, but not /build/trunk/libcpp/expr.c or /build/trunk/gcc/x-expr.c.

function

Specifies the line that begins the body of the function function. For example, in C, this is the line with the open brace.

function:label

Specifies the line where label appears in function.

filename:function

Specifies the line that begins the body of the function function in the file filename. You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files.

label

Specifies the line at which the label named label appears. GDB searches for the label in the function corresponding to the currently selected stack frame. If there is no current selected stack frame (for instance, if the inferior is not running), then GDB will not search for a label.

*address

Specifies the program address address. For line-oriented commands, such as list and edit, this specifies a source line that contains address. For break and other breakpoint oriented commands, this can be used to set breakpoints in parts of your program which do not have debugging information or source files.

Here address may be any expression valid in the current working language (see working language) that specifies a code address. In addition, as a convenience, GDB extends the semantics of expressions used in locations to cover the situations that frequently happen during debugging. Here are the various forms of address:

expression

Any expression valid in the current working language.

funcaddr

An address of a function or procedure derived from its name. In C, C++, Java, Objective-C, Fortran, minimal, and assembly, this is simply the function’s name function (and actually a special case of a valid expression). In Pascal and Modula-2, this is &function. In Ada, this is function'Address (although the Pascal form also works).

This form specifies the address of the function’s first instruction, before the stack frame and arguments have been set up.

'filename'::funcaddr

Like funcaddr above, but also specifies the name of the source file explicitly. This is useful if the name of the function does not specify the function unambiguously, e.g., if there are several functions with identical names in different source files.

-pstap|-probe-stap [objfile:[provider:]]name

The GNU/Linux tool SystemTap provides a way for applications to embed static probes. See Static Probe Points, for more information on finding and using static probes. This form of linespec specifies the location of such a static probe.

If objfile is given, only probes coming from that shared library or executable matching objfile as a regular expression are considered. If provider is given, then only probes from that provider are considered. If several probes match the spec, GDB will insert a breakpoint at each one of those probes.


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9.3 Editing Source Files

To edit the lines in a source file, use the edit command. The editing program of your choice is invoked with the current line set to the active line in the program. Alternatively, there are several ways to specify what part of the file you want to print if you want to see other parts of the program:

edit location

Edit the source file specified by location. Editing starts at that location, e.g., at the specified source line of the specified file. See Specify Location, for all the possible forms of the location argument; here are the forms of the edit command most commonly used:

edit number

Edit the current source file with number as the active line number.

edit function

Edit the file containing function at the beginning of its definition.

9.3.1 Choosing your Editor

You can customize GDB to use any editor you want 7. By default, it is /bin/ex, but you can change this by setting the environment variable EDITOR before using GDB. For example, to configure GDB to use the vi editor, you could use these commands with the sh shell:

EDITOR=/usr/bin/vi
export EDITOR
gdb …

or in the csh shell,

setenv EDITOR /usr/bin/vi
gdb …

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9.4 Searching Source Files

There are two commands for searching through the current source file for a regular expression.

forward-search regexp
search regexp

The command ‘forward-search regexp’ checks each line, starting with the one following the last line listed, for a match for regexp. It lists the line that is found. You can use the synonym ‘search regexp’ or abbreviate the command name as fo.

reverse-search regexp

The command ‘reverse-search regexp’ checks each line, starting with the one before the last line listed and going backward, for a match for regexp. It lists the line that is found. You can abbreviate this command as rev.


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9.5 Specifying Source Directories

Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names. Even when they do, the directories could be moved between the compilation and your debugging session. GDB has a list of directories to search for source files; this is called the source path. Each time GDB wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name.

For example, suppose an executable references the file /usr/src/foo-1.0/lib/foo.c, and our source path is /mnt/cross. The file is first looked up literally; if this fails, /mnt/cross/usr/src/foo-1.0/lib/foo.c is tried; if this fails, /mnt/cross/foo.c is opened; if this fails, an error message is printed. GDB does not look up the parts of the source file name, such as /mnt/cross/src/foo-1.0/lib/foo.c. Likewise, the subdirectories of the source path are not searched: if the source path is /mnt/cross, and the binary refers to foo.c, GDB would not find it under /mnt/cross/usr/src/foo-1.0/lib.

Plain file names, relative file names with leading directories, file names containing dots, etc. are all treated as described above; for instance, if the source path is /mnt/cross, and the source file is recorded as ../lib/foo.c, GDB would first try ../lib/foo.c, then /mnt/cross/../lib/foo.c, and after that—/mnt/cross/foo.c.

Note that the executable search path is not used to locate the source files.

Whenever you reset or rearrange the source path, GDB clears out any information it has cached about where source files are found and where each line is in the file.

When you start GDB, its source path includes only ‘cdir’ and ‘cwd’, in that order. To add other directories, use the directory command.

The search path is used to find both program source files and GDB script files (read using the ‘-command’ option and ‘source’ command).

In addition to the source path, GDB provides a set of commands that manage a list of source path substitution rules. A substitution rule specifies how to rewrite source directories stored in the program’s debug information in case the sources were moved to a different directory between compilation and debugging. A rule is made of two strings, the first specifying what needs to be rewritten in the path, and the second specifying how it should be rewritten. In set substitute-path, we name these two parts from and to respectively. GDB does a simple string replacement of from with to at the start of the directory part of the source file name, and uses that result instead of the original file name to look up the sources.

Using the previous example, suppose the foo-1.0 tree has been moved from /usr/src to /mnt/cross, then you can tell GDB to replace /usr/src in all source path names with /mnt/cross. The first lookup will then be /mnt/cross/foo-1.0/lib/foo.c in place of the original location of /usr/src/foo-1.0/lib/foo.c. To define a source path substitution rule, use the set substitute-path command (see set substitute-path).

To avoid unexpected substitution results, a rule is applied only if the from part of the directory name ends at a directory separator. For instance, a rule substituting /usr/source into /mnt/cross will be applied to /usr/source/foo-1.0 but not to /usr/sourceware/foo-2.0. And because the substitution is applied only at the beginning of the directory name, this rule will not be applied to /root/usr/source/baz.c either.

In many cases, you can achieve the same result using the directory command. However, set substitute-path can be more efficient in the case where the sources are organized in a complex tree with multiple subdirectories. With the directory command, you need to add each subdirectory of your project. If you moved the entire tree while preserving its internal organization, then set substitute-path allows you to direct the debugger to all the sources with one single command.

set substitute-path is also more than just a shortcut command. The source path is only used if the file at the original location no longer exists. On the other hand, set substitute-path modifies the debugger behavior to look at the rewritten location instead. So, if for any reason a source file that is not relevant to your executable is located at the original location, a substitution rule is the only method available to point GDB at the new location.

You can configure a default source path substitution rule by configuring GDB with the ‘--with-relocated-sources=dir’ option. The dir should be the name of a directory under GDB’s configured prefix (set with ‘--prefix’ or ‘--exec-prefix’), and directory names in debug information under dir will be adjusted automatically if the installed GDB is moved to a new location. This is useful if GDB, libraries or executables with debug information and corresponding source code are being moved together.

directory dirname
dir dirname

Add directory dirname to the front of the source path. Several directory names may be given to this command, separated by ‘:’ (‘;’ on MS-DOS and MS-Windows, where ‘:’ usually appears as part of absolute file names) or whitespace. You may specify a directory that is already in the source path; this moves it forward, so GDB searches it sooner.

You can use the string ‘$cdir’ to refer to the compilation directory (if one is recorded), and ‘$cwd’ to refer to the current working directory. ‘$cwd’ is not the same as ‘.’—the former tracks the current working directory as it changes during your GDB session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path.

directory

Reset the source path to its default value (‘$cdir:$cwd’ on Unix systems). This requires confirmation.

set directories path-list

Set the source path to path-list. ‘$cdir:$cwd’ are added if missing.

show directories

Print the source path: show which directories it contains.

set substitute-path from to

Define a source path substitution rule, and add it at the end of the current list of existing substitution rules. If a rule with the same from was already defined, then the old rule is also deleted.

For example, if the file /foo/bar/baz.c was moved to /mnt/cross/baz.c, then the command

(gdb) set substitute-path /usr/src /mnt/cross

will tell GDB to replace ‘/usr/src’ with ‘/mnt/cross’, which will allow GDB to find the file baz.c even though it was moved.

In the case when more than one substitution rule have been defined, the rules are evaluated one by one in the order where they have been defined. The first one matching, if any, is selected to perform the substitution.

For instance, if we had entered the following commands:

(gdb) set substitute-path /usr/src/include /mnt/include
(gdb) set substitute-path /usr/src /mnt/src

GDB would then rewrite /usr/src/include/defs.h into /mnt/include/defs.h by using the first rule. However, it would use the second rule to rewrite /usr/src/lib/foo.c into /mnt/src/lib/foo.c.

unset substitute-path [path]

If a path is specified, search the current list of substitution rules for a rule that would rewrite that path. Delete that rule if found. A warning is emitted by the debugger if no rule could be found.

If no path is specified, then all substitution rules are deleted.

show substitute-path [path]

If a path is specified, then print the source path substitution rule which would rewrite that path, if any.

If no path is specified, then print all existing source path substitution rules.

If your source path is cluttered with directories that are no longer of interest, GDB may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows:

  1. Use directory with no argument to reset the source path to its default value.
  2. Use directory with suitable arguments to reinstall the directories you want in the source path. You can add all the directories in one command.

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9.6 Source and Machine Code

You can use the command info line to map source lines to program addresses (and vice versa), and the command disassemble to display a range of addresses as machine instructions. You can use the command set disassemble-next-line to set whether to disassemble next source line when execution stops. When run under GNU Emacs mode, the info line command causes the arrow to point to the line specified. Also, info line prints addresses in symbolic form as well as hex.

info line linespec

Print the starting and ending addresses of the compiled code for source line linespec. You can specify source lines in any of the ways documented in Specify Location.

For example, we can use info line to discover the location of the object code for the first line of function m4_changequote:

(gdb) info line m4_changequote
Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.

We can also inquire (using *addr as the form for linespec) what source line covers a particular address:

(gdb) info line *0x63ff
Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.

After info line, the default address for the x command is changed to the starting address of the line, so that ‘x/i’ is sufficient to begin examining the machine code (see Examining Memory). Also, this address is saved as the value of the convenience variable $_ (see Convenience Variables).

disassemble
disassemble /m
disassemble /r

This specialized command dumps a range of memory as machine instructions. It can also print mixed source+disassembly by specifying the /m modifier and print the raw instructions in hex as well as in symbolic form by specifying the /r. The default memory range is the function surrounding the program counter of the selected frame. A single argument to this command is a program counter value; GDB dumps the function surrounding this value. When two arguments are given, they should be separated by a comma, possibly surrounded by whitespace. The arguments specify a range of addresses to dump, in one of two forms:

start,end

the addresses from start (inclusive) to end (exclusive)

start,+length

the addresses from start (inclusive) to start+length (exclusive).

When 2 arguments are specified, the name of the function is also printed (since there could be several functions in the given range).

The argument(s) can be any expression yielding a numeric value, such as ‘0x32c4’, ‘&main+10’ or ‘$pc - 8’.

If the range of memory being disassembled contains current program counter, the instruction at that location is shown with a => marker.

The following example shows the disassembly of a range of addresses of HP PA-RISC 2.0 code:

(gdb) disas 0x32c4, 0x32e4
Dump of assembler code from 0x32c4 to 0x32e4:
   0x32c4 <main+204>:      addil 0,dp
   0x32c8 <main+208>:      ldw 0x22c(sr0,r1),r26
   0x32cc <main+212>:      ldil 0x3000,r31
   0x32d0 <main+216>:      ble 0x3f8(sr4,r31)
   0x32d4 <main+220>:      ldo 0(r31),rp
   0x32d8 <main+224>:      addil -0x800,dp
   0x32dc <main+228>:      ldo 0x588(r1),r26
   0x32e0 <main+232>:      ldil 0x3000,r31
End of assembler dump.

Here is an example showing mixed source+assembly for Intel x86, when the program is stopped just after function prologue:

(gdb) disas /m main
Dump of assembler code for function main:
5       {
   0x08048330 <+0>:    push   %ebp
   0x08048331 <+1>:    mov    %esp,%ebp
   0x08048333 <+3>:    sub    $0x8,%esp
   0x08048336 <+6>:    and    $0xfffffff0,%esp
   0x08048339 <+9>:    sub    $0x10,%esp

6         printf ("Hello.\n");
=> 0x0804833c <+12>:   movl   $0x8048440,(%esp)
   0x08048343 <+19>:   call   0x8048284 <puts@plt>

7         return 0;
8       }
   0x08048348 <+24>:   mov    $0x0,%eax
   0x0804834d <+29>:   leave
   0x0804834e <+30>:   ret

End of assembler dump.

Here is another example showing raw instructions in hex for AMD x86-64,

(gdb) disas /r 0x400281,+10
Dump of assembler code from 0x400281 to 0x40028b:
   0x0000000000400281:  38 36  cmp    %dh,(%rsi)
   0x0000000000400283:  2d 36 34 2e 73 sub    $0x732e3436,%eax
   0x0000000000400288:  6f     outsl  %ds:(%rsi),(%dx)
   0x0000000000400289:  2e 32 00       xor    %cs:(%rax),%al
End of assembler dump.

Addresses cannot be specified as a linespec (see Specify Location). So, for example, if you want to disassemble function bar in file foo.c, you must type ‘disassemble 'foo.c'::bar’ and not ‘disassemble foo.c:bar’.

Some architectures have more than one commonly-used set of instruction mnemonics or other syntax.

For programs that were dynamically linked and use shared libraries, instructions that call functions or branch to locations in the shared libraries might show a seemingly bogus location—it’s actually a location of the relocation table. On some architectures, GDB might be able to resolve these to actual function names.

set disassembly-flavor instruction-set

Select the instruction set to use when disassembling the program via the disassemble or x/i commands.

Currently this command is only defined for the Intel x86 family. You can set instruction-set to either intel or att. The default is att, the AT&T flavor used by default by Unix assemblers for x86-based targets.

show disassembly-flavor

Show the current setting of the disassembly flavor.

set disassemble-next-line
show disassemble-next-line

Control whether or not GDB will disassemble the next source line or instruction when execution stops. If ON, GDB will display disassembly of the next source line when execution of the program being debugged stops. This is in addition to displaying the source line itself, which GDB always does if possible. If the next source line cannot be displayed for some reason (e.g., if GDB cannot find the source file, or there’s no line info in the debug info), GDB will display disassembly of the next instruction instead of showing the next source line. If AUTO, GDB will display disassembly of next instruction only if the source line cannot be displayed. This setting causes GDB to display some feedback when you step through a function with no line info or whose source file is unavailable. The default is OFF, which means never display the disassembly of the next line or instruction.


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10 Examining Data

The usual way to examine data in your program is with the print command (abbreviated p), or its synonym inspect. It evaluates and prints the value of an expression of the language your program is written in (see Using GDB with Different Languages). It may also print the expression using a Python-based pretty-printer (see Pretty Printing).

print expr
print /f expr

expr is an expression (in the source language). By default the value of expr is printed in a format appropriate to its data type; you can choose a different format by specifying ‘/f’, where f is a letter specifying the format; see Output Formats.

print
print /f

If you omit expr, GDB displays the last value again (from the value history; see Value History). This allows you to conveniently inspect the same value in an alternative format.

A more low-level way of examining data is with the x command. It examines data in memory at a specified address and prints it in a specified format. See Examining Memory.

If you are interested in information about types, or about how the fields of a struct or a class are declared, use the ptype exp command rather than print. See Examining the Symbol Table.

Another way of examining values of expressions and type information is through the Python extension command explore (available only if the GDB build is configured with --with-python). It offers an interactive way to start at the highest level (or, the most abstract level) of the data type of an expression (or, the data type itself) and explore all the way down to leaf scalar values/fields embedded in the higher level data types.

explore arg

arg is either an expression (in the source language), or a type visible in the current context of the program being debugged.

The working of the explore command can be illustrated with an example. If a data type struct ComplexStruct is defined in your C program as

struct SimpleStruct
{
  int i;
  double d;
};

struct ComplexStruct
{
  struct SimpleStruct *ss_p;
  int arr[10];
};

followed by variable declarations as

struct SimpleStruct ss = { 10, 1.11 };
struct ComplexStruct cs = { &ss, { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } };

then, the value of the variable cs can be explored using the explore command as follows.

(gdb) explore cs
The value of `cs' is a struct/class of type `struct ComplexStruct' with
the following fields:

  ss_p = <Enter 0 to explore this field of type `struct SimpleStruct *'>
   arr = <Enter 1 to explore this field of type `int [10]'>

Enter the field number of choice:

Since the fields of cs are not scalar values, you are being prompted to chose the field you want to explore. Let’s say you choose the field ss_p by entering 0. Then, since this field is a pointer, you will be asked if it is pointing to a single value. From the declaration of cs above, it is indeed pointing to a single value, hence you enter y. If you enter n, then you will be asked if it were pointing to an array of values, in which case this field will be explored as if it were an array.

`cs.ss_p' is a pointer to a value of type `struct SimpleStruct'
Continue exploring it as a pointer to a single value [y/n]: y
The value of `*(cs.ss_p)' is a struct/class of type `struct
SimpleStruct' with the following fields:

  i = 10 .. (Value of type `int')
  d = 1.1100000000000001 .. (Value of type `double')

Press enter to return to parent value:

If the field arr of cs was chosen for exploration by entering 1 earlier, then since it is as array, you will be prompted to enter the index of the element in the array that you want to explore.

`cs.arr' is an array of `int'.
Enter the index of the element you want to explore in `cs.arr': 5

`(cs.arr)[5]' is a scalar value of type `int'.

(cs.arr)[5] = 4

Press enter to return to parent value: 

In general, at any stage of exploration, you can go deeper towards the leaf values by responding to the prompts appropriately, or hit the return key to return to the enclosing data structure (the higher level data structure).

Similar to exploring values, you can use the explore command to explore types. Instead of specifying a value (which is typically a variable name or an expression valid in the current context of the program being debugged), you specify a type name. If you consider the same example as above, your can explore the type struct ComplexStruct by passing the argument struct ComplexStruct to the explore command.

(gdb) explore struct ComplexStruct

By responding to the prompts appropriately in the subsequent interactive session, you can explore the type struct ComplexStruct in a manner similar to how the value cs was explored in the above example.

The explore command also has two sub-commands, explore value and explore type. The former sub-command is a way to explicitly specify that value exploration of the argument is being invoked, while the latter is a way to explicitly specify that type exploration of the argument is being invoked.

explore value expr

This sub-command of explore explores the value of the expression expr (if expr is an expression valid in the current context of the program being debugged). The behavior of this command is identical to that of the behavior of the explore command being passed the argument expr.

explore type arg

This sub-command of explore explores the type of arg (if arg is a type visible in the current context of program being debugged), or the type of the value/expression arg (if arg is an expression valid in the current context of the program being debugged). If arg is a type, then the behavior of this command is identical to that of the explore command being passed the argument arg. If arg is an expression, then the behavior of this command will be identical to that of the explore command being passed the type of arg as the argument.


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10.1 Expressions

print and many other GDB commands accept an expression and compute its value. Any kind of constant, variable or operator defined by the programming language you are using is valid in an expression in GDB. This includes conditional expressions, function calls, casts, and string constants. It also includes preprocessor macros, if you compiled your program to include this information; see Compilation.

GDB supports array constants in expressions input by the user. The syntax is {element, element…}. For example, you can use the command print {1, 2, 3} to create an array of three integers. If you pass an array to a function or assign it to a program variable, GDB copies the array to memory that is malloced in the target program.

Because C is so widespread, most of the expressions shown in examples in this manual are in C. See Using GDB with Different Languages, for information on how to use expressions in other languages.

In this section, we discuss operators that you can use in GDB expressions regardless of your programming language.

Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer in order to examine a structure at that address in memory.

GDB supports these operators, in addition to those common to programming languages:

@

@’ is a binary operator for treating parts of memory as arrays. See Artificial Arrays, for more information.

::

::’ allows you to specify a variable in terms of the file or function where it is defined. See Program Variables.

{type} addr

Refers to an object of type type stored at address addr in memory. The address addr may be any expression whose value is an integer or pointer (but parentheses are required around binary operators, just as in a cast). This construct is allowed regardless of what kind of data is normally supposed to reside at addr.


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10.2 Ambiguous Expressions

Expressions can sometimes contain some ambiguous elements. For instance, some programming languages (notably Ada, C++ and Objective-C) permit a single function name to be defined several times, for application in different contexts. This is called overloading. Another example involving Ada is generics. A generic package is similar to C++ templates and is typically instantiated several times, resulting in the same function name being defined in different contexts.

In some cases and depending on the language, it is possible to adjust the expression to remove the ambiguity. For instance in C++, you can specify the signature of the function you want to break on, as in break function(types). In Ada, using the fully qualified name of your function often makes the expression unambiguous as well.

When an ambiguity that needs to be resolved is detected, the debugger has the capability to display a menu of numbered choices for each possibility, and then waits for the selection with the prompt ‘>’. The first option is always ‘[0] cancel’, and typing 0 RET aborts the current command. If the command in which the expression was used allows more than one choice to be selected, the next option in the menu is ‘[1] all’, and typing 1 RET selects all possible choices.

For example, the following session excerpt shows an attempt to set a breakpoint at the overloaded symbol String::after. We choose three particular definitions of that function name:

(gdb) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
> 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the "delete" command to delete unwanted
 breakpoints.
(gdb)
set multiple-symbols mode

This option allows you to adjust the debugger behavior when an expression is ambiguous.

By default, mode is set to all. If the command with which the expression is used allows more than one choice, then GDB automatically selects all possible choices. For instance, inserting a breakpoint on a function using an ambiguous name results in a breakpoint inserted on each possible match. However, if a unique choice must be made, then GDB uses the menu to help you disambiguate the expression. For instance, printing the address of an overloaded function will result in the use of the menu.

When mode is set to ask, the debugger always uses the menu when an ambiguity is detected.

Finally, when mode is set to cancel, the debugger reports an error due to the ambiguity and the command is aborted.

show multiple-symbols

Show the current value of the multiple-symbols setting.


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10.3 Program Variables

The most common kind of expression to use is the name of a variable in your program.

Variables in expressions are understood in the selected stack frame (see Selecting a Frame); they must be either:

or

This means that in the function

foo (a)
     int a;
{
  bar (a);
  {
    int b = test ();
    bar (b);
  }
}

you can examine and use the variable a whenever your program is executing within the function foo, but you can only use or examine the variable b while your program is executing inside the block where b is declared.

There is an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point is not in this file. But it is possible to have more than one such variable or function with the same name (in different source files). If that happens, referring to that name has unpredictable effects. If you wish, you can specify a static variable in a particular function or file by using the colon-colon (::) notation:

file::variable
function::variable

Here file or function is the name of the context for the static variable. In the case of file names, you can use quotes to make sure GDB parses the file name as a single word—for example, to print a global value of x defined in f2.c:

(gdb) p 'f2.c'::x

The :: notation is normally used for referring to static variables, since you typically disambiguate uses of local variables in functions by selecting the appropriate frame and using the simple name of the variable. However, you may also use this notation to refer to local variables in frames enclosing the selected frame:

void
foo (int a)
{
  if (a < 10)
    bar (a);
  else
    process (a);    /* Stop here */
}

int
bar (int a)
{
  foo (a + 5);
}

For example, if there is a breakpoint at the commented line, here is what you might see when the program stops after executing the call bar(0):

(gdb) p a
$1 = 10
(gdb) p bar::a
$2 = 5
(gdb) up 2
#2  0x080483d0 in foo (a=5) at foobar.c:12
(gdb) p a
$3 = 5
(gdb) p bar::a
$4 = 0

These uses of ‘::’ are very rarely in conflict with the very similar use of the same notation in C++. When they are in conflict, the C++ meaning takes precedence; however, this can be overridden by quoting the file or function name with single quotes.

For example, suppose the program is stopped in a method of a class that has a field named includefile, and there is also an include file named includefile that defines a variable, some_global.

(gdb) p includefile
$1 = 23
(gdb) p includefile::some_global
A syntax error in expression, near `'.
(gdb) p 'includefile'::some_global
$2 = 27

Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function—just after entry to a new scope, and just before exit.

You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.

This may also happen when the compiler does significant optimizations. To be sure of always seeing accurate values, turn off all optimization when compiling.

Another possible effect of compiler optimizations is to optimize unused variables out of existence, or assign variables to registers (as opposed to memory addresses). Depending on the support for such cases offered by the debug info format used by the compiler, GDB might not be able to display values for such local variables. If that happens, GDB will print a message like this:

No symbol "foo" in current context.

To solve such problems, either recompile without optimizations, or use a different debug info format, if the compiler supports several such formats. See Compilation, for more information on choosing compiler options. See C and C++, for more information about debug info formats that are best suited to C++ programs.

If you ask to print an object whose contents are unknown to GDB, e.g., because its data type is not completely specified by the debug information, GDB will say ‘<incomplete type>’. See incomplete type, for more about this.

If you append @entry string to a function parameter name you get its value at the time the function got called. If the value is not available an error message is printed. Entry values are available only with some compilers. Entry values are normally also printed at the function parameter list according to set print entry-values.

Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
29	  i++;
(gdb) next
30	  e (i);
(gdb) print i
$1 = 31
(gdb) print i@entry
$2 = 30

Strings are identified as arrays of char values without specified signedness. Arrays of either signed char or unsigned char get printed as arrays of 1 byte sized integers. -fsigned-char or -funsigned-char GCC options have no effect as GDB defines literal string type "char" as char without a sign. For program code

char var0[] = "A";
signed char var1[] = "A";

You get during debugging

(gdb) print var0
$1 = "A"
(gdb) print var1
$2 = {65 'A', 0 '\0'}

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10.4 Artificial Arrays

It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program.

You can do this by referring to a contiguous span of memory as an artificial array, using the binary operator ‘@’. The left operand of ‘@’ should be the first element of the desired array and be an individual object. The right operand should be the desired length of the array. The result is an array value whose elements are all of the type of the left argument. The first element is actually the left argument; the second element comes from bytes of memory immediately following those that hold the first element, and so on. Here is an example. If a program says

int *array = (int *) malloc (len * sizeof (int));

you can print the contents of array with

p *array@len

The left operand of ‘@’ must reside in memory. Array values made with ‘@’ in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (see Value History), after printing one out.

Another way to create an artificial array is to use a cast. This re-interprets a value as if it were an array. The value need not be in memory:

(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}

As a convenience, if you leave the array length out (as in ‘(type[])value’) GDB calculates the size to fill the value (as ‘sizeof(value)/sizeof(type)’:

(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}

Sometimes the artificial array mechanism is not quite enough; in moderately complex data structures, the elements of interest may not actually be adjacent—for example, if you are interested in the values of pointers in an array. One useful work-around in this situation is to use a convenience variable (see Convenience Variables) as a counter in an expression that prints the first interesting value, and then repeat that expression via RET. For instance, suppose you have an array dtab of pointers to structures, and you are interested in the values of a field fv in each structure. Here is an example of what you might type:

set $i = 0
p dtab[$i++]->fv
RET
RET

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10.5 Output Formats

By default, GDB prints a value according to its data type. Sometimes this is not what you want. For example, you might want to print a number in hex, or a pointer in decimal. Or you might want to view data in memory at a certain address as a character string or as an instruction. To do these things, specify an output format when you print a value.

The simplest use of output formats is to say how to print a value already computed. This is done by starting the arguments of the print command with a slash and a format letter. The format letters supported are:

x

Regard the bits of the value as an integer, and print the integer in hexadecimal.

d

Print as integer in signed decimal.

u

Print as integer in unsigned decimal.

o

Print as integer in octal.

t

Print as integer in binary. The letter ‘t’ stands for “two”. 8

a

Print as an address, both absolute in hexadecimal and as an offset from the nearest preceding symbol. You can use this format used to discover where (in what function) an unknown address is located:

(gdb) p/a 0x54320
$3 = 0x54320 <_initialize_vx+396>

The command info symbol 0x54320 yields similar results. See info symbol.

c

Regard as an integer and print it as a character constant. This prints both the numerical value and its character representation. The character representation is replaced with the octal escape ‘\nnn’ for characters outside the 7-bit ASCII range.

Without this format, GDB displays char, unsigned char, and signed char data as character constants. Single-byte members of vectors are displayed as integer data.

f

Regard the bits of the value as a floating point number and print using typical floating point syntax.

s

Regard as a string, if possible. With this format, pointers to single-byte data are displayed as null-terminated strings and arrays of single-byte data are displayed as fixed-length strings. Other values are displayed in their natural types.

Without this format, GDB displays pointers to and arrays of char, unsigned char, and signed char as strings. Single-byte members of a vector are displayed as an integer array.

z

Like ‘x’ formatting, the value is treated as an integer and printed as hexadecimal, but leading zeros are printed to pad the value to the size of the integer type.

r

Print using the ‘raw’ formatting. By default, GDB will use a Python-based pretty-printer, if one is available (see Pretty Printing). This typically results in a higher-level display of the value’s contents. The ‘r’ format bypasses any Python pretty-printer which might exist.

For example, to print the program counter in hex (see Registers), type

p/x $pc

Note that no space is required before the slash; this is because command names in GDB cannot contain a slash.

To reprint the last value in the value history with a different format, you can use the print command with just a format and no expression. For example, ‘p/x’ reprints the last value in hex.


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10.6 Examining Memory

You can use the command x (for “examine”) to examine memory in any of several formats, independently of your program’s data types.

x/nfu addr
x addr
x

Use the x command to examine memory.

n, f, and u are all optional parameters that specify how much memory to display and how to format it; addr is an expression giving the address where you want to start displaying memory. If you use defaults for nfu, you need not type the slash ‘/’. Several commands set convenient defaults for addr.

n, the repeat count

The repeat count is a decimal integer; the default is 1. It specifies how much memory (counting by units u) to display.

f, the display format

The display format is one of the formats used by print (‘x’, ‘d’, ‘u’, ‘o’, ‘t’, ‘a’, ‘c’, ‘f’, ‘s’), and in addition ‘i’ (for machine instructions). The default is ‘x’ (hexadecimal) initially. The default changes each time you use either x or print.

u, the unit size

The unit size is any of

b

Bytes.

h

Halfwords (two bytes).

w

Words (four bytes). This is the initial default.

g

Giant words (eight bytes).

Each time you specify a unit size with x, that size becomes the default unit the next time you use x. For the ‘i’ format, the unit size is ignored and is normally not written. For the ‘s’ format, the unit size defaults to ‘b’, unless it is explicitly given. Use x /hs to display 16-bit char strings and x /ws to display 32-bit strings. The next use of x /s will again display 8-bit strings. Note that the results depend on the programming language of the current compilation unit. If the language is C, the ‘s’ modifier will use the UTF-16 encoding while ‘w’ will use UTF-32. The encoding is set by the programming language and cannot be altered.

addr, starting display address

addr is the address where you want GDB to begin displaying memory. The expression need not have a pointer value (though it may); it is always interpreted as an integer address of a byte of memory. See Expressions, for more information on expressions. The default for addr is usually just after the last address examined—but several other commands also set the default address: info breakpoints (to the address of the last breakpoint listed), info line (to the starting address of a line), and print (if you use it to display a value from memory).

For example, ‘x/3uh 0x54320’ is a request to display three halfwords (h) of memory, formatted as unsigned decimal integers (‘u’), starting at address 0x54320. ‘x/4xw $sp’ prints the four words (‘w’) of memory above the stack pointer (here, ‘$sp’; see Registers) in hexadecimal (‘x’).

Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order works. The output specifications ‘4xw’ and ‘4wx’ mean exactly the same thing. (However, the count n must come first; ‘wx4’ does not work.)

Even though the unit size u is ignored for the formats ‘s’ and ‘i’, you might still want to use a count n; for example, ‘3i’ specifies that you want to see three machine instructions, including any operands. For convenience, especially when used with the display command, the ‘i’ format also prints branch delay slot instructions, if any, beyond the count specified, which immediately follow the last instruction that is within the count. The command disassemble gives an alternative way of inspecting machine instructions; see Source and Machine Code.

All the defaults for the arguments to x are designed to make it easy to continue scanning memory with minimal specifications each time you use x. For example, after you have inspected three machine instructions with ‘x/3i addr’, you can inspect the next seven with just ‘x/7’. If you use RET to repeat the x command, the repeat count n is used again; the other arguments default as for successive uses of x.

When examining machine instructions, the instruction at current program counter is shown with a => marker. For example:

(gdb) x/5i $pc-6
   0x804837f <main+11>: mov    %esp,%ebp
   0x8048381 <main+13>: push   %ecx
   0x8048382 <main+14>: sub    $0x4,%esp
=> 0x8048385 <main+17>: movl   $0x8048460,(%esp)
   0x804838c <main+24>: call   0x80482d4 <puts@plt>

The addresses and contents printed by the x command are not saved in the value history because there is often too much of them and they would get in the way. Instead, GDB makes these values available for subsequent use in expressions as values of the convenience variables $_ and $__. After an x command, the last address examined is available for use in expressions in the convenience variable $_. The contents of that address, as examined, are available in the convenience variable $__.

If the x command has a repeat count, the address and contents saved are from the last memory unit printed; this is not the same as the last address printed if several units were printed on the last line of output.

When you are debugging a program running on a remote target machine (see Remote Debugging), you may wish to verify the program’s image in the remote machine’s memory against the executable file you downloaded to the target. Or, on any target, you may want to check whether the program has corrupted its own read-only sections. The compare-sections command is provided for such situations.

compare-sections [section-name|-r]

Compare the data of a loadable section section-name in the executable file of the program being debugged with the same section in the target machine’s memory, and report any mismatches. With no arguments, compares all loadable sections. With an argument of -r, compares all loadable read-only sections.

Note: for remote targets, this command can be accelerated if the target supports computing the CRC checksum of a block of memory (see qCRC packet).


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10.7 Automatic Display

If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the automatic display list so that GDB prints its value each time your program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like this:

2: foo = 38
3: bar[5] = (struct hack *) 0x3804

This display shows item numbers, expressions and their current values. As with displays you request manually using x or print, you can specify the output format you prefer; in fact, display decides whether to use print or x depending your format specification—it uses x if you specify either the ‘i’ or ‘s’ format, or a unit size; otherwise it uses print.

display expr

Add the expression expr to the list of expressions to display each time your program stops. See Expressions.

display does not repeat if you press RET again after using it.

display/fmt expr

For fmt specifying only a display format and not a size or count, add the expression expr to the auto-display list but arrange to display it each time in the specified format fmt. See Output Formats.

display/fmt addr

For fmti’ or ‘s’, or including a unit-size or a number of units, add the expression addr as a memory address to be examined each time your program stops. Examining means in effect doing ‘x/fmt addr’. See Examining Memory.

For example, ‘display/i $pc’ can be helpful, to see the machine instruction about to be executed each time execution stops (‘$pc’ is a common name for the program counter; see Registers).

undisplay dnums
delete display dnums

Remove items from the list of expressions to display. Specify the numbers of the displays that you want affected with the command argument dnums. It can be a single display number, one of the numbers shown in the first field of the ‘info display’ display; or it could be a range of display numbers, as in 2-4.

undisplay does not repeat if you press RET after using it. (Otherwise you would just get the error ‘No display number …’.)

disable display dnums

Disable the display of item numbers dnums. A disabled display item is not printed automatically, but is not forgotten. It may be enabled again later. Specify the numbers of the displays that you want affected with the command argument dnums. It can be a single display number, one of the numbers shown in the first field of the ‘info display’ display; or it could be a range of display numbers, as in 2-4.

enable display dnums

Enable display of item numbers dnums. It becomes effective once again in auto display of its expression, until you specify otherwise. Specify the numbers of the displays that you want affected with the command argument dnums. It can be a single display number, one of the numbers shown in the first field of the ‘info display’ display; or it could be a range of display numbers, as in 2-4.

display

Display the current values of the expressions on the list, just as is done when your program stops.

info display

Print the list of expressions previously set up to display automatically, each one with its item number, but without showing the values. This includes disabled expressions, which are marked as such. It also includes expressions which would not be displayed right now because they refer to automatic variables not currently available.

If a display expression refers to local variables, then it does not make sense outside the lexical context for which it was set up. Such an expression is disabled when execution enters a context where one of its variables is not defined. For example, if you give the command display last_char while inside a function with an argument last_char, GDB displays this argument while your program continues to stop inside that function. When it stops elsewhere—where there is no variable last_char—the display is disabled automatically. The next time your program stops where last_char is meaningful, you can enable the display expression once again.


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10.8 Print Settings

GDB provides the following ways to control how arrays, structures, and symbols are printed.

These settings are useful for debugging programs in any language:

set print address
set print address on

GDB prints memory addresses showing the location of stack traces, structure values, pointer values, breakpoints, and so forth, even when it also displays the contents of those addresses. The default is on. For example, this is what a stack frame display looks like with set print address on:

(gdb) f
#0  set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
    at input.c:530
530         if (lquote != def_lquote)
set print address off

Do not print addresses when displaying their contents. For example, this is the same stack frame displayed with set print address off:

(gdb) set print addr off
(gdb) f
#0  set_quotes (lq="<<", rq=">>") at input.c:530
530         if (lquote != def_lquote)

You can use ‘set print address off’ to eliminate all machine dependent displays from the GDB interface. For example, with print address off, you should get the same text for backtraces on all machines—whether or not they involve pointer arguments.

show print address

Show whether or not addresses are to be printed.

When GDB prints a symbolic address, it normally prints the closest earlier symbol plus an offset. If that symbol does not uniquely identify the address (for example, it is a name whose scope is a single source file), you may need to clarify. One way to do this is with info line, for example ‘info line *0x4537’. Alternately, you can set GDB to print the source file and line number when it prints a symbolic address:

set print symbol-filename on

Tell GDB to print the source file name and line number of a symbol in the symbolic form of an address.

set print symbol-filename off

Do not print source file name and line number of a symbol. This is the default.

show print symbol-filename

Show whether or not GDB will print the source file name and line number of a symbol in the symbolic form of an address.

Another situation where it is helpful to show symbol filenames and line numbers is when disassembling code; GDB shows you the line number and source file that corresponds to each instruction.

Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol:

set print max-symbolic-offset max-offset
set print max-symbolic-offset unlimited

Tell GDB to only display the symbolic form of an address if the offset between the closest earlier symbol and the address is less than max-offset. The default is unlimited, which tells GDB to always print the symbolic form of an address if any symbol precedes it. Zero is equivalent to unlimited.

show print max-symbolic-offset

Ask how large the maximum offset is that GDB prints in a symbolic address.

If you have a pointer and you are not sure where it points, try ‘set print symbol-filename on’. Then you can determine the name and source file location of the variable where it points, using ‘p/a pointer’. This interprets the address in symbolic form. For example, here GDB shows that a variable ptt points at another variable t, defined in hi2.c:

(gdb) set print symbol-filename on
(gdb) p/a ptt
$4 = 0xe008 <t in hi2.c>

Warning: For pointers that point to a local variable, ‘p/a’ does not show the symbol name and filename of the referent, even with the appropriate set print options turned on.

You can also enable ‘/a’-like formatting all the time using ‘set print symbol on’:

set print symbol on

Tell GDB to print the symbol corresponding to an address, if one exists.

set print symbol off

Tell GDB not to print the symbol corresponding to an address. In this mode, GDB will still print the symbol corresponding to pointers to functions. This is the default.

show print symbol

Show whether GDB will display the symbol corresponding to an address.

Other settings control how different kinds of objects are printed:

set print array
set print array on

Pretty print arrays. This format is more convenient to read, but uses more space. The default is off.

set print array off

Return to compressed format for arrays.

show print array

Show whether compressed or pretty format is selected for displaying arrays.

set print array-indexes
set print array-indexes on

Print the index of each element when displaying arrays. May be more convenient to locate a given element in the array or quickly find the index of a given element in that printed array. The default is off.

set print array-indexes off

Stop printing element indexes when displaying arrays.

show print array-indexes

Show whether the index of each element is printed when displaying arrays.

set print elements number-of-elements
set print elements unlimited

Set a limit on how many elements of an array GDB will print. If GDB is printing a large array, it stops printing after it has printed the number of elements set by the set print elements command. This limit also applies to the display of strings. When GDB starts, this limit is set to 200. Setting number-of-elements to unlimited or zero means that the number of elements to print is unlimited.

show print elements

Display the number of elements of a large array that GDB will print. If the number is 0, then the printing is unlimited.

set print frame-arguments value

This command allows to control how the values of arguments are printed when the debugger prints a frame (see Frames). The possible values are:

all

The values of all arguments are printed.

scalars

Print the value of an argument only if it is a scalar. The value of more complex arguments such as arrays, structures, unions, etc, is replaced by . This is the default. Here is an example where only scalar arguments are shown:

#1  0x08048361 in call_me (i=3, s=…, ss=0xbf8d508c, u=…, e=green)
  at frame-args.c:23
none

None of the argument values are printed. Instead, the value of each argument is replaced by . In this case, the example above now becomes:

#1  0x08048361 in call_me (i=…, s=…, ss=…, u=…, e=…)
  at frame-args.c:23

By default, only scalar arguments are printed. This command can be used to configure the debugger to print the value of all arguments, regardless of their type. However, it is often advantageous to not print the value of more complex parameters. For instance, it reduces the amount of information printed in each frame, making the backtrace more readable. Also, it improves performance when displaying Ada frames, because the computation of large arguments can sometimes be CPU-intensive, especially in large applications. Setting print frame-arguments to scalars (the default) or none avoids this computation, thus speeding up the display of each Ada frame.

show print frame-arguments

Show how the value of arguments should be displayed when printing a frame.

set print raw frame-arguments on

Print frame arguments in raw, non pretty-printed, form.

set print raw frame-arguments off

Print frame arguments in pretty-printed form, if there is a pretty-printer for the value (see Pretty Printing), otherwise print the value in raw form. This is the default.

show print raw frame-arguments

Show whether to print frame arguments in raw form.

set print entry-values value

Set printing of frame argument values at function entry. In some cases GDB can determine the value of function argument which was passed by the function caller, even if the value was modified inside the called function and therefore is different. With optimized code, the current value could be unavailable, but the entry value may still be known.

The default value is default (see below for its description). Older GDB behaved as with the setting no. Compilers not supporting this feature will behave in the default setting the same way as with the no setting.

This functionality is currently supported only by DWARF 2 debugging format and the compiler has to produce ‘DW_TAG_GNU_call_site’ tags. With GCC, you need to specify -O -g during compilation, to get this information.

The value parameter can be one of the following:

no

Print only actual parameter values, never print values from function entry point.

#0  equal (val=5)
#0  different (val=6)
#0  lost (val=<optimized out>)
#0  born (val=10)
#0  invalid (val=<optimized out>)
only

Print only parameter values from function entry point. The actual parameter values are never printed.

#0  equal (val@entry=5)
#0  different (val@entry=5)
#0  lost (val@entry=5)
#0  born (val@entry=<optimized out>)
#0  invalid (val@entry=<optimized out>)
preferred

Print only parameter values from function entry point. If value from function entry point is not known while the actual value is known, print the actual value for such parameter.

#0  equal (val@entry=5)
#0  different (val@entry=5)
#0  lost (val@entry=5)
#0  born (val=10)
#0  invalid (val@entry=<optimized out>)
if-needed

Print actual parameter values. If actual parameter value is not known while value from function entry point is known, print the entry point value for such parameter.

#0  equal (val=5)
#0  different (val=6)
#0  lost (val@entry=5)
#0  born (val=10)
#0  invalid (val=<optimized out>)
both

Always print both the actual parameter value and its value from function entry point, even if values of one or both are not available due to compiler optimizations.

#0  equal (val=5, val@entry=5)
#0  different (val=6, val@entry=5)
#0  lost (val=<optimized out>, val@entry=5)
#0  born (val=10, val@entry=<optimized out>)
#0  invalid (val=<optimized out>, val@entry=<optimized out>)
compact

Print the actual parameter value if it is known and also its value from function entry point if it is known. If neither is known, print for the actual value <optimized out>. If not in MI mode (see GDB/MI) and if both values are known and identical, print the shortened param=param@entry=VALUE notation.

#0  equal (val=val@entry=5)
#0  different (val=6, val@entry=5)
#0  lost (val@entry=5)
#0  born (val=10)
#0  invalid (val=<optimized out>)
default

Always print the actual parameter value. Print also its value from function entry point, but only if it is known. If not in MI mode (see GDB/MI) and if both values are known and identical, print the shortened param=param@entry=VALUE notation.

#0  equal (val=val@entry=5)
#0  different (val=6, val@entry=5)
#0  lost (val=<optimized out>, val@entry=5)
#0  born (val=10)
#0  invalid (val=<optimized out>)

For analysis messages on possible failures of frame argument values at function entry resolution see set debug entry-values.

show print entry-values

Show the method being used for printing of frame argument values at function entry.

set print repeats number-of-repeats
set print repeats unlimited

Set the threshold for suppressing display of repeated array elements. When the number of consecutive identical elements of an array exceeds the threshold, GDB prints the string "<repeats n times>", where n is the number of identical repetitions, instead of displaying the identical elements themselves. Setting the threshold to unlimited or zero will cause all elements to be individually printed. The default threshold is 10.

show print repeats

Display the current threshold for printing repeated identical elements.

set print null-stop

Cause GDB to stop printing the characters of an array when the first NULL is encountered. This is useful when large arrays actually contain only short strings. The default is off.

show print null-stop

Show whether GDB stops printing an array on the first NULL character.

set print pretty on

Cause GDB to print structures in an indented format with one member per line, like this:

$1 = {
  next = 0x0,
  flags = {
    sweet = 1,
    sour = 1
  },
  meat = 0x54 "Pork"
}
set print pretty off

Cause GDB to print structures in a compact format, like this:

$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}

This is the default format.

show print pretty

Show which format GDB is using to print structures.

set print sevenbit-strings on

Print using only seven-bit characters; if this option is set, GDB displays any eight-bit characters (in strings or character values) using the notation \nnn. This setting is best if you are working in English (ASCII) and you use the high-order bit of characters as a marker or “meta” bit.

set print sevenbit-strings off

Print full eight-bit characters. This allows the use of more international character sets, and is the default.

show print sevenbit-strings

Show whether or not GDB is printing only seven-bit characters.

set print union on

Tell GDB to print unions which are contained in structures and other unions. This is the default setting.

set print union off

Tell GDB not to print unions which are contained in structures and other unions. GDB will print "{...}" instead.

show print union

Ask GDB whether or not it will print unions which are contained in structures and other unions.

For example, given the declarations

typedef enum {Tree, Bug} Species;
typedef enum {Big_tree, Acorn, Seedling} Tree_forms;
typedef enum {Caterpillar, Cocoon, Butterfly}
              Bug_forms;

struct thing {
  Species it;
  union {
    Tree_forms tree;
    Bug_forms bug;
  } form;
};

struct thing foo = {Tree, {Acorn}};

with set print union on in effect ‘p foo’ would print

$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}

and with set print union off in effect it would print

$1 = {it = Tree, form = {...}}

set print union affects programs written in C-like languages and in Pascal.

These settings are of interest when debugging C++ programs:

set print demangle
set print demangle on

Print C++ names in their source form rather than in the encoded (“mangled”) form passed to the assembler and linker for type-safe linkage. The default is on.

show print demangle

Show whether C++ names are printed in mangled or demangled form.

set print asm-demangle
set print asm-demangle on

Print C++ names in their source form rather than their mangled form, even in assembler code printouts such as instruction disassemblies. The default is off.

show print asm-demangle

Show whether C++ names in assembly listings are printed in mangled or demangled form.

set demangle-style style

Choose among several encoding schemes used by different compilers to represent C++ names. The choices for style are currently:

auto

Allow GDB to choose a decoding style by inspecting your program. This is the default.

gnu

Decode based on the GNU C++ compiler (g++) encoding algorithm.

hp

Decode based on the HP ANSI C++ (aCC) encoding algorithm.

lucid

Decode based on the Lucid C++ compiler (lcc) encoding algorithm.

arm

Decode using the algorithm in the C++ Annotated Reference Manual. Warning: this setting alone is not sufficient to allow debugging cfront-generated executables. GDB would require further enhancement to permit that.

If you omit style, you will see a list of possible formats.

show demangle-style

Display the encoding style currently in use for decoding C++ symbols.

set print object
set print object on

When displaying a pointer to an object, identify the actual (derived) type of the object rather than the declared type, using the virtual function table. Note that the virtual function table is required—this feature can only work for objects that have run-time type identification; a single virtual method in the object’s declared type is sufficient. Note that this setting is also taken into account when working with variable objects via MI (see GDB/MI).

set print object off

Display only the declared type of objects, without reference to the virtual function table. This is the default setting.

show print object

Show whether actual, or declared, object types are displayed.

set print static-members
set print static-members on

Print static members when displaying a C++ object. The default is on.

set print static-members off

Do not print static members when displaying a C++ object.

show print static-members

Show whether C++ static members are printed or not.

set print pascal_static-members
set print pascal_static-members on

Print static members when displaying a Pascal object. The default is on.

set print pascal_static-members off

Do not print static members when displaying a Pascal object.

show print pascal_static-members

Show whether Pascal static members are printed or not.

set print vtbl
set print vtbl on

Pretty print C++ virtual function tables. The default is off. (The vtbl commands do not work on programs compiled with the HP ANSI C++ compiler (aCC).)

set print vtbl off

Do not pretty print C++ virtual function tables.

show print vtbl

Show whether C++ virtual function tables are pretty printed, or not.


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10.9 Pretty Printing

GDB provides a mechanism to allow pretty-printing of values using Python code. It greatly simplifies the display of complex objects. This mechanism works for both MI and the CLI.


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10.9.1 Pretty-Printer Introduction

When GDB prints a value, it first sees if there is a pretty-printer registered for the value. If there is then GDB invokes the pretty-printer to print the value. Otherwise the value is printed normally.

Pretty-printers are normally named. This makes them easy to manage. The ‘info pretty-printer’ command will list all the installed pretty-printers with their names. If a pretty-printer can handle multiple data types, then its subprinters are the printers for the individual data types. Each such subprinter has its own name. The format of the name is printer-name;subprinter-name.

Pretty-printers are installed by registering them with GDB. Typically they are automatically loaded and registered when the corresponding debug information is loaded, thus making them available without having to do anything special.

There are three places where a pretty-printer can be registered.

See Selecting Pretty-Printers, for further information on how pretty-printers are selected,

See Writing a Pretty-Printer, for implementing pretty printers for new types.


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10.9.2 Pretty-Printer Example

Here is how a C++ std::string looks without a pretty-printer:

(gdb) print s
$1 = {
  static npos = 4294967295, 
  _M_dataplus = {
    <std::allocator<char>> = {
      <__gnu_cxx::new_allocator<char>> = {
        <No data fields>}, <No data fields>
      },
    members of std::basic_string<char, std::char_traits<char>,
      std::allocator<char> >::_Alloc_hider:
    _M_p = 0x804a014 "abcd"
  }
}

With a pretty-printer for std::string only the contents are printed:

(gdb) print s
$2 = "abcd"

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10.9.3 Pretty-Printer Commands

info pretty-printer [object-regexp [name-regexp]]

Print the list of installed pretty-printers. This includes disabled pretty-printers, which are marked as such.

object-regexp is a regular expression matching the objects whose pretty-printers to list. Objects can be global, the program space’s file (see Progspaces In Python), and the object files within that program space (see Objfiles In Python). See Selecting Pretty-Printers, for details on how GDB looks up a printer from these three objects.

name-regexp is a regular expression matching the name of the printers to list.

disable pretty-printer [object-regexp [name-regexp]]

Disable pretty-printers matching object-regexp and name-regexp. A disabled pretty-printer is not forgotten, it may be enabled again later.

enable pretty-printer [object-regexp [name-regexp]]

Enable pretty-printers matching object-regexp and name-regexp.

Example:

Suppose we have three pretty-printers installed: one from library1.so named foo that prints objects of type foo, and another from library2.so named bar that prints two types of objects, bar1 and bar2.

(gdb) info pretty-printer
library1.so:
  foo
library2.so:
  bar
    bar1
    bar2
(gdb) info pretty-printer library2
library2.so:
  bar
    bar1
    bar2
(gdb) disable pretty-printer library1
1 printer disabled
2 of 3 printers enabled
(gdb) info pretty-printer
library1.so:
  foo [disabled]
library2.so:
  bar
    bar1
    bar2
(gdb) disable pretty-printer library2 bar:bar1
1 printer disabled
1 of 3 printers enabled
(gdb) info pretty-printer library2
library1.so:
  foo [disabled]
library2.so:
  bar
    bar1 [disabled]
    bar2
(gdb) disable pretty-printer library2 bar
1 printer disabled
0 of 3 printers enabled
(gdb) info pretty-printer library2
library1.so:
  foo [disabled]
library2.so:
  bar [disabled]
    bar1 [disabled]
    bar2

Note that for bar the entire printer can be disabled, as can each individual subprinter.


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10.10 Value History

Values printed by the print command are saved in the GDB value history. This allows you to refer to them in other expressions. Values are kept until the symbol table is re-read or discarded (for example with the file or symbol-file commands). When the symbol table changes, the value history is discarded, since the values may contain pointers back to the types defined in the symbol table.

The values printed are given history numbers by which you can refer to them. These are successive integers starting with one. print shows you the history number assigned to a value by printing ‘$num = ’ before the value; here num is the history number.

To refer to any previous value, use ‘$’ followed by the value’s history number. The way print labels its output is designed to remind you of this. Just $ refers to the most recent value in the history, and $$ refers to the value before that. $$n refers to the nth value from the end; $$2 is the value just prior to $$, $$1 is equivalent to $$, and $$0 is equivalent to $.

For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure. It suffices to type

p *$

If you have a chain of structures where the component next points to the next one, you can print the contents of the next one with this:

p *$.next

You can print successive links in the chain by repeating this command—which you can do by just typing RET.

Note that the history records values, not expressions. If the value of x is 4 and you type these commands:

print x
set x=5

then the value recorded in the value history by the print command remains 4 even though the value of x has changed.

show values

Print the last ten values in the value history, with their item numbers. This is like ‘p $$9’ repeated ten times, except that show values does not change the history.

show values n

Print ten history values centered on history item number n.

show values +

Print ten history values just after the values last printed. If no more values are available, show values + produces no display.

Pressing RET to repeat show values n has exactly the same effect as ‘show values +’.


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10.11 Convenience Variables

GDB provides convenience variables that you can use within GDB to hold on to a value and refer to it later. These variables exist entirely within GDB; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is why you can use them freely.

Convenience variables are prefixed with ‘$’. Any name preceded by ‘$’ can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see Registers). (Value history references, in contrast, are numbers preceded by ‘$’. See Value History.)

You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example:

set $foo = *object_ptr

would save in $foo the value contained in the object pointed to by object_ptr.

Using a convenience variable for the first time creates it, but its value is void until you assign a new value. You can alter the value with another assignment at any time.

Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value.

show convenience

Print a list of convenience variables used so far, and their values, as well as a list of the convenience functions. Abbreviated show conv.

init-if-undefined $variable = expression

Set a convenience variable if it has not already been set. This is useful for user-defined commands that keep some state. It is similar, in concept, to using local static variables with initializers in C (except that convenience variables are global). It can also be used to allow users to override default values used in a command script.

If the variable is already defined then the expression is not evaluated so any side-effects do not occur.

One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For example, to print a field from successive elements of an array of structures:

set $i = 0
print bar[$i++]->contents

Repeat that command by typing RET.

Some convenience variables are created automatically by GDB and given values likely to be useful.

$_

The variable $_ is automatically set by the x command to the last address examined (see Examining Memory). Other commands which provide a default address for x to examine also set $_ to that address; these commands include info line and info breakpoint. The type of $_ is void * except when set by the x command, in which case it is a pointer to the type of $__.

$__

The variable $__ is automatically set by the x command to the value found in the last address examined. Its type is chosen to match the format in which the data was printed.

$_exitcode

When the program being debugged terminates normally, GDB automatically sets this variable to the exit code of the program, and resets $_exitsignal to void.

$_exitsignal

When the program being debugged dies due to an uncaught signal, GDB automatically sets this variable to that signal’s number, and resets $_exitcode to void.

To distinguish between whether the program being debugged has exited (i.e., $_exitcode is not void) or signalled (i.e., $_exitsignal is not void), the convenience function $_isvoid can be used (see Convenience Functions). For example, considering the following source code:

#include <signal.h>

int
main (int argc, char *argv[])
{
  raise (SIGALRM);
  return 0;
}

A valid way of telling whether the program being debugged has exited or signalled would be:

(gdb) define has_exited_or_signalled
Type commands for definition of ``has_exited_or_signalled''.
End with a line saying just ``end''.
>if $_isvoid ($_exitsignal)
 >echo The program has exited\n
 >else
 >echo The program has signalled\n
 >end
>end
(gdb) run
Starting program:

Program terminated with signal SIGALRM, Alarm clock.
The program no longer exists.
(gdb) has_exited_or_signalled
The program has signalled

As can be seen, GDB correctly informs that the program being debugged has signalled, since it calls raise and raises a SIGALRM signal. If the program being debugged had not called raise, then GDB would report a normal exit:

(gdb) has_exited_or_signalled
The program has exited
$_exception

The variable $_exception is set to the exception object being thrown at an exception-related catchpoint. See Set Catchpoints.

$_probe_argc
$_probe_arg0…$_probe_arg11

Arguments to a static probe. See Static Probe Points.

$_sdata

The variable $_sdata contains extra collected static tracepoint data. See Tracepoint Action Lists. Note that $_sdata could be empty, if not inspecting a trace buffer, or if extra static tracepoint data has not been collected.

$_siginfo

The variable $_siginfo contains extra signal information (see extra signal information). Note that $_siginfo could be empty, if the application has not yet received any signals. For example, it will be empty before you execute the run command.

$_tlb

The variable $_tlb is automatically set when debugging applications running on MS-Windows in native mode or connected to gdbserver that supports the qGetTIBAddr request. See General Query Packets. This variable contains the address of the thread information block.

On HP-UX systems, if you refer to a function or variable name that begins with a dollar sign, GDB searches for a user or system name first, before it searches for a convenience variable.


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10.12 Convenience Functions

GDB also supplies some convenience functions. These have a syntax similar to convenience variables. A convenience function can be used in an expression just like an ordinary function; however, a convenience function is implemented internally to GDB.

These functions do not require GDB to be configured with Python support, which means that they are always available.

$_isvoid (expr)

Return one if the expression expr is void. Otherwise it returns zero.

A void expression is an expression where the type of the result is void. For example, you can examine a convenience variable (see Convenience Variables) to check whether it is void:

(gdb) print $_exitcode
$1 = void
(gdb) print $_isvoid ($_exitcode)
$2 = 1
(gdb) run
Starting program: ./a.out
[Inferior 1 (process 29572) exited normally]
(gdb) print $_exitcode
$3 = 0
(gdb) print $_isvoid ($_exitcode)
$4 = 0

In the example above, we used $_isvoid to check whether $_exitcode is void before and after the execution of the program being debugged. Before the execution there is no exit code to be examined, therefore $_exitcode is void. After the execution the program being debugged returned zero, therefore $_exitcode is zero, which means that it is not void anymore.

The void expression can also be a call of a function from the program being debugged. For example, given the following function:

void
foo (void)
{
}

The result of calling it inside GDB is void:

(gdb) print foo ()
$1 = void
(gdb) print $_isvoid (foo ())
$2 = 1
(gdb) set $v = foo ()
(gdb) print $v
$3 = void
(gdb) print $_isvoid ($v)
$4 = 1

These functions require GDB to be configured with Python support.

$_memeq(buf1, buf2, length)

Returns one if the length bytes at the addresses given by buf1 and buf2 are equal. Otherwise it returns zero.

$_regex(str, regex)

Returns one if the string str matches the regular expression regex. Otherwise it returns zero. The syntax of the regular expression is that specified by Python’s regular expression support.

$_streq(str1, str2)

Returns one if the strings str1 and str2 are equal. Otherwise it returns zero.

$_strlen(str)

Returns the length of string str.

$_caller_is(name[, number_of_frames])

Returns one if the calling function’s name is equal to name. Otherwise it returns zero.

If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.

Example:

(gdb) backtrace
#0  bottom_func ()
    at testsuite/gdb.python/py-caller-is.c:21
#1  0x00000000004005a0 in middle_func ()
    at testsuite/gdb.python/py-caller-is.c:27
#2  0x00000000004005ab in top_func ()
    at testsuite/gdb.python/py-caller-is.c:33
#3  0x00000000004005b6 in main ()
    at testsuite/gdb.python/py-caller-is.c:39
(gdb) print $_caller_is ("middle_func")
$1 = 1
(gdb) print $_caller_is ("top_func", 2)
$1 = 1
$_caller_matches(regexp[, number_of_frames])

Returns one if the calling function’s name matches the regular expression regexp. Otherwise it returns zero.

If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.

$_any_caller_is(name[, number_of_frames])

Returns one if any calling function’s name is equal to name. Otherwise it returns zero.

If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.

This function differs from $_caller_is in that this function checks all stack frames from the immediate caller to the frame specified by number_of_frames, whereas $_caller_is only checks the frame specified by number_of_frames.

$_any_caller_matches(regexp[, number_of_frames])

Returns one if any calling function’s name matches the regular expression regexp. Otherwise it returns zero.

If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.

This function differs from $_caller_matches in that this function checks all stack frames from the immediate caller to the frame specified by number_of_frames, whereas $_caller_matches only checks the frame specified by number_of_frames.

GDB provides the ability to list and get help on convenience functions.

help function

Print a list of all convenience functions.


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10.13 Registers

You can refer to machine register contents, in expressions, as variables with names starting with ‘$’. The names of registers are different for each machine; use info registers to see the names used on your machine.

info registers

Print the names and values of all registers except floating-point and vector registers (in the selected stack frame).

info all-registers

Print the names and values of all registers, including floating-point and vector registers (in the selected stack frame).

info registers regname

Print the relativized value of each specified register regname. As discussed in detail below, register values are normally relative to the selected stack frame. The regname may be any register name valid on the machine you are using, with or without the initial ‘$’.

GDB has four “standard” register names that are available (in expressions) on most machines—whenever they do not conflict with an architecture’s canonical mnemonics for registers. The register names $pc and $sp are used for the program counter register and the stack pointer. $fp is used for a register that contains a pointer to the current stack frame, and $ps is used for a register that contains the processor status. For example, you could print the program counter in hex with

p/x $pc

or print the instruction to be executed next with

x/i $pc

or add four to the stack pointer9 with

set $sp += 4

Whenever possible, these four standard register names are available on your machine even though the machine has different canonical mnemonics, so long as there is no conflict. The info registers command shows the canonical names. For example, on the SPARC, info registers displays the processor status register as $psr but you can also refer to it as $ps; and on x86-based machines $ps is an alias for the EFLAGS register.

GDB always considers the contents of an ordinary register as an integer when the register is examined in this way. Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values. There is no way to refer to the contents of an ordinary register as floating point value (although you can print it as a floating point value with ‘print/f $regname’).

Some registers have distinct “raw” and “virtual” data formats. This means that the data format in which the register contents are saved by the operating system is not the same one that your program normally sees. For example, the registers of the 68881 floating point coprocessor are always saved in “extended” (raw) format, but all C programs expect to work with “double” (virtual) format. In such cases, GDB normally works with the virtual format only (the format that makes sense for your program), but the info registers command prints the data in both formats.

Some machines have special registers whose contents can be interpreted in several different ways. For example, modern x86-based machines have SSE and MMX registers that can hold several values packed together in several different formats. GDB refers to such registers in struct notation:

(gdb) print $xmm1
$1 = {
  v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044},
  v2_double = {9.92129282474342e-303, 2.7585945287983262e-313},
  v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000",
  v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0},
  v4_int32 = {0, 20657912, 11, 13},
  v2_int64 = {88725056443645952, 55834574859},
  uint128 = 0x0000000d0000000b013b36f800000000
}

To set values of such registers, you need to tell GDB which view of the register you wish to change, as if you were assigning value to a struct member:

 (gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF

Normally, register values are relative to the selected stack frame (see Selecting a Frame). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with ‘frame 0’).

Usually ABIs reserve some registers as not needed to be saved by the callee (a.k.a.: “caller-saved”, “call-clobbered” or “volatile” registers). It may therefore not be possible for GDB to know the value a register had before the call (in other words, in the outer frame), if the register value has since been changed by the callee. GDB tries to deduce where the inner frame saved (“callee-saved”) registers, from the debug info, unwind info, or the machine code generated by your compiler. If some register is not saved, and GDB knows the register is “caller-saved” (via its own knowledge of the ABI, or because the debug/unwind info explicitly says the register’s value is undefined), GDB displays ‘<not saved> as the register’s value. With targets that GDB has no knowledge of the register saving convention, if a register was not saved by the callee, then its value and location in the outer frame are assumed to be the same of the inner frame. This is usually harmless, because if the register is call-clobbered, the caller either does not care what is in the register after the call, or has code to restore the value that it does care about. Note, however, that if you change such a register in the outer frame, you may also be affecting the inner frame. Also, the more “outer” the frame is you’re looking at, the more likely a call-clobbered register’s value is to be wrong, in the sense that it doesn’t actually represent the value the register had just before the call.


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10.14 Floating Point Hardware

Depending on the configuration, GDB may be able to give you more information about the status of the floating point hardware.

info float

Display hardware-dependent information about the floating point unit. The exact contents and layout vary depending on the floating point chip. Currently, ‘info float’ is supported on the ARM and x86 machines.


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10.15 Vector Unit

Depending on the configuration, GDB may be able to give you more information about the status of the vector unit.

info vector

Display information about the vector unit. The exact contents and layout vary depending on the hardware.


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10.16 Operating System Auxiliary Information

GDB provides interfaces to useful OS facilities that can help you debug your program.

Some operating systems supply an auxiliary vector to programs at startup. This is akin to the arguments and environment that you specify for a program, but contains a system-dependent variety of binary values that tell system libraries important details about the hardware, operating system, and process. Each value’s purpose is identified by an integer tag; the meanings are well-known but system-specific. Depending on the configuration and operating system facilities, GDB may be able to show you this information. For remote targets, this functionality may further depend on the remote stub’s support of the ‘qXfer:auxv:read’ packet, see qXfer auxiliary vector read.

info auxv

Display the auxiliary vector of the inferior, which can be either a live process or a core dump file. GDB prints each tag value numerically, and also shows names and text descriptions for recognized tags. Some values in the vector are numbers, some bit masks, and some pointers to strings or other data. GDB displays each value in the most appropriate form for a recognized tag, and in hexadecimal for an unrecognized tag.

On some targets, GDB can access operating system-specific information and show it to you. The types of information available will differ depending on the type of operating system running on the target. The mechanism used to fetch the data is described in Operating System Information. For remote targets, this functionality depends on the remote stub’s support of the ‘qXfer:osdata:read’ packet, see qXfer osdata read.

info os infotype

Display OS information of the requested type.

On GNU/Linux, the following values of infotype are valid:

processes

Display the list of processes on the target. For each process, GDB prints the process identifier, the name of the user, the command corresponding to the process, and the list of processor cores that the process is currently running on. (To understand what these properties mean, for this and the following info types, please consult the general GNU/Linux documentation.)

procgroups

Display the list of process groups on the target. For each process, GDB prints the identifier of the process group that it belongs to, the command corresponding to the process group leader, the process identifier, and the command line of the process. The list is sorted first by the process group identifier, then by the process identifier, so that processes belonging to the same process group are grouped together and the process group leader is listed first.

threads

Display the list of threads running on the target. For each thread, GDB prints the identifier of the process that the thread belongs to, the command of the process, the thread identifier, and the processor core that it is currently running on. The main thread of a process is not listed.

files

Display the list of open file descriptors on the target. For each file descriptor, GDB prints the identifier of the process owning the descriptor, the command of the owning process, the value of the descriptor, and the target of the descriptor.

sockets

Display the list of Internet-domain sockets on the target. For each socket, GDB prints the address and port of the local and remote endpoints, the current state of the connection, the creator of the socket, the IP address family of the socket, and the type of the connection.

shm

Display the list of all System V shared-memory regions on the target. For each shared-memory region, GDB prints the region key, the shared-memory identifier, the access permissions, the size of the region, the process that created the region, the process that last attached to or detached from the region, the current number of live attaches to the region, and the times at which the region was last attached to, detach from, and changed.

semaphores

Display the list of all System V semaphore sets on the target. For each semaphore set, GDB prints the semaphore set key, the semaphore set identifier, the access permissions, the number of semaphores in the set, the user and group of the owner and creator of the semaphore set, and the times at which the semaphore set was operated upon and changed.

msg

Display the list of all System V message queues on the target. For each message queue, GDB prints the message queue key, the message queue identifier, the access permissions, the current number of bytes on the queue, the current number of messages on the queue, the processes that last sent and received a message on the queue, the user and group of the owner and creator of the message queue, the times at which a message was last sent and received on the queue, and the time at which the message queue was last changed.

modules

Display the list of all loaded kernel modules on the target. For each module, GDB prints the module name, the size of the module in bytes, the number of times the module is used, the dependencies of the module, the status of the module, and the address of the loaded module in memory.

info os

If infotype is omitted, then list the possible values for infotype and the kind of OS information available for each infotype. If the target does not return a list of possible types, this command will report an error.


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10.17 Memory Region Attributes

Memory region attributes allow you to describe special handling required by regions of your target’s memory. GDB uses attributes to determine whether to allow certain types of memory accesses; whether to use specific width accesses; and whether to cache target memory. By default the description of memory regions is fetched from the target (if the current target supports this), but the user can override the fetched regions.

Defined memory regions can be individually enabled and disabled. When a memory region is disabled, GDB uses the default attributes when accessing memory in that region. Similarly, if no memory regions have been defined, GDB uses the default attributes when accessing all memory.

When a memory region is defined, it is given a number to identify it; to enable, disable, or remove a memory region, you specify that number.

mem lower upper attributes

Define a memory region bounded by lower and upper with attributes attributes…, and add it to the list of regions monitored by GDB. Note that upper == 0 is a special case: it is treated as the target’s maximum memory address. (0xffff on 16 bit targets, 0xffffffff on 32 bit targets, etc.)

mem auto

Discard any user changes to the memory regions and use target-supplied regions, if available, or no regions if the target does not support.

delete mem nums

Remove memory regions nums… from the list of regions monitored by GDB.

disable mem nums

Disable monitoring of memory regions nums…. A disabled memory region is not forgotten. It may be enabled again later.

enable mem nums

Enable monitoring of memory regions nums….

info mem

Print a table of all defined memory regions, with the following columns for each region:

Memory Region Number
Enabled or Disabled.

Enabled memory regions are marked with ‘y’. Disabled memory regions are marked with ‘n’.

Lo Address

The address defining the inclusive lower bound of the memory region.

Hi Address

The address defining the exclusive upper bound of the memory region.

Attributes

The list of attributes set for this memory region.

10.17.1 Attributes

10.17.1.1 Memory Access Mode

The access mode attributes set whether GDB may make read or write accesses to a memory region.

While these attributes prevent GDB from performing invalid memory accesses, they do nothing to prevent the target system, I/O DMA, etc. from accessing memory.

ro

Memory is read only.

wo

Memory is write only.

rw

Memory is read/write. This is the default.

10.17.1.2 Memory Access Size

The access size attribute tells GDB to use specific sized accesses in the memory region. Often memory mapped device registers require specific sized accesses. If no access size attribute is specified, GDB may use accesses of any size.

8

Use 8 bit memory accesses.

16

Use 16 bit memory accesses.

32

Use 32 bit memory accesses.

64

Use 64 bit memory accesses.

10.17.1.3 Data Cache

The data cache attributes set whether GDB will cache target memory. While this generally improves performance by reducing debug protocol overhead, it can lead to incorrect results because GDB does not know about volatile variables or memory mapped device registers.

cache

Enable GDB to cache target memory.

nocache

Disable GDB from caching target memory. This is the default.

10.17.2 Memory Access Checking

GDB can be instructed to refuse accesses to memory that is not explicitly described. This can be useful if accessing such regions has undesired effects for a specific target, or to provide better error checking. The following commands control this behaviour.

set mem inaccessible-by-default [on|off]

If on is specified, make GDB treat memory not explicitly described by the memory ranges as non-existent and refuse accesses to such memory. The checks are only performed if there’s at least one memory range defined. If off is specified, make GDB treat the memory not explicitly described by the memory ranges as RAM. The default value is on.

show mem inaccessible-by-default

Show the current handling of accesses to unknown memory.


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10.18 Copy Between Memory and a File

You can use the commands dump, append, and restore to copy data between target memory and a file. The dump and append commands write data to a file, and the restore command reads data from a file back into the inferior’s memory. Files may be in binary, Motorola S-record, Intel hex, or Tektronix Hex format; however, GDB can only append to binary files.

dump [format] memory filename start_addr end_addr
dump [format] value filename expr

Dump the contents of memory from start_addr to end_addr, or the value of expr, to filename in the given format.

The format parameter may be any one of:

binary

Raw binary form.

ihex

Intel hex format.

srec

Motorola S-record format.

tekhex

Tektronix Hex format.

GDB uses the same definitions of these formats as the GNU binary utilities, like ‘objdump’ and ‘objcopy’. If format is omitted, GDB dumps the data in raw binary form.

append [binary] memory filename start_addr end_addr
append [binary] value filename expr

Append the contents of memory from start_addr to end_addr, or the value of expr, to the file filename, in raw binary form. (GDB can only append data to files in raw binary form.)

restore filename [binary] bias start end

Restore the contents of file filename into memory. The restore command can automatically recognize any known BFD file format, except for raw binary. To restore a raw binary file you must specify the optional keyword binary after the filename.

If bias is non-zero, its value will be added to the addresses contained in the file. Binary files always start at address zero, so they will be restored at address bias. Other bfd files have a built-in location; they will be restored at offset bias from that location.

If start and/or end are non-zero, then only data between file offset start and file offset end will be restored. These offsets are relative to the addresses in the file, before the bias argument is applied.


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10.19 How to Produce a Core File from Your Program

A core file or core dump is a file that records the memory image of a running process and its process status (register values etc.). Its primary use is post-mortem debugging of a program that crashed while it ran outside a debugger. A program that crashes automatically produces a core file, unless this feature is disabled by the user. See Files, for information on invoking GDB in the post-mortem debugging mode.

Occasionally, you may wish to produce a core file of the program you are debugging in order to preserve a snapshot of its state. GDB has a special command for that.

generate-core-file [file]
gcore [file]

Produce a core dump of the inferior process. The optional argument file specifies the file name where to put the core dump. If not specified, the file name defaults to core.pid, where pid is the inferior process ID.

Note that this command is implemented only for some systems (as of this writing, GNU/Linux, FreeBSD, Solaris, and S390).


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10.20 Character Sets

If the program you are debugging uses a different character set to represent characters and strings than the one GDB uses itself, GDB can automatically translate between the character sets for you. The character set GDB uses we call the host character set; the one the inferior program uses we call the target character set.

For example, if you are running GDB on a GNU/Linux system, which uses the ISO Latin 1 character set, but you are using GDB’s remote protocol (see Remote Debugging) to debug a program running on an IBM mainframe, which uses the EBCDIC character set, then the host character set is Latin-1, and the target character set is EBCDIC. If you give GDB the command set target-charset EBCDIC-US, then GDB translates between EBCDIC and Latin 1 as you print character or string values, or use character and string literals in expressions.

GDB has no way to automatically recognize which character set the inferior program uses; you must tell it, using the set target-charset command, described below.

Here are the commands for controlling GDB’s character set support:

set target-charset charset

Set the current target character set to charset. To display the list of supported target character sets, type set target-charset TABTAB.

set host-charset charset

Set the current host character set to charset.

By default, GDB uses a host character set appropriate to the system it is running on; you can override that default using the set host-charset command. On some systems, GDB cannot automatically determine the appropriate host character set. In this case, GDB uses ‘UTF-8’.

GDB can only use certain character sets as its host character set. If you type set host-charset TABTAB, GDB will list the host character sets it supports.

set charset charset

Set the current host and target character sets to charset. As above, if you type set charset TABTAB, GDB will list the names of the character sets that can be used for both host and target.

show charset

Show the names of the current host and target character sets.

show host-charset

Show the name of the current host character set.

show target-charset

Show the name of the current target character set.

set target-wide-charset charset

Set the current target’s wide character set to charset. This is the character set used by the target’s wchar_t type. To display the list of supported wide character sets, type set target-wide-charset TABTAB.

show target-wide-charset

Show the name of the current target’s wide character set.

Here is an example of GDB’s character set support in action. Assume that the following source code has been placed in the file charset-test.c:

#include <stdio.h>

char ascii_hello[]
  = {72, 101, 108, 108, 111, 44, 32, 119,
     111, 114, 108, 100, 33, 10, 0};
char ibm1047_hello[]
  = {200, 133, 147, 147, 150, 107, 64, 166,
     150, 153, 147, 132, 90, 37, 0};

main ()
{
  printf ("Hello, world!\n");
}

In this program, ascii_hello and ibm1047_hello are arrays containing the string ‘Hello, world!’ followed by a newline, encoded in the ASCII and IBM1047 character sets.

We compile the program, and invoke the debugger on it:

$ gcc -g charset-test.c -o charset-test
$ gdb -nw charset-test
GNU gdb 2001-12-19-cvs
Copyright 2001 Free Software Foundation, Inc.
…
(gdb)

We can use the show charset command to see what character sets GDB is currently using to interpret and display characters and strings:

(gdb) show charset
The current host and target character set is `ISO-8859-1'.
(gdb)

For the sake of printing this manual, let’s use ASCII as our initial character set:

(gdb) set charset ASCII
(gdb) show charset
The current host and target character set is `ASCII'.
(gdb)

Let’s assume that ASCII is indeed the correct character set for our host system — in other words, let’s assume that if GDB prints characters using the ASCII character set, our terminal will display them properly. Since our current target character set is also ASCII, the contents of ascii_hello print legibly:

(gdb) print ascii_hello
$1 = 0x401698 "Hello, world!\n"
(gdb) print ascii_hello[0]
$2 = 72 'H'
(gdb)

GDB uses the target character set for character and string literals you use in expressions:

(gdb) print '+'
$3 = 43 '+'
(gdb)

The ASCII character set uses the number 43 to encode the ‘+’ character.

GDB relies on the user to tell it which character set the target program uses. If we print ibm1047_hello while our target character set is still ASCII, we get jibberish:

(gdb) print ibm1047_hello
$4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%"
(gdb) print ibm1047_hello[0]
$5 = 200 '\310'
(gdb)

If we invoke the set target-charset followed by TABTAB, GDB tells us the character sets it supports:

(gdb) set target-charset
ASCII       EBCDIC-US   IBM1047     ISO-8859-1
(gdb) set target-charset

We can select IBM1047 as our target character set, and examine the program’s strings again. Now the ASCII string is wrong, but GDB translates the contents of ibm1047_hello from the target character set, IBM1047, to the host character set, ASCII, and they display correctly:

(gdb) set target-charset IBM1047
(gdb) show charset
The current host character set is `ASCII'.
The current target character set is `IBM1047'.
(gdb) print ascii_hello
$6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012"
(gdb) print ascii_hello[0]
$7 = 72 '\110'
(gdb) print ibm1047_hello
$8 = 0x4016a8 "Hello, world!\n"
(gdb) print ibm1047_hello[0]
$9 = 200 'H'
(gdb)

As above, GDB uses the target character set for character and string literals you use in expressions:

(gdb) print '+'
$10 = 78 '+'
(gdb)

The IBM1047 character set uses the number 78 to encode the ‘+’ character.


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10.21 Caching Data of Targets

GDB caches data exchanged between the debugger and a target. Each cache is associated with the address space of the inferior. See Inferiors and Programs, about inferior and address space. Such caching generally improves performance in remote debugging (see Remote Debugging), because it reduces the overhead of the remote protocol by bundling memory reads and writes into large chunks. Unfortunately, simply caching everything would lead to incorrect results, since GDB does not necessarily know anything about volatile values, memory-mapped I/O addresses, etc. Furthermore, in non-stop mode (see Non-Stop Mode) memory can be changed while a gdb command is executing. Therefore, by default, GDB only caches data known to be on the stack10 or in the code segment. Other regions of memory can be explicitly marked as cacheable; see Memory Region Attributes.

set remotecache on
set remotecache off

This option no longer does anything; it exists for compatibility with old scripts.

show remotecache

Show the current state of the obsolete remotecache flag.

set stack-cache on
set stack-cache off

Enable or disable caching of stack accesses. When on, use caching. By default, this option is on.

show stack-cache

Show the current state of data caching for memory accesses.

set code-cache on
set code-cache off

Enable or disable caching of code segment accesses. When on, use caching. By default, this option is on. This improves performance of disassembly in remote debugging.

show code-cache

Show the current state of target memory cache for code segment accesses.

info dcache [line]

Print the information about the performance of data cache of the current inferior’s address space. The information displayed includes the dcache width and depth, and for each cache line, its number, address, and how many times it was referenced. This command is useful for debugging the data cache operation.

If a line number is specified, the contents of that line will be printed in hex.

set dcache size size

Set maximum number of entries in dcache (dcache depth above).

set dcache line-size line-size

Set number of bytes each dcache entry caches (dcache width above). Must be a power of 2.

show dcache size

Show maximum number of dcache entries. See info dcache.

show dcache line-size

Show default size of dcache lines.


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10.22 Search Memory

Memory can be searched for a particular sequence of bytes with the find command.

find [/sn] start_addr, +len, val1 [, val2, …]
find [/sn] start_addr, end_addr, val1 [, val2, …]

Search memory for the sequence of bytes specified by val1, val2, etc. The search begins at address start_addr and continues for either len bytes or through to end_addr inclusive.

s and n are optional parameters. They may be specified in either order, apart or together.

s, search query size

The size of each search query value.

b

bytes

h

halfwords (two bytes)

w

words (four bytes)

g

giant words (eight bytes)

All values are interpreted in the current language. This means, for example, that if the current source language is C/C++ then searching for the string “hello” includes the trailing ’\0’.

If the value size is not specified, it is taken from the value’s type in the current language. This is useful when one wants to specify the search pattern as a mixture of types. Note that this means, for example, that in the case of C-like languages a search for an untyped 0x42 will search for ‘(int) 0x42’ which is typically four bytes.

n, maximum number of finds

The maximum number of matches to print. The default is to print all finds.

You can use strings as search values. Quote them with double-quotes ("). The string value is copied into the search pattern byte by byte, regardless of the endianness of the target and the size specification.

The address of each match found is printed as well as a count of the number of matches found.

The address of the last value found is stored in convenience variable ‘$_’. A count of the number of matches is stored in ‘$numfound’.

For example, if stopped at the printf in this function:

void
hello ()
{
  static char hello[] = "hello-hello";
  static struct { char c; short s; int i; }
    __attribute__ ((packed)) mixed
    = { 'c', 0x1234, 0x87654321 };
  printf ("%s\n", hello);
}

you get during debugging:

(gdb) find &hello[0], +sizeof(hello), "hello"
0x804956d <hello.1620+6>
1 pattern found
(gdb) find &hello[0], +sizeof(hello), 'h', 'e', 'l', 'l', 'o'
0x8049567 <hello.1620>
0x804956d <hello.1620+6>
2 patterns found
(gdb) find /b1 &hello[0], +sizeof(hello), 'h', 0x65, 'l'
0x8049567 <hello.1620>
1 pattern found
(gdb) find &mixed, +sizeof(mixed), (char) 'c', (short) 0x1234, (int) 0x87654321
0x8049560 <mixed.1625>
1 pattern found
(gdb) print $numfound
$1 = 1
(gdb) print $_
$2 = (void *) 0x8049560

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11 Debugging Optimized Code

Almost all compilers support optimization. With optimization disabled, the compiler generates assembly code that corresponds directly to your source code, in a simplistic way. As the compiler applies more powerful optimizations, the generated assembly code diverges from your original source code. With help from debugging information generated by the compiler, GDB can map from the running program back to constructs from your original source.

GDB is more accurate with optimization disabled. If you can recompile without optimization, it is easier to follow the progress of your program during debugging. But, there are many cases where you may need to debug an optimized version.

When you debug a program compiled with ‘-g -O’, remember that the optimizer has rearranged your code; the debugger shows you what is really there. Do not be too surprised when the execution path does not exactly match your source file! An extreme example: if you define a variable, but never use it, GDB never sees that variable—because the compiler optimizes it out of existence.

Some things do not work as well with ‘-g -O’ as with just ‘-g’, particularly on machines with instruction scheduling. If in doubt, recompile with ‘-g’ alone, and if this fixes the problem, please report it to us as a bug (including a test case!). See Variables, for more information about debugging optimized code.


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11.1 Inline Functions

Inlining is an optimization that inserts a copy of the function body directly at each call site, instead of jumping to a shared routine. GDB displays inlined functions just like non-inlined functions. They appear in backtraces. You can view their arguments and local variables, step into them with step, skip them with next, and escape from them with finish. You can check whether a function was inlined by using the info frame command.

For GDB to support inlined functions, the compiler must record information about inlining in the debug information — GCC using the DWARF 2 format does this, and several other compilers do also. GDB only supports inlined functions when using DWARF 2. Versions of GCC before 4.1 do not emit two required attributes (‘DW_AT_call_file’ and ‘DW_AT_call_line’); GDB does not display inlined function calls with earlier versions of GCC. It instead displays the arguments and local variables of inlined functions as local variables in the caller.

The body of an inlined function is directly included at its call site; unlike a non-inlined function, there are no instructions devoted to the call. GDB still pretends that the call site and the start of the inlined function are different instructions. Stepping to the call site shows the call site, and then stepping again shows the first line of the inlined function, even though no additional instructions are executed.

This makes source-level debugging much clearer; you can see both the context of the call and then the effect of the call. Only stepping by a single instruction using stepi or nexti does not do this; single instruction steps always show the inlined body.

There are some ways that GDB does not pretend that inlined function calls are the same as normal calls:


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11.2 Tail Call Frames

Function B can call function C in its very last statement. In unoptimized compilation the call of C is immediately followed by return instruction at the end of B code. Optimizing compiler may replace the call and return in function B into one jump to function C instead. Such use of a jump instruction is called tail call.

During execution of function C, there will be no indication in the function call stack frames that it was tail-called from B. If function A regularly calls function B which tail-calls function C, then GDB will see A as the caller of C. However, in some cases GDB can determine that C was tail-called from B, and it will then create fictitious call frame for that, with the return address set up as if B called C normally.

This functionality is currently supported only by DWARF 2 debugging format and the compiler has to produce ‘DW_TAG_GNU_call_site’ tags. With GCC, you need to specify -O -g during compilation, to get this information.

info frame command (see Frame Info) will indicate the tail call frame kind by text tail call frame such as in this sample GDB output:

(gdb) x/i $pc - 2
   0x40066b <b(int, double)+11>: jmp 0x400640 <c(int, double)>
(gdb) info frame
Stack level 1, frame at 0x7fffffffda30:
 rip = 0x40066d in b (amd64-entry-value.cc:59); saved rip 0x4004c5
 tail call frame, caller of frame at 0x7fffffffda30
 source language c++.
 Arglist at unknown address.
 Locals at unknown address, Previous frame's sp is 0x7fffffffda30

The detection of all the possible code path executions can find them ambiguous. There is no execution history stored (possible Reverse Execution is never used for this purpose) and the last known caller could have reached the known callee by multiple different jump sequences. In such case GDB still tries to show at least all the unambiguous top tail callers and all the unambiguous bottom tail calees, if any.

set debug entry-values

When set to on, enables printing of analysis messages for both frame argument values at function entry and tail calls. It will show all the possible valid tail calls code paths it has considered. It will also print the intersection of them with the final unambiguous (possibly partial or even empty) code path result.

show debug entry-values

Show the current state of analysis messages printing for both frame argument values at function entry and tail calls.

The analysis messages for tail calls can for example show why the virtual tail call frame for function c has not been recognized (due to the indirect reference by variable x):

static void __attribute__((noinline, noclone)) c (void);
void (*x) (void) = c;
static void __attribute__((noinline, noclone)) a (void) { x++; }
static void __attribute__((noinline, noclone)) c (void) { a (); }
int main (void) { x (); return 0; }

Breakpoint 1, DW_OP_GNU_entry_value resolving cannot find
DW_TAG_GNU_call_site 0x40039a in main
a () at t.c:3
3	static void __attribute__((noinline, noclone)) a (void) { x++; }
(gdb) bt
#0  a () at t.c:3
#1  0x000000000040039a in main () at t.c:5

Another possibility is an ambiguous virtual tail call frames resolution:

int i;
static void __attribute__((noinline, noclone)) f (void) { i++; }
static void __attribute__((noinline, noclone)) e (void) { f (); }
static void __attribute__((noinline, noclone)) d (void) { f (); }
static void __attribute__((noinline, noclone)) c (void) { d (); }
static void __attribute__((noinline, noclone)) b (void)
{ if (i) c (); else e (); }
static void __attribute__((noinline, noclone)) a (void) { b (); }
int main (void) { a (); return 0; }

tailcall: initial: 0x4004d2(a) 0x4004ce(b) 0x4004b2(c) 0x4004a2(d)
tailcall: compare: 0x4004d2(a) 0x4004cc(b) 0x400492(e)
tailcall: reduced: 0x4004d2(a) |
(gdb) bt
#0  f () at t.c:2
#1  0x00000000004004d2 in a () at t.c:8
#2  0x0000000000400395 in main () at t.c:9

Frames #0 and #2 are real, #1 is a virtual tail call frame. The code can have possible execution paths main→a→b→c→d→f or main→a→b→e→f, GDB cannot find which one from the inferior state.

initial: state shows some random possible calling sequence GDB has found. It then finds another possible calling sequcen - that one is prefixed by compare:. The non-ambiguous intersection of these two is printed as the reduced: calling sequence. That one could have many futher compare: and reduced: statements as long as there remain any non-ambiguous sequence entries.

For the frame of function b in both cases there are different possible $pc values (0x4004cc or 0x4004ce), therefore this frame is also ambigous. The only non-ambiguous frame is the one for function a, therefore this one is displayed to the user while the ambiguous frames are omitted.

There can be also reasons why printing of frame argument values at function entry may fail:

int v;
static void __attribute__((noinline, noclone)) c (int i) { v++; }
static void __attribute__((noinline, noclone)) a (int i);
static void __attribute__((noinline, noclone)) b (int i) { a (i); }
static void __attribute__((noinline, noclone)) a (int i)
{ if (i) b (i - 1); else c (0); }
int main (void) { a (5); return 0; }

(gdb) bt
#0  c (i=i@entry=0) at t.c:2
#1  0x0000000000400428 in a (DW_OP_GNU_entry_value resolving has found
function "a" at 0x400420 can call itself via tail calls
i=<optimized out>) at t.c:6
#2  0x000000000040036e in main () at t.c:7

GDB cannot find out from the inferior state if and how many times did function a call itself (via function b) as these calls would be tail calls. Such tail calls would modify thue i variable, therefore GDB cannot be sure the value it knows would be right - GDB prints <optimized out> instead.


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12 C Preprocessor Macros

Some languages, such as C and C++, provide a way to define and invoke “preprocessor macros” which expand into strings of tokens. GDB can evaluate expressions containing macro invocations, show the result of macro expansion, and show a macro’s definition, including where it was defined.

You may need to compile your program specially to provide GDB with information about preprocessor macros. Most compilers do not include macros in their debugging information, even when you compile with the -g flag. See Compilation.

A program may define a macro at one point, remove that definition later, and then provide a different definition after that. Thus, at different points in the program, a macro may have different definitions, or have no definition at all. If there is a current stack frame, GDB uses the macros in scope at that frame’s source code line. Otherwise, GDB uses the macros in scope at the current listing location; see List.

Whenever GDB evaluates an expression, it always expands any macro invocations present in the expression. GDB also provides the following commands for working with macros explicitly.

macro expand expression
macro exp expression

Show the results of expanding all preprocessor macro invocations in expression. Since GDB simply expands macros, but does not parse the result, expression need not be a valid expression; it can be any string of tokens.

macro expand-once expression
macro exp1 expression

(This command is not yet implemented.) Show the results of expanding those preprocessor macro invocations that appear explicitly in expression. Macro invocations appearing in that expansion are left unchanged. This command allows you to see the effect of a particular macro more clearly, without being confused by further expansions. Since GDB simply expands macros, but does not parse the result, expression need not be a valid expression; it can be any string of tokens.

info macro [-a|-all] [--] macro

Show the current definition or all definitions of the named macro, and describe the source location or compiler command-line where that definition was established. The optional double dash is to signify the end of argument processing and the beginning of macro for non C-like macros where the macro may begin with a hyphen.

info macros linespec

Show all macro definitions that are in effect at the location specified by linespec, and describe the source location or compiler command-line where those definitions were established.

macro define macro replacement-list
macro define macro(arglist) replacement-list

Introduce a definition for a preprocessor macro named macro, invocations of which are replaced by the tokens given in replacement-list. The first form of this command defines an “object-like” macro, which takes no arguments; the second form defines a “function-like” macro, which takes the arguments given in arglist.

A definition introduced by this command is in scope in every expression evaluated in GDB, until it is removed with the macro undef command, described below. The definition overrides all definitions for macro present in the program being debugged, as well as any previous user-supplied definition.

macro undef macro

Remove any user-supplied definition for the macro named macro. This command only affects definitions provided with the macro define command, described above; it cannot remove definitions present in the program being debugged.

macro list

List all the macros defined using the macro define command.

Here is a transcript showing the above commands in action. First, we show our source files:

$ cat sample.c
#include <stdio.h>
#include "sample.h"

#define M 42
#define ADD(x) (M + x)

main ()
{
#define N 28
  printf ("Hello, world!\n");
#undef N
  printf ("We're so creative.\n");
#define N 1729
  printf ("Goodbye, world!\n");
}
$ cat sample.h
#define Q <
$

Now, we compile the program using the GNU C compiler, GCC. We pass the -gdwarf-211 and -g3 flags to ensure the compiler includes information about preprocessor macros in the debugging information.

$ gcc -gdwarf-2 -g3 sample.c -o sample
$

Now, we start GDB on our sample program:

$ gdb -nw sample
GNU gdb 2002-05-06-cvs
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, …
(gdb)

We can expand macros and examine their definitions, even when the program is not running. GDB uses the current listing position to decide which macro definitions are in scope:

(gdb) list main
3
4       #define M 42
5       #define ADD(x) (M + x)
6
7       main ()
8       {
9       #define N 28
10        printf ("Hello, world!\n");
11      #undef N
12        printf ("We're so creative.\n");
(gdb) info macro ADD
Defined at /home/jimb/gdb/macros/play/sample.c:5
#define ADD(x) (M + x)
(gdb) info macro Q
Defined at /home/jimb/gdb/macros/play/sample.h:1
  included at /home/jimb/gdb/macros/play/sample.c:2
#define Q <
(gdb) macro expand ADD(1)
expands to: (42 + 1)
(gdb) macro expand-once ADD(1)
expands to: once (M + 1)
(gdb)

In the example above, note that macro expand-once expands only the macro invocation explicit in the original text — the invocation of ADD — but does not expand the invocation of the macro M, which was introduced by ADD.

Once the program is running, GDB uses the macro definitions in force at the source line of the current stack frame:

(gdb) break main
Breakpoint 1 at 0x8048370: file sample.c, line 10.
(gdb) run
Starting program: /home/jimb/gdb/macros/play/sample

Breakpoint 1, main () at sample.c:10
10        printf ("Hello, world!\n");
(gdb)

At line 10, the definition of the macro N at line 9 is in force:

(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:9
#define N 28
(gdb) macro expand N Q M
expands to: 28 < 42
(gdb) print N Q M
$1 = 1
(gdb)

As we step over directives that remove N’s definition, and then give it a new definition, GDB finds the definition (or lack thereof) in force at each point:

(gdb) next
Hello, world!
12        printf ("We're so creative.\n");
(gdb) info macro N
The symbol `N' has no definition as a C/C++ preprocessor macro
at /home/jimb/gdb/macros/play/sample.c:12
(gdb) next
We're so creative.
14        printf ("Goodbye, world!\n");
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:13
#define N 1729
(gdb) macro expand N Q M
expands to: 1729 < 42
(gdb) print N Q M
$2 = 0
(gdb)

In addition to source files, macros can be defined on the compilation command line using the -Dname=value syntax. For macros defined in such a way, GDB displays the location of their definition as line zero of the source file submitted to the compiler.

(gdb) info macro __STDC__
Defined at /home/jimb/gdb/macros/play/sample.c:0
-D__STDC__=1
(gdb)

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13 Tracepoints

In some applications, it is not feasible for the debugger to interrupt the program’s execution long enough for the developer to learn anything helpful about its behavior. If the program’s correctness depends on its real-time behavior, delays introduced by a debugger might cause the program to change its behavior drastically, or perhaps fail, even when the code itself is correct. It is useful to be able to observe the program’s behavior without interrupting it.

Using GDB’s trace and collect commands, you can specify locations in the program, called tracepoints, and arbitrary expressions to evaluate when those tracepoints are reached. Later, using the tfind command, you can examine the values those expressions had when the program hit the tracepoints. The expressions may also denote objects in memory—structures or arrays, for example—whose values GDB should record; while visiting a particular tracepoint, you may inspect those objects as if they were in memory at that moment. However, because GDB records these values without interacting with you, it can do so quickly and unobtrusively, hopefully not disturbing the program’s behavior.

The tracepoint facility is currently available only for remote targets. See Targets. In addition, your remote target must know how to collect trace data. This functionality is implemented in the remote stub; however, none of the stubs distributed with GDB support tracepoints as of this writing. The format of the remote packets used to implement tracepoints are described in Tracepoint Packets.

It is also possible to get trace data from a file, in a manner reminiscent of corefiles; you specify the filename, and use tfind to search through the file. See Trace Files, for more details.

This chapter describes the tracepoint commands and features.


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13.1 Commands to Set Tracepoints

Before running such a trace experiment, an arbitrary number of tracepoints can be set. A tracepoint is actually a special type of breakpoint (see Set Breaks), so you can manipulate it using standard breakpoint commands. For instance, as with breakpoints, tracepoint numbers are successive integers starting from one, and many of the commands associated with tracepoints take the tracepoint number as their argument, to identify which tracepoint to work on.

For each tracepoint, you can specify, in advance, some arbitrary set of data that you want the target to collect in the trace buffer when it hits that tracepoint. The collected data can include registers, local variables, or global data. Later, you can use GDB commands to examine the values these data had at the time the tracepoint was hit.

Tracepoints do not support every breakpoint feature. Ignore counts on tracepoints have no effect, and tracepoints cannot run GDB commands when they are hit. Tracepoints may not be thread-specific either.

Some targets may support fast tracepoints, which are inserted in a different way (such as with a jump instead of a trap), that is faster but possibly restricted in where they may be installed.

Regular and fast tracepoints are dynamic tracing facilities, meaning that they can be used to insert tracepoints at (almost) any location in the target. Some targets may also support controlling static tracepoints from GDB. With static tracing, a set of instrumentation points, also known as markers, are embedded in the target program, and can be activated or deactivated by name or address. These are usually placed at locations which facilitate investigating what the target is actually doing. GDB’s support for static tracing includes being able to list instrumentation points, and attach them with GDB defined high level tracepoints that expose the whole range of convenience of GDB’s tracepoints support. Namely, support for collecting registers values and values of global or local (to the instrumentation point) variables; tracepoint conditions and trace state variables. The act of installing a GDB static tracepoint on an instrumentation point, or marker, is referred to as probing a static tracepoint marker.

gdbserver supports tracepoints on some target systems. See Tracepoints support in gdbserver.

This section describes commands to set tracepoints and associated conditions and actions.


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13.1.1 Create and Delete Tracepoints

trace location

The trace command is very similar to the break command. Its argument location can be a source line, a function name, or an address in the target program. See Specify Location. The trace command defines a tracepoint, which is a point in the target program where the debugger will briefly stop, collect some data, and then allow the program to continue. Setting a tracepoint or changing its actions takes effect immediately if the remote stub supports the ‘InstallInTrace’ feature (see install tracepoint in tracing). If remote stub doesn’t support the ‘InstallInTrace’ feature, all these changes don’t take effect until the next tstart command, and once a trace experiment is running, further changes will not have any effect until the next trace experiment starts. In addition, GDB supports pending tracepoints—tracepoints whose address is not yet resolved. (This is similar to pending breakpoints.) Pending tracepoints are not downloaded to the target and not installed until they are resolved. The resolution of pending tracepoints requires GDB support—when debugging with the remote target, and GDB disconnects from the remote stub (see disconnected tracing), pending tracepoints can not be resolved (and downloaded to the remote stub) while GDB is disconnected.

Here are some examples of using the trace command:

(gdb) trace foo.c:121    // a source file and line number

(gdb) trace +2           // 2 lines forward

(gdb) trace my_function  // first source line of function

(gdb) trace *my_function // EXACT start address of function

(gdb) trace *0x2117c4    // an address

You can abbreviate trace as tr.

trace location if cond

Set a tracepoint with condition cond; evaluate the expression cond each time the tracepoint is reached, and collect data only if the value is nonzero—that is, if cond evaluates as true. See Tracepoint Conditions, for more information on tracepoint conditions.

ftrace location [ if cond ]

The ftrace command sets a fast tracepoint. For targets that support them, fast tracepoints will use a more efficient but possibly less general technique to trigger data collection, such as a jump instruction instead of a trap, or some sort of hardware support. It may not be possible to create a fast tracepoint at the desired location, in which case the command will exit with an explanatory message.

GDB handles arguments to ftrace exactly as for trace.

On 32-bit x86-architecture systems, fast tracepoints normally need to be placed at an instruction that is 5 bytes or longer, but can be placed at 4-byte instructions if the low 64K of memory of the target program is available to install trampolines. Some Unix-type systems, such as GNU/Linux, exclude low addresses from the program’s address space; but for instance with the Linux kernel it is possible to let GDB use this area by doing a sysctl command to set the mmap_min_addr kernel parameter, as in

sudo sysctl -w vm.mmap_min_addr=32768

which sets the low address to 32K, which leaves plenty of room for trampolines. The minimum address should be set to a page boundary.

strace location [ if cond ]

The strace command sets a static tracepoint. For targets that support it, setting a static tracepoint probes a static instrumentation point, or marker, found at location. It may not be possible to set a static tracepoint at the desired location, in which case the command will exit with an explanatory message.

GDB handles arguments to strace exactly as for trace, with the addition that the user can also specify -m marker as location. This probes the marker identified by the marker string identifier. This identifier depends on the static tracepoint backend library your program is using. You can find all the marker identifiers in the ‘ID’ field of the info static-tracepoint-markers command output. See Listing Static Tracepoint Markers. For example, in the following small program using the UST tracing engine:

main ()
{
  trace_mark(ust, bar33, "str %s", "FOOBAZ");
}

the marker id is composed of joining the first two arguments to the trace_mark call with a slash, which translates to:

(gdb) info static-tracepoint-markers
Cnt Enb ID         Address            What
1   n   ust/bar33  0x0000000000400ddc in main at stexample.c:22
         Data: "str %s"
[etc...]

so you may probe the marker above with:

(gdb) strace -m ust/bar33

Static tracepoints accept an extra collect action — collect $_sdata. This collects arbitrary user data passed in the probe point call to the tracing library. In the UST example above, you’ll see that the third argument to trace_mark is a printf-like format string. The user data is then the result of running that formating string against the following arguments. Note that info static-tracepoint-markers command output lists that format string in the ‘Data:’ field.

You can inspect this data when analyzing the trace buffer, by printing the $_sdata variable like any other variable available to GDB. See Tracepoint Action Lists.

The convenience variable $tpnum records the tracepoint number of the most recently set tracepoint.

delete tracepoint [num]

Permanently delete one or more tracepoints. With no argument, the default is to delete all tracepoints. Note that the regular delete command can remove tracepoints also.

Examples:

(gdb) delete trace 1 2 3 // remove three tracepoints

(gdb) delete trace       // remove all tracepoints

You can abbreviate this command as del tr.


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13.1.2 Enable and Disable Tracepoints

These commands are deprecated; they are equivalent to plain disable and enable.

disable tracepoint [num]

Disable tracepoint num, or all tracepoints if no argument num is given. A disabled tracepoint will have no effect during a trace experiment, but it is not forgotten. You can re-enable a disabled tracepoint using the enable tracepoint command. If the command is issued during a trace experiment and the debug target has support for disabling tracepoints during a trace experiment, then the change will be effective immediately. Otherwise, it will be applied to the next trace experiment.

enable tracepoint [num]

Enable tracepoint num, or all tracepoints. If this command is issued during a trace experiment and the debug target supports enabling tracepoints during a trace experiment, then the enabled tracepoints will become effective immediately. Otherwise, they will become effective the next time a trace experiment is run.


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13.1.3 Tracepoint Passcounts

passcount [n [num]]

Set the passcount of a tracepoint. The passcount is a way to automatically stop a trace experiment. If a tracepoint’s passcount is n, then the trace experiment will be automatically stopped on the n’th time that tracepoint is hit. If the tracepoint number num is not specified, the passcount command sets the passcount of the most recently defined tracepoint. If no passcount is given, the trace experiment will run until stopped explicitly by the user.

Examples:

(gdb) passcount 5 2 // Stop on the 5th execution of
                                   // tracepoint 2
(gdb) passcount 12  // Stop on the 12th execution of the
                                   // most recently defined tracepoint.
(gdb) trace foo
(gdb) pass 3
(gdb) trace bar
(gdb) pass 2
(gdb) trace baz
(gdb) pass 1        // Stop tracing when foo has been
                                    // executed 3 times OR when bar has
                                    // been executed 2 times
                                    // OR when baz has been executed 1 time.

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13.1.4 Tracepoint Conditions

The simplest sort of tracepoint collects data every time your program reaches a specified place. You can also specify a condition for a tracepoint. A condition is just a Boolean expression in your programming language (see Expressions). A tracepoint with a condition evaluates the expression each time your program reaches it, and data collection happens only if the condition is true.

Tracepoint conditions can be specified when a tracepoint is set, by using ‘if’ in the arguments to the trace command. See Setting Tracepoints. They can also be set or changed at any time with the condition command, just as with breakpoints.

Unlike breakpoint conditions, GDB does not actually evaluate the conditional expression itself. Instead, GDB encodes the expression into an agent expression (see Agent Expressions) suitable for execution on the target, independently of GDB. Global variables become raw memory locations, locals become stack accesses, and so forth.

For instance, suppose you have a function that is usually called frequently, but should not be called after an error has occurred. You could use the following tracepoint command to collect data about calls of that function that happen while the error code is propagating through the program; an unconditional tracepoint could end up collecting thousands of useless trace frames that you would have to search through.

(gdb) trace normal_operation if errcode > 0

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13.1.5 Trace State Variables

A trace state variable is a special type of variable that is created and managed by target-side code. The syntax is the same as that for GDB’s convenience variables (a string prefixed with “$”), but they are stored on the target. They must be created explicitly, using a tvariable command. They are always 64-bit signed integers.

Trace state variables are remembered by GDB, and downloaded to the target along with tracepoint information when the trace experiment starts. There are no intrinsic limits on the number of trace state variables, beyond memory limitations of the target.

Although trace state variables are managed by the target, you can use them in print commands and expressions as if they were convenience variables; GDB will get the current value from the target while the trace experiment is running. Trace state variables share the same namespace as other “$” variables, which means that you cannot have trace state variables with names like $23 or $pc, nor can you have a trace state variable and a convenience variable with the same name.

tvariable $name [ = expression ]

The tvariable command creates a new trace state variable named $name, and optionally gives it an initial value of expression. The expression is evaluated when this command is entered; the result will be converted to an integer if possible, otherwise GDB will report an error. A subsequent tvariable command specifying the same name does not create a variable, but instead assigns the supplied initial value to the existing variable of that name, overwriting any previous initial value. The default initial value is 0.

info tvariables

List all the trace state variables along with their initial values. Their current values may also be displayed, if the trace experiment is currently running.

delete tvariable [ $name]

Delete the given trace state variables, or all of them if no arguments are specified.


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13.1.6 Tracepoint Action Lists

actions [num]

This command will prompt for a list of actions to be taken when the tracepoint is hit. If the tracepoint number num is not specified, this command sets the actions for the one that was most recently defined (so that you can define a tracepoint and then say actions without bothering about its number). You specify the actions themselves on the following lines, one action at a time, and terminate the actions list with a line containing just end. So far, the only defined actions are collect, teval, and while-stepping.

actions is actually equivalent to commands (see Breakpoint Command Lists), except that only the defined actions are allowed; any other GDB command is rejected.

To remove all actions from a tracepoint, type ‘actions num’ and follow it immediately with ‘end’.

(gdb) collect data // collect some data

(gdb) while-stepping 5 // single-step 5 times, collect data

(gdb) end              // signals the end of actions.

In the following example, the action list begins with collect commands indicating the things to be collected when the tracepoint is hit. Then, in order to single-step and collect additional data following the tracepoint, a while-stepping command is used, followed by the list of things to be collected after each step in a sequence of single steps. The while-stepping command is terminated by its own separate end command. Lastly, the action list is terminated by an end command.

(gdb) trace foo
(gdb) actions
Enter actions for tracepoint 1, one per line:
> collect bar,baz
> collect $regs
> while-stepping 12
  > collect $pc, arr[i]
  > end
end
collect[/mods] expr1, expr2, …

Collect values of the given expressions when the tracepoint is hit. This command accepts a comma-separated list of any valid expressions. In addition to global, static, or local variables, the following special arguments are supported:

$regs

Collect all registers.

$args

Collect all function arguments.

$locals

Collect all local variables.

$_ret

Collect the return address. This is helpful if you want to see more of a backtrace.

$_probe_argc

Collects the number of arguments from the static probe at which the tracepoint is located. See Static Probe Points.

$_probe_argn

n is an integer between 0 and 11. Collects the nth argument from the static probe at which the tracepoint is located. See Static Probe Points.

$_sdata

Collect static tracepoint marker specific data. Only available for static tracepoints. See Tracepoint Action Lists. On the UST static tracepoints library backend, an instrumentation point resembles a printf function call. The tracing library is able to collect user specified data formatted to a character string using the format provided by the programmer that instrumented the program. Other backends have similar mechanisms. Here’s an example of a UST marker call:

 const char master_name[] = "$your_name";
 trace_mark(channel1, marker1, "hello %s", master_name)

In this case, collecting $_sdata collects the string ‘hello $yourname’. When analyzing the trace buffer, you can inspect ‘$_sdata’ like any other variable available to GDB.

You can give several consecutive collect commands, each one with a single argument, or one collect command with several arguments separated by commas; the effect is the same.

The optional mods changes the usual handling of the arguments. s requests that pointers to chars be handled as strings, in particular collecting the contents of the memory being pointed at, up to the first zero. The upper bound is by default the value of the print elements variable; if s is followed by a decimal number, that is the upper bound instead. So for instance ‘collect/s25 mystr’ collects as many as 25 characters at ‘mystr’.

The command info scope (see info scope) is particularly useful for figuring out what data to collect.

teval expr1, expr2, …

Evaluate the given expressions when the tracepoint is hit. This command accepts a comma-separated list of expressions. The results are discarded, so this is mainly useful for assigning values to trace state variables (see Trace State Variables) without adding those values to the trace buffer, as would be the case if the collect action were used.

while-stepping n

Perform n single-step instruction traces after the tracepoint, collecting new data after each step. The while-stepping command is followed by the list of what to collect while stepping (followed by its own end command):

> while-stepping 12
  > collect $regs, myglobal
  > end
>

Note that $pc is not automatically collected by while-stepping; you need to explicitly collect that register if you need it. You may abbreviate while-stepping as ws or stepping.

set default-collect expr1, expr2, …

This variable is a list of expressions to collect at each tracepoint hit. It is effectively an additional collect action prepended to every tracepoint action list. The expressions are parsed individually for each tracepoint, so for instance a variable named xyz may be interpreted as a global for one tracepoint, and a local for another, as appropriate to the tracepoint’s location.

show default-collect

Show the list of expressions that are collected by default at each tracepoint hit.


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13.1.7 Listing Tracepoints

info tracepoints [num]

Display information about the tracepoint num. If you don’t specify a tracepoint number, displays information about all the tracepoints defined so far. The format is similar to that used for info breakpoints; in fact, info tracepoints is the same command, simply restricting itself to tracepoints.

A tracepoint’s listing may include additional information specific to tracing:

(gdb) info trace
Num     Type           Disp Enb Address    What
1       tracepoint     keep y   0x0804ab57 in foo() at main.cxx:7
        while-stepping 20
          collect globfoo, $regs
        end
        collect globfoo2
        end
        pass count 1200 
2       tracepoint     keep y   <MULTIPLE>
        collect $eip
2.1                         y     0x0804859c in func4 at change-loc.h:35
        installed on target
2.2                         y     0xb7ffc480 in func4 at change-loc.h:35
        installed on target
2.3                         y     <PENDING>  set_tracepoint
3       tracepoint     keep y   0x080485b1 in foo at change-loc.c:29
        not installed on target
(gdb)

This command can be abbreviated info tp.


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13.1.8 Listing Static Tracepoint Markers

info static-tracepoint-markers

Display information about all static tracepoint markers defined in the program.

For each marker, the following columns are printed:

Count

An incrementing counter, output to help readability. This is not a stable identifier.

ID

The marker ID, as reported by the target.

Enabled or Disabled

Probed markers are tagged with ‘y’. ‘n’ identifies marks that are not enabled.

Address

Where the marker is in your program, as a memory address.

What

Where the marker is in the source for your program, as a file and line number. If the debug information included in the program does not allow GDB to locate the source of the marker, this column will be left blank.

In addition, the following information may be printed for each marker:

Data

User data passed to the tracing library by the marker call. In the UST backend, this is the format string passed as argument to the marker call.

Static tracepoints probing the marker

The list of static tracepoints attached to the marker.

(gdb) info static-tracepoint-markers
Cnt ID         Enb Address            What
1   ust/bar2   y   0x0000000000400e1a in main at stexample.c:25
     Data: number1 %d number2 %d
     Probed by static tracepoints: #2
2   ust/bar33  n   0x0000000000400c87 in main at stexample.c:24
     Data: str %s
(gdb)

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13.1.9 Starting and Stopping Trace Experiments

tstart

This command starts the trace experiment, and begins collecting data. It has the side effect of discarding all the data collected in the trace buffer during the previous trace experiment. If any arguments are supplied, they are taken as a note and stored with the trace experiment’s state. The notes may be arbitrary text, and are especially useful with disconnected tracing in a multi-user context; the notes can explain what the trace is doing, supply user contact information, and so forth.

tstop

This command stops the trace experiment. If any arguments are supplied, they are recorded with the experiment as a note. This is useful if you are stopping a trace started by someone else, for instance if the trace is interfering with the system’s behavior and needs to be stopped quickly.

Note: a trace experiment and data collection may stop automatically if any tracepoint’s passcount is reached (see Tracepoint Passcounts), or if the trace buffer becomes full.

tstatus

This command displays the status of the current trace data collection.

Here is an example of the commands we described so far:

(gdb) trace gdb_c_test
(gdb) actions
Enter actions for tracepoint #1, one per line.
> collect $regs,$locals,$args
> while-stepping 11
  > collect $regs
  > end
> end
(gdb) tstart
	[time passes …]
(gdb) tstop

You can choose to continue running the trace experiment even if GDB disconnects from the target, voluntarily or involuntarily. For commands such as detach, the debugger will ask what you want to do with the trace. But for unexpected terminations (GDB crash, network outage), it would be unfortunate to lose hard-won trace data, so the variable disconnected-tracing lets you decide whether the trace should continue running without GDB.

set disconnected-tracing on
set disconnected-tracing off

Choose whether a tracing run should continue to run if GDB has disconnected from the target. Note that detach or quit will ask you directly what to do about a running trace no matter what this variable’s setting, so the variable is mainly useful for handling unexpected situations, such as loss of the network.

show disconnected-tracing

Show the current choice for disconnected tracing.

When you reconnect to the target, the trace experiment may or may not still be running; it might have filled the trace buffer in the meantime, or stopped for one of the other reasons. If it is running, it will continue after reconnection.

Upon reconnection, the target will upload information about the tracepoints in effect. GDB will then compare that information to the set of tracepoints currently defined, and attempt to match them up, allowing for the possibility that the numbers may have changed due to creation and deletion in the meantime. If one of the target’s tracepoints does not match any in GDB, the debugger will create a new tracepoint, so that you have a number with which to specify that tracepoint. This matching-up process is necessarily heuristic, and it may result in useless tracepoints being created; you may simply delete them if they are of no use.

If your target agent supports a circular trace buffer, then you can run a trace experiment indefinitely without filling the trace buffer; when space runs out, the agent deletes already-collected trace frames, oldest first, until there is enough room to continue collecting. This is especially useful if your tracepoints are being hit too often, and your trace gets terminated prematurely because the buffer is full. To ask for a circular trace buffer, simply set ‘circular-trace-buffer’ to on. You can set this at any time, including during tracing; if the agent can do it, it will change buffer handling on the fly, otherwise it will not take effect until the next run.

set circular-trace-buffer on
set circular-trace-buffer off

Choose whether a tracing run should use a linear or circular buffer for trace data. A linear buffer will not lose any trace data, but may fill up prematurely, while a circular buffer will discard old trace data, but it will have always room for the latest tracepoint hits.

show circular-trace-buffer

Show the current choice for the trace buffer. Note that this may not match the agent’s current buffer handling, nor is it guaranteed to match the setting that might have been in effect during a past run, for instance if you are looking at frames from a trace file.

set trace-buffer-size n
set trace-buffer-size unlimited

Request that the target use a trace buffer of n bytes. Not all targets will honor the request; they may have a compiled-in size for the trace buffer, or some other limitation. Set to a value of unlimited or -1 to let the target use whatever size it likes. This is also the default.

show trace-buffer-size

Show the current requested size for the trace buffer. Note that this will only match the actual size if the target supports size-setting, and was able to handle the requested size. For instance, if the target can only change buffer size between runs, this variable will not reflect the change until the next run starts. Use tstatus to get a report of the actual buffer size.

set trace-user text
show trace-user
set trace-notes text

Set the trace run’s notes.

show trace-notes

Show the trace run’s notes.

set trace-stop-notes text

Set the trace run’s stop notes. The handling of the note is as for tstop arguments; the set command is convenient way to fix a stop note that is mistaken or incomplete.

show trace-stop-notes

Show the trace run’s stop notes.


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13.1.10 Tracepoint Restrictions

There are a number of restrictions on the use of tracepoints. As described above, tracepoint data gathering occurs on the target without interaction from GDB. Thus the full capabilities of the debugger are not available during data gathering, and then at data examination time, you will be limited by only having what was collected. The following items describe some common problems, but it is not exhaustive, and you may run into additional difficulties not mentioned here.


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13.2 Using the Collected Data

After the tracepoint experiment ends, you use GDB commands for examining the trace data. The basic idea is that each tracepoint collects a trace snapshot every time it is hit and another snapshot every time it single-steps. All these snapshots are consecutively numbered from zero and go into a buffer, and you can examine them later. The way you examine them is to focus on a specific trace snapshot. When the remote stub is focused on a trace snapshot, it will respond to all GDB requests for memory and registers by reading from the buffer which belongs to that snapshot, rather than from real memory or registers of the program being debugged. This means that all GDB commands (print, info registers, backtrace, etc.) will behave as if we were currently debugging the program state as it was when the tracepoint occurred. Any requests for data that are not in the buffer will fail.


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13.2.1 tfind n

The basic command for selecting a trace snapshot from the buffer is tfind n, which finds trace snapshot number n, counting from zero. If no argument n is given, the next snapshot is selected.

Here are the various forms of using the tfind command.

tfind start

Find the first snapshot in the buffer. This is a synonym for tfind 0 (since 0 is the number of the first snapshot).

tfind none

Stop debugging trace snapshots, resume live debugging.

tfind end

Same as ‘tfind none’.

tfind

No argument means find the next trace snapshot.

tfind -

Find the previous trace snapshot before the current one. This permits retracing earlier steps.

tfind tracepoint num

Find the next snapshot associated with tracepoint num. Search proceeds forward from the last examined trace snapshot. If no argument num is given, it means find the next snapshot collected for the same tracepoint as the current snapshot.

tfind pc addr

Find the next snapshot associated with the value addr of the program counter. Search proceeds forward from the last examined trace snapshot. If no argument addr is given, it means find the next snapshot with the same value of PC as the current snapshot.

tfind outside addr1, addr2

Find the next snapshot whose PC is outside the given range of addresses (exclusive).

tfind range addr1, addr2

Find the next snapshot whose PC is between addr1 and addr2 (inclusive).

tfind line [file:]n

Find the next snapshot associated with the source line n. If the optional argument file is given, refer to line n in that source file. Search proceeds forward from the last examined trace snapshot. If no argument n is given, it means find the next line other than the one currently being examined; thus saying tfind line repeatedly can appear to have the same effect as stepping from line to line in a live debugging session.

The default arguments for the tfind commands are specifically designed to make it easy to scan through the trace buffer. For instance, tfind with no argument selects the next trace snapshot, and tfind - with no argument selects the previous trace snapshot. So, by giving one tfind command, and then simply hitting RET repeatedly you can examine all the trace snapshots in order. Or, by saying tfind - and then hitting RET repeatedly you can examine the snapshots in reverse order. The tfind line command with no argument selects the snapshot for the next source line executed. The tfind pc command with no argument selects the next snapshot with the same program counter (PC) as the current frame. The tfind tracepoint command with no argument selects the next trace snapshot collected by the same tracepoint as the current one.

In addition to letting you scan through the trace buffer manually, these commands make it easy to construct GDB scripts that scan through the trace buffer and print out whatever collected data you are interested in. Thus, if we want to examine the PC, FP, and SP registers from each trace frame in the buffer, we can say this:

(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
          $trace_frame, $pc, $sp, $fp
> tfind
> end

Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14

Or, if we want to examine the variable X at each source line in the buffer:

(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, X == %d\n", $trace_frame, X
> tfind line
> end

Frame 0, X = 1
Frame 7, X = 2
Frame 13, X = 255

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13.2.2 tdump

This command takes no arguments. It prints all the data collected at the current trace snapshot.

(gdb) trace 444
(gdb) actions
Enter actions for tracepoint #2, one per line:
> collect $regs, $locals, $args, gdb_long_test
> end

(gdb) tstart

(gdb) tfind line 444
#0  gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
at gdb_test.c:444
444        printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )

(gdb) tdump
Data collected at tracepoint 2, trace frame 1:
d0             0xc4aa0085       -995491707
d1             0x18     24
d2             0x80     128
d3             0x33     51
d4             0x71aea3d        119204413
d5             0x22     34
d6             0xe0     224
d7             0x380035 3670069
a0             0x19e24a 1696330
a1             0x3000668        50333288
a2             0x100    256
a3             0x322000 3284992
a4             0x3000698        50333336
a5             0x1ad3cc 1758156
fp             0x30bf3c 0x30bf3c
sp             0x30bf34 0x30bf34
ps             0x0      0
pc             0x20b2c8 0x20b2c8
fpcontrol      0x0      0
fpstatus       0x0      0
fpiaddr        0x0      0
p = 0x20e5b4 "gdb-test"
p1 = (void *) 0x11
p2 = (void *) 0x22
p3 = (void *) 0x33
p4 = (void *) 0x44
p5 = (void *) 0x55
p6 = (void *) 0x66
gdb_long_test = 17 '\021'

(gdb)

tdump works by scanning the tracepoint’s current collection actions and printing the value of each expression listed. So tdump can fail, if after a run, you change the tracepoint’s actions to mention variables that were not collected during the run.

Also, for tracepoints with while-stepping loops, tdump uses the collected value of $pc to distinguish between trace frames that were collected at the tracepoint hit, and frames that were collected while stepping. This allows it to correctly choose whether to display the basic list of collections, or the collections from the body of the while-stepping loop. However, if $pc was not collected, then tdump will always attempt to dump using the basic collection list, and may fail if a while-stepping frame does not include all the same data that is collected at the tracepoint hit.


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13.2.3 save tracepoints filename

This command saves all current tracepoint definitions together with their actions and passcounts, into a file filename suitable for use in a later debugging session. To read the saved tracepoint definitions, use the source command (see Command Files). The save-tracepoints command is a deprecated alias for save tracepoints


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13.3 Convenience Variables for Tracepoints

(int) $trace_frame

The current trace snapshot (a.k.a. frame) number, or -1 if no snapshot is selected.

(int) $tracepoint

The tracepoint for the current trace snapshot.

(int) $trace_line

The line number for the current trace snapshot.

(char []) $trace_file

The source file for the current trace snapshot.

(char []) $trace_func

The name of the function containing $tracepoint.

Note: $trace_file is not suitable for use in printf, use output instead.

Here’s a simple example of using these convenience variables for stepping through all the trace snapshots and printing some of their data. Note that these are not the same as trace state variables, which are managed by the target.

(gdb) tfind start

(gdb) while $trace_frame != -1
> output $trace_file
> printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
> tfind
> end

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13.4 Using Trace Files

In some situations, the target running a trace experiment may no longer be available; perhaps it crashed, or the hardware was needed for a different activity. To handle these cases, you can arrange to dump the trace data into a file, and later use that file as a source of trace data, via the target tfile command.

tsave [ -r ] filename
tsave [-ctf] dirname

Save the trace data to filename. By default, this command assumes that filename refers to the host filesystem, so if necessary GDB will copy raw trace data up from the target and then save it. If the target supports it, you can also supply the optional argument -r (“remote”) to direct the target to save the data directly into filename in its own filesystem, which may be more efficient if the trace buffer is very large. (Note, however, that target tfile can only read from files accessible to the host.) By default, this command will save trace frame in tfile format. You can supply the optional argument -ctf to save date in CTF format. The Common Trace Format (CTF) is proposed as a trace format that can be shared by multiple debugging and tracing tools. Please go to ‘http://www.efficios.com/ctf’ to get more information.

target tfile filename
target ctf dirname

Use the file named filename or directory named dirname as a source of trace data. Commands that examine data work as they do with a live target, but it is not possible to run any new trace experiments. tstatus will report the state of the trace run at the moment the data was saved, as well as the current trace frame you are examining. Both filename and dirname must be on a filesystem accessible to the host.

(gdb) target ctf ctf.ctf
(gdb) tfind
Found trace frame 0, tracepoint 2
39            ++a;  /* set tracepoint 1 here */
(gdb) tdump
Data collected at tracepoint 2, trace frame 0:
i = 0
a = 0
b = 1 '\001'
c = {"123", "456", "789", "123", "456", "789"}
d = {{{a = 1, b = 2}, {a = 3, b = 4}}, {{a = 5, b = 6}, {a = 7, b = 8}}}
(gdb) p b
$1 = 1

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14 Debugging Programs That Use Overlays

If your program is too large to fit completely in your target system’s memory, you can sometimes use overlays to work around this problem. GDB provides some support for debugging programs that use overlays.


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14.1 How Overlays Work

Suppose you have a computer whose instruction address space is only 64 kilobytes long, but which has much more memory which can be accessed by other means: special instructions, segment registers, or memory management hardware, for example. Suppose further that you want to adapt a program which is larger than 64 kilobytes to run on this system.

One solution is to identify modules of your program which are relatively independent, and need not call each other directly; call these modules overlays. Separate the overlays from the main program, and place their machine code in the larger memory. Place your main program in instruction memory, but leave at least enough space there to hold the largest overlay as well.

Now, to call a function located in an overlay, you must first copy that overlay’s machine code from the large memory into the space set aside for it in the instruction memory, and then jump to its entry point there.

    Data             Instruction            Larger
Address Space       Address Space        Address Space
+-----------+       +-----------+        +-----------+
|           |       |           |        |           |
+-----------+       +-----------+        +-----------+<-- overlay 1
| program   |       |   main    |   .----| overlay 1 | load address
| variables |       |  program  |   |    +-----------+
| and heap  |       |           |   |    |           |
+-----------+       |           |   |    +-----------+<-- overlay 2
|           |       +-----------+   |    |           | load address
+-----------+       |           |   |  .-| overlay 2 |
                    |           |   |  | |           |
         mapped --->+-----------+   |  | +-----------+
         address    |           |   |  | |           |
                    |  overlay  | <-'  | |           |
                    |   area    |  <---' +-----------+<-- overlay 3
                    |           | <---.  |           | load address
                    +-----------+     `--| overlay 3 |
                    |           |        |           |
                    +-----------+        |           |
                                         +-----------+
                                         |           |
                                         +-----------+

                    A code overlay

The diagram (see A code overlay) shows a system with separate data and instruction address spaces. To map an overlay, the program copies its code from the larger address space to the instruction address space. Since the overlays shown here all use the same mapped address, only one may be mapped at a time. For a system with a single address space for data and instructions, the diagram would be similar, except that the program variables and heap would share an address space with the main program and the overlay area.

An overlay loaded into instruction memory and ready for use is called a mapped overlay; its mapped address is its address in the instruction memory. An overlay not present (or only partially present) in instruction memory is called unmapped; its load address is its address in the larger memory. The mapped address is also called the virtual memory address, or VMA; the load address is also called the load memory address, or LMA.

Unfortunately, overlays are not a completely transparent way to adapt a program to limited instruction memory. They introduce a new set of global constraints you must keep in mind as you design your program:

The overlay system described above is rather simple, and could be improved in many ways:


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14.2 Overlay Commands

To use GDB’s overlay support, each overlay in your program must correspond to a separate section of the executable file. The section’s virtual memory address and load memory address must be the overlay’s mapped and load addresses. Identifying overlays with sections allows GDB to determine the appropriate address of a function or variable, depending on whether the overlay is mapped or not.

GDB’s overlay commands all start with the word overlay; you can abbreviate this as ov or ovly. The commands are:

overlay off

Disable GDB’s overlay support. When overlay support is disabled, GDB assumes that all functions and variables are always present at their mapped addresses. By default, GDB’s overlay support is disabled.

overlay manual

Enable manual overlay debugging. In this mode, GDB relies on you to tell it which overlays are mapped, and which are not, using the overlay map-overlay and overlay unmap-overlay commands described below.

overlay map-overlay overlay
overlay map overlay

Tell GDB that overlay is now mapped; overlay must be the name of the object file section containing the overlay. When an overlay is mapped, GDB assumes it can find the overlay’s functions and variables at their mapped addresses. GDB assumes that any other overlays whose mapped ranges overlap that of overlay are now unmapped.

overlay unmap-overlay overlay
overlay unmap overlay

Tell GDB that overlay is no longer mapped; overlay must be the name of the object file section containing the overlay. When an overlay is unmapped, GDB assumes it can find the overlay’s functions and variables at their load addresses.

overlay auto

Enable automatic overlay debugging. In this mode, GDB consults a data structure the overlay manager maintains in the inferior to see which overlays are mapped. For details, see Automatic Overlay Debugging.

overlay load-target
overlay load

Re-read the overlay table from the inferior. Normally, GDB re-reads the table GDB automatically each time the inferior stops, so this command should only be necessary if you have changed the overlay mapping yourself using GDB. This command is only useful when using automatic overlay debugging.

overlay list-overlays
overlay list

Display a list of the overlays currently mapped, along with their mapped addresses, load addresses, and sizes.

Normally, when GDB prints a code address, it includes the name of the function the address falls in:

(gdb) print main
$3 = {int ()} 0x11a0 <main>

When overlay debugging is enabled, GDB recognizes code in unmapped overlays, and prints the names of unmapped functions with asterisks around them. For example, if foo is a function in an unmapped overlay, GDB prints it this way:

(gdb) overlay list
No sections are mapped.
(gdb) print foo
$5 = {int (int)} 0x100000 <*foo*>

When foo’s overlay is mapped, GDB prints the function’s name normally:

(gdb) overlay list
Section .ov.foo.text, loaded at 0x100000 - 0x100034,
        mapped at 0x1016 - 0x104a
(gdb) print foo
$6 = {int (int)} 0x1016 <foo>

When overlay debugging is enabled, GDB can find the correct address for functions and variables in an overlay, whether or not the overlay is mapped. This allows most GDB commands, like break and disassemble, to work normally, even on unmapped code. However, GDB’s breakpoint support has some limitations:


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14.3 Automatic Overlay Debugging

GDB can automatically track which overlays are mapped and which are not, given some simple co-operation from the overlay manager in the inferior. If you enable automatic overlay debugging with the overlay auto command (see Overlay Commands), GDB looks in the inferior’s memory for certain variables describing the current state of the overlays.

Here are the variables your overlay manager must define to support GDB’s automatic overlay debugging:

_ovly_table:

This variable must be an array of the following structures:

struct
{
  /* The overlay's mapped address.  */
  unsigned long vma;

  /* The size of the overlay, in bytes.  */
  unsigned long size;

  /* The overlay's load address.  */
  unsigned long lma;

  /* Non-zero if the overlay is currently mapped;
     zero otherwise.  */
  unsigned long mapped;
}
_novlys:

This variable must be a four-byte signed integer, holding the total number of elements in _ovly_table.

To decide whether a particular overlay is mapped or not, GDB looks for an entry in _ovly_table whose vma and lma members equal the VMA and LMA of the overlay’s section in the executable file. When GDB finds a matching entry, it consults the entry’s mapped member to determine whether the overlay is currently mapped.

In addition, your overlay manager may define a function called _ovly_debug_event. If this function is defined, GDB will silently set a breakpoint there. If the overlay manager then calls this function whenever it has changed the overlay table, this will enable GDB to accurately keep track of which overlays are in program memory, and update any breakpoints that may be set in overlays. This will allow breakpoints to work even if the overlays are kept in ROM or other non-writable memory while they are not being executed.


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14.4 Overlay Sample Program

When linking a program which uses overlays, you must place the overlays at their load addresses, while relocating them to run at their mapped addresses. To do this, you must write a linker script (see Overlay Description in Using ld: the GNU linker). Unfortunately, since linker scripts are specific to a particular host system, target architecture, and target memory layout, this manual cannot provide portable sample code demonstrating GDB’s overlay support.

However, the GDB source distribution does contain an overlaid program, with linker scripts for a few systems, as part of its test suite. The program consists of the following files from gdb/testsuite/gdb.base:

overlays.c

The main program file.

ovlymgr.c

A simple overlay manager, used by overlays.c.

foo.c
bar.c
baz.c
grbx.c

Overlay modules, loaded and used by overlays.c.

d10v.ld
m32r.ld

Linker scripts for linking the test program on the d10v-elf and m32r-elf targets.

You can build the test program using the d10v-elf GCC cross-compiler like this:

$ d10v-elf-gcc -g -c overlays.c
$ d10v-elf-gcc -g -c ovlymgr.c
$ d10v-elf-gcc -g -c foo.c
$ d10v-elf-gcc -g -c bar.c
$ d10v-elf-gcc -g -c baz.c
$ d10v-elf-gcc -g -c grbx.c
$ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \
                  baz.o grbx.o -Wl,-Td10v.ld -o overlays

The build process is identical for any other architecture, except that you must substitute the appropriate compiler and linker script for the target system for d10v-elf-gcc and d10v.ld.


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15 Using GDB with Different Languages

Although programming languages generally have common aspects, they are rarely expressed in the same manner. For instance, in ANSI C, dereferencing a pointer p is accomplished by *p, but in Modula-2, it is accomplished by p^. Values can also be represented (and displayed) differently. Hex numbers in C appear as ‘0x1ae’, while in Modula-2 they appear as ‘1AEH’.

Language-specific information is built into GDB for some languages, allowing you to express operations like the above in your program’s native language, and allowing GDB to output values in a manner consistent with the syntax of your program’s native language. The language you use to build expressions is called the working language.


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15.1 Switching Between Source Languages

There are two ways to control the working language—either have GDB set it automatically, or select it manually yourself. You can use the set language command for either purpose. On startup, GDB defaults to setting the language automatically. The working language is used to determine how expressions you type are interpreted, how values are printed, etc.

In addition to the working language, every source file that GDB knows about has its own working language. For some object file formats, the compiler might indicate which language a particular source file is in. However, most of the time GDB infers the language from the name of the file. The language of a source file controls whether C++ names are demangled—this way backtrace can show each frame appropriately for its own language. There is no way to set the language of a source file from within GDB, but you can set the language associated with a filename extension. See Displaying the Language.

This is most commonly a problem when you use a program, such as cfront or f2c, that generates C but is written in another language. In that case, make the program use #line directives in its C output; that way GDB will know the correct language of the source code of the original program, and will display that source code, not the generated C code.


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15.1.1 List of Filename Extensions and Languages

If a source file name ends in one of the following extensions, then GDB infers that its language is the one indicated.

.ada
.ads
.adb
.a

Ada source file.

.c

C source file

.C
.cc
.cp
.cpp
.cxx
.c++

C++ source file

.d

D source file

.m

Objective-C source file

.f
.F

Fortran source file

.mod

Modula-2 source file

.s
.S

Assembler source file. This actually behaves almost like C, but GDB does not skip over function prologues when stepping.

In addition, you may set the language associated with a filename extension. See Displaying the Language.


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15.1.2 Setting the Working Language

If you allow GDB to set the language automatically, expressions are interpreted the same way in your debugging session and your program.

If you wish, you may set the language manually. To do this, issue the command ‘set language lang’, where lang is the name of a language, such as c or modula-2. For a list of the supported languages, type ‘set language’.

Setting the language manually prevents GDB from updating the working language automatically. This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages—but means different things. For instance, if the current source file were written in C, and GDB was parsing Modula-2, a command such as:

print a = b + c

might not have the effect you intended. In C, this means to add b and c and place the result in a. The result printed would be the value of a. In Modula-2, this means to compare a to the result of b+c, yielding a BOOLEAN value.


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15.1.3 Having GDB Infer the Source Language

To have GDB set the working language automatically, use ‘set language local’ or ‘set language auto’. GDB then infers the working language. That is, when your program stops in a frame (usually by encountering a breakpoint), GDB sets the working language to the language recorded for the function in that frame. If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and GDB issues a warning.

This may not seem necessary for most programs, which are written entirely in one source language. However, program modules and libraries written in one source language can be used by a main program written in a different source language. Using ‘set language auto’ in this case frees you from having to set the working language manually.


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15.2 Displaying the Language

The following commands help you find out which language is the working language, and also what language source files were written in.

show language

Display the current working language. This is the language you can use with commands such as print to build and compute expressions that may involve variables in your program.

info frame

Display the source language for this frame. This language becomes the working language if you use an identifier from this frame. See Information about a Frame, to identify the other information listed here.

info source

Display the source language of this source file. See Examining the Symbol Table, to identify the other information listed here.

In unusual circumstances, you may have source files with extensions not in the standard list. You can then set the extension associated with a language explicitly:

set extension-language ext language

Tell GDB that source files with extension ext are to be assumed as written in the source language language.

info extensions

List all the filename extensions and the associated languages.


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15.3 Type and Range Checking

Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks. These include checking the type of arguments to functions and operators and making sure mathematical overflows are caught at run time. Checks such as these help to ensure a program’s correctness once it has been compiled by eliminating type mismatches and providing active checks for range errors when your program is running.

By default GDB checks for these errors according to the rules of the current source language. Although GDB does not check the statements in your program, it can check expressions entered directly into GDB for evaluation via the print command, for example.


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15.3.1 An Overview of Type Checking

Some languages, such as C and C++, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs. These checks prevent type mismatch errors from ever causing any run-time problems. For example,

int klass::my_method(char *b) { return  b ? 1 : 2; }

(gdb) print obj.my_method (0)
$1 = 2
but
(gdb) print obj.my_method (0x1234)
Cannot resolve method klass::my_method to any overloaded instance

The second example fails because in C++ the integer constant ‘0x1234’ is not type-compatible with the pointer parameter type.

For the expressions you use in GDB commands, you can tell GDB to not enforce strict type checking or to treat any mismatches as errors and abandon the expression; When type checking is disabled, GDB successfully evaluates expressions like the second example above.

Even if type checking is off, there may be other reasons related to type that prevent GDB from evaluating an expression. For instance, GDB does not know how to add an int and a struct foo. These particular type errors have nothing to do with the language in use and usually arise from expressions which make little sense to evaluate anyway.

GDB provides some additional commands for controlling type checking:

set check type on
set check type off

Set strict type checking on or off. If any type mismatches occur in evaluating an expression while type checking is on, GDB prints a message and aborts evaluation of the expression.

show check type

Show the current setting of type checking and whether GDB is enforcing strict type checking rules.


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15.3.2 An Overview of Range Checking

In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks. Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array.

For expressions you use in GDB commands, you can tell GDB to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway.

A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type. Some languages, however, do not treat overflows as an error. In many implementations of C, mathematical overflow causes the result to “wrap around” to lower values—for example, if m is the largest integer value, and s is the smallest, then

m + 1 ⇒ s

This, too, is specific to individual languages, and in some cases specific to individual compilers or machines. See Supported Languages, for further details on specific languages.

GDB provides some additional commands for controlling the range checker:

set check range auto

Set range checking on or off based on the current working language. See Supported Languages, for the default settings for each language.

set check range on
set check range off

Set range checking on or off, overriding the default setting for the current working language. A warning is issued if the setting does not match the language default. If a range error occurs and range checking is on, then a message is printed and evaluation of the expression is aborted.

set check range warn

Output messages when the GDB range checker detects a range error, but attempt to evaluate the expression anyway. Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many Unix systems).

show range

Show the current setting of the range checker, and whether or not it is being set automatically by GDB.


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15.4 Supported Languages

GDB supports C, C++, D, Go, Objective-C, Fortran, Java, OpenCL C, Pascal, assembly, Modula-2, and Ada. Some GDB features may be used in expressions regardless of the language you use: the GDB @ and :: operators, and the ‘{type}addr’ construct (see Expressions) can be used with the constructs of any supported language.

The following sections detail to what degree each source language is supported by GDB. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDB expression parser accepts, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial.


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15.4.1 C and C++

Since C and C++ are so closely related, many features of GDB apply to both languages. Whenever this is the case, we discuss those languages together.

The C++ debugging facilities are jointly implemented by the C++ compiler and GDB. Therefore, to debug your C++ code effectively, you must compile your C++ programs with a supported C++ compiler, such as GNU g++, or the HP ANSI C++ compiler (aCC).


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15.4.1.1 C and C++ Operators

Operators must be defined on values of specific types. For instance, + is defined on numbers, but not on structures. Operators are often defined on groups of types.

For the purposes of C and C++, the following definitions hold:

The following operators are supported. They are listed here in order of increasing precedence:

,

The comma or sequencing operator. Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated.

=

Assignment. The value of an assignment expression is the value assigned. Defined on scalar types.

op=

Used in an expression of the form a opb, and translated to a = a op b. op= and = have the same precedence. The operator op is any one of the operators |, ^, &, <<, >>, +, -, *, /, %.

?:

The ternary operator. a ? b : c can be thought of as: if a then b else c. The argument a should be of an integral type.

||

Logical OR. Defined on integral types.

&&

Logical AND. Defined on integral types.

|

Bitwise OR. Defined on integral types.

^

Bitwise exclusive-OR. Defined on integral types.

&

Bitwise AND. Defined on integral types.

==, !=

Equality and inequality. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.

<, >, <=, >=

Less than, greater than, less than or equal, greater than or equal. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.

<<, >>

left shift, and right shift. Defined on integral types.

@

The GDB “artificial array” operator (see Expressions).

+, -

Addition and subtraction. Defined on integral types, floating-point types and pointer types.

*, /, %

Multiplication, division, and modulus. Multiplication and division are defined on integral and floating-point types. Modulus is defined on integral types.

++, --

Increment and decrement. When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable’s value is used before the operation takes place.

*

Pointer dereferencing. Defined on pointer types. Same precedence as ++.

&

Address operator. Defined on variables. Same precedence as ++.

For debugging C++, GDB implements a use of ‘&’ beyond what is allowed in the C++ language itself: you can use ‘&(&ref)’ to examine the address where a C++ reference variable (declared with ‘&ref’) is stored.

-

Negative. Defined on integral and floating-point types. Same precedence as ++.

!

Logical negation. Defined on integral types. Same precedence as ++.

~

Bitwise complement operator. Defined on integral types. Same precedence as ++.

., ->

Structure member, and pointer-to-structure member. For convenience, GDB regards the two as equivalent, choosing whether to dereference a pointer based on the stored type information. Defined on struct and union data.

.*, ->*

Dereferences of pointers to members.

[]

Array indexing. a[i] is defined as *(a+i). Same precedence as ->.

()

Function parameter list. Same precedence as ->.

::

C++ scope resolution operator. Defined on struct, union, and class types.

::

Doubled colons also represent the GDB scope operator (see Expressions). Same precedence as ::, above.

If an operator is redefined in the user code, GDB usually attempts to invoke the redefined version instead of using the operator’s predefined meaning.


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15.4.1.2 C and C++ Constants

GDB allows you to express the constants of C and C++ in the following ways:


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15.4.1.3 C++ Expressions

GDB expression handling can interpret most C++ expressions.

Warning: GDB can only debug C++ code if you use the proper compiler and the proper debug format. Currently, GDB works best when debugging C++ code that is compiled with the most recent version of GCC possible. The DWARF debugging format is preferred; GCC defaults to this on most popular platforms. Other compilers and/or debug formats are likely to work badly or not at all when using GDB to debug C++ code. See Compilation.

  1. Member function calls are allowed; you can use expressions like
    count = aml->GetOriginal(x, y)
    
  2. While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, GDB allows implicit references to the class instance pointer this following the same rules as C++. using declarations in the current scope are also respected by GDB.
  3. You can call overloaded functions; GDB resolves the function call to the right definition, with some restrictions. GDB does not perform overload resolution involving user-defined type conversions, calls to constructors, or instantiations of templates that do not exist in the program. It also cannot handle ellipsis argument lists or default arguments.

    It does perform integral conversions and promotions, floating-point promotions, arithmetic conversions, pointer conversions, conversions of class objects to base classes, and standard conversions such as those of functions or arrays to pointers; it requires an exact match on the number of function arguments.

    Overload resolution is always performed, unless you have specified set overload-resolution off. See GDB Features for C++.

    You must specify set overload-resolution off in order to use an explicit function signature to call an overloaded function, as in

    p 'foo(char,int)'('x', 13)
    

    The GDB command-completion facility can simplify this; see Command Completion.

  4. GDB understands variables declared as C++ references; you can use them in expressions just as you do in C++ source—they are automatically dereferenced.

    In the parameter list shown when GDB displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you have specified ‘set print address off’.

  5. GDB supports the C++ name resolution operator ::—your expressions can use it just as expressions in your program do. Since one scope may be defined in another, you can use :: repeatedly if necessary, for example in an expression like ‘scope1::scope2::name’. GDB also allows resolving name scope by reference to source files, in both C and C++ debugging (see Program Variables).
  6. GDB performs argument-dependent lookup, following the C++ specification.

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15.4.1.4 C and C++ Defaults

If you allow GDB to set range checking automatically, it defaults to off whenever the working language changes to C or C++. This happens regardless of whether you or GDB selects the working language.

If you allow GDB to set the language automatically, it recognizes source files whose names end with .c, .C, or .cc, etc, and when GDB enters code compiled from one of these files, it sets the working language to C or C++. See Having GDB Infer the Source Language, for further details.


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15.4.1.5 C and C++ Type and Range Checks

By default, when GDB parses C or C++ expressions, strict type checking is used. However, if you turn type checking off, GDB will allow certain non-standard conversions, such as promoting integer constants to pointers.

Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array.


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15.4.1.6 GDB and C

The set print union and show print union commands apply to the union type. When set to ‘on’, any union that is inside a struct or class is also printed. Otherwise, it appears as ‘{...}’.

The @ operator aids in the debugging of dynamic arrays, formed with pointers and a memory allocation function. See Expressions.


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15.4.1.7 GDB Features for C++

Some GDB commands are particularly useful with C++, and some are designed specifically for use with C++. Here is a summary:

breakpoint menus

When you want a breakpoint in a function whose name is overloaded, GDB has the capability to display a menu of possible breakpoint locations to help you specify which function definition you want. See Ambiguous Expressions.

rbreak regex

Setting breakpoints using regular expressions is helpful for setting breakpoints on overloaded functions that are not members of any special classes. See Setting Breakpoints.

catch throw
catch rethrow
catch catch

Debug C++ exception handling using these commands. See Setting Catchpoints.

ptype typename

Print inheritance relationships as well as other information for type typename. See Examining the Symbol Table.

info vtbl expression.

The info vtbl command can be used to display the virtual method tables of the object computed by expression. This shows one entry per virtual table; there may be multiple virtual tables when multiple inheritance is in use.

set print demangle
show print demangle
set print asm-demangle
show print asm-demangle

Control whether C++ symbols display in their source form, both when displaying code as C++ source and when displaying disassemblies. See Print Settings.

set print object
show print object

Choose whether to print derived (actual) or declared types of objects. See Print Settings.

set print vtbl
show print vtbl

Control the format for printing virtual function tables. See Print Settings. (The vtbl commands do not work on programs compiled with the HP ANSI C++ compiler (aCC).)

set overload-resolution on

Enable overload resolution for C++ expression evaluation. The default is on. For overloaded functions, GDB evaluates the arguments and searches for a function whose signature matches the argument types, using the standard C++ conversion rules (see C++ Expressions, for details). If it cannot find a match, it emits a message.

set overload-resolution off

Disable overload resolution for C++ expression evaluation. For overloaded functions that are not class member functions, GDB chooses the first function of the specified name that it finds in the symbol table, whether or not its arguments are of the correct type. For overloaded functions that are class member functions, GDB searches for a function whose signature exactly matches the argument types.

show overload-resolution

Show the current setting of overload resolution.

Overloaded symbol names

You can specify a particular definition of an overloaded symbol, using the same notation that is used to declare such symbols in C++: type symbol(types) rather than just symbol. You can also use the GDB command-line word completion facilities to list the available choices, or to finish the type list for you. See Command Completion, for details on how to do this.


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15.4.1.8 Decimal Floating Point format

GDB can examine, set and perform computations with numbers in decimal floating point format, which in the C language correspond to the _Decimal32, _Decimal64 and _Decimal128 types as specified by the extension to support decimal floating-point arithmetic.

There are two encodings in use, depending on the architecture: BID (Binary Integer Decimal) for x86 and x86-64, and DPD (Densely Packed Decimal) for PowerPC and S/390. GDB will use the appropriate encoding for the configured target.

Because of a limitation in libdecnumber, the library used by GDB to manipulate decimal floating point numbers, it is not possible to convert (using a cast, for example) integers wider than 32-bit to decimal float.

In addition, in order to imitate GDB’s behaviour with binary floating point computations, error checking in decimal float operations ignores underflow, overflow and divide by zero exceptions.

In the PowerPC architecture, GDB provides a set of pseudo-registers to inspect _Decimal128 values stored in floating point registers. See PowerPC for more details.


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15.4.2 D

GDB can be used to debug programs written in D and compiled with GDC, LDC or DMD compilers. Currently GDB supports only one D specific feature — dynamic arrays.


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15.4.3 Go

GDB can be used to debug programs written in Go and compiled with gccgo or 6g compilers.

Here is a summary of the Go-specific features and restrictions:

The current Go package

The name of the current package does not need to be specified when specifying global variables and functions.

For example, given the program:

package main
var myglob = "Shall we?"
func main () {
  // ...
}

When stopped inside main either of these work:

(gdb) p myglob
(gdb) p main.myglob
Builtin Go types

The string type is recognized by GDB and is printed as a string.

Builtin Go functions

The GDB expression parser recognizes the unsafe.Sizeof function and handles it internally.

Restrictions on Go expressions

All Go operators are supported except &^. The Go _ “blank identifier” is not supported. Automatic dereferencing of pointers is not supported.


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15.4.4 Objective-C

This section provides information about some commands and command options that are useful for debugging Objective-C code. See also info classes, and info selectors, for a few more commands specific to Objective-C support.


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15.4.4.1 Method Names in Commands

The following commands have been extended to accept Objective-C method names as line specifications:

A fully qualified Objective-C method name is specified as

-[Class methodName]

where the minus sign is used to indicate an instance method and a plus sign (not shown) is used to indicate a class method. The class name Class and method name methodName are enclosed in brackets, similar to the way messages are specified in Objective-C source code. For example, to set a breakpoint at the create instance method of class Fruit in the program currently being debugged, enter:

break -[Fruit create]

To list ten program lines around the initialize class method, enter:

list +[NSText initialize]

In the current version of GDB, the plus or minus sign is required. In future versions of GDB, the plus or minus sign will be optional, but you can use it to narrow the search. It is also possible to specify just a method name:

break create

You must specify the complete method name, including any colons. If your program’s source files contain more than one create method, you’ll be presented with a numbered list of classes that implement that method. Indicate your choice by number, or type ‘0’ to exit if none apply.

As another example, to clear a breakpoint established at the makeKeyAndOrderFront: method of the NSWindow class, enter:

clear -[NSWindow makeKeyAndOrderFront:]

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15.4.4.2 The Print Command With Objective-C

The print command has also been extended to accept methods. For example:

print -[object hash]

will tell GDB to send the hash message to object and print the result. Also, an additional command has been added, print-object or po for short, which is meant to print the description of an object. However, this command may only work with certain Objective-C libraries that have a particular hook function, _NSPrintForDebugger, defined.


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15.4.5 OpenCL C

This section provides information about GDBs OpenCL C support.


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15.4.5.1 OpenCL C Datatypes

GDB supports the builtin scalar and vector datatypes specified by OpenCL 1.1. In addition the half- and double-precision floating point data types of the cl_khr_fp16 and cl_khr_fp64 OpenCL extensions are also known to GDB.


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15.4.5.2 OpenCL C Expressions

GDB supports accesses to vector components including the access as lvalue where possible. Since OpenCL C is based on C99 most C expressions supported by GDB can be used as well.


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15.4.5.3 OpenCL C Operators

GDB supports the operators specified by OpenCL 1.1 for scalar and vector data types.


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15.4.6 Fortran

GDB can be used to debug programs written in Fortran, but it currently supports only the features of Fortran 77 language.

Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers among them) append an underscore to the names of variables and functions. When you debug programs compiled by those compilers, you will need to refer to variables and functions with a trailing underscore.


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15.4.6.1 Fortran Operators and Expressions

Operators must be defined on values of specific types. For instance, + is defined on numbers, but not on characters or other non- arithmetic types. Operators are often defined on groups of types.

**

The exponentiation operator. It raises the first operand to the power of the second one.

:

The range operator. Normally used in the form of array(low:high) to represent a section of array.

%

The access component operator. Normally used to access elements in derived types. Also suitable for unions. As unions aren’t part of regular Fortran, this can only happen when accessing a register that uses a gdbarch-defined union type.


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15.4.6.2 Fortran Defaults

Fortran symbols are usually case-insensitive, so GDB by default uses case-insensitive matches for Fortran symbols. You can change that with the ‘set case-insensitive’ command, see Symbols, for the details.


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15.4.6.3 Special Fortran Commands

GDB has some commands to support Fortran-specific features, such as displaying common blocks.

info common [common-name]

This command prints the values contained in the Fortran COMMON block whose name is common-name. With no argument, the names of all COMMON blocks visible at the current program location are printed.


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15.4.7 Pascal

Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. GDB does not support entering expressions, printing values, or similar features using Pascal syntax.

The Pascal-specific command set print pascal_static-members controls whether static members of Pascal objects are displayed. See pascal_static-members.


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15.4.8 Modula-2

The extensions made to GDB to support Modula-2 only support output from the GNU Modula-2 compiler (which is currently being developed). Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them is most likely to give an error as GDB reads in the executable’s symbol table.


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15.4.8.1 Operators

Operators must be defined on values of specific types. For instance, + is defined on numbers, but not on structures. Operators are often defined on groups of types. For the purposes of Modula-2, the following definitions hold:

The following operators are supported, and appear in order of increasing precedence:

,

Function argument or array index separator.

:=

Assignment. The value of var := value is value.

<, >

Less than, greater than on integral, floating-point, or enumerated types.

<=, >=

Less than or equal to, greater than or equal to on integral, floating-point and enumerated types, or set inclusion on set types. Same precedence as <.

=, <>, #

Equality and two ways of expressing inequality, valid on scalar types. Same precedence as <. In GDB scripts, only <> is available for inequality, since # conflicts with the script comment character.

IN

Set membership. Defined on set types and the types of their members. Same precedence as <.

OR

Boolean disjunction. Defined on boolean types.

AND, &

Boolean conjunction. Defined on boolean types.

@

The GDB “artificial array” operator (see Expressions).

+, -

Addition and subtraction on integral and floating-point types, or union and difference on set types.

*

Multiplication on integral and floating-point types, or set intersection on set types.

/

Division on floating-point types, or symmetric set difference on set types. Same precedence as *.

DIV, MOD

Integer division and remainder. Defined on integral types. Same precedence as *.

-

Negative. Defined on INTEGER and REAL data.

^

Pointer dereferencing. Defined on pointer types.

NOT

Boolean negation. Defined on boolean types. Same precedence as ^.

.

RECORD field selector. Defined on RECORD data. Same precedence as ^.

[]

Array indexing. Defined on ARRAY data. Same precedence as ^.

()

Procedure argument list. Defined on PROCEDURE objects. Same precedence as ^.

::, .

GDB and Modula-2 scope operators.

Warning: Set expressions and their operations are not yet supported, so GDB treats the use of the operator IN, or the use of operators +, -, *, /, =, , <>, #, <=, and >= on sets as an error.


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15.4.8.2 Built-in Functions and Procedures

Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:

a

represents an ARRAY variable.

c

represents a CHAR constant or variable.

i

represents a variable or constant of integral type.

m

represents an identifier that belongs to a set. Generally used in the same function with the metavariable s. The type of s should be SET OF mtype (where mtype is the type of m).

n

represents a variable or constant of integral or floating-point type.

r

represents a variable or constant of floating-point type.

t

represents a type.

v

represents a variable.

x

represents a variable or constant of one of many types. See the explanation of the function for details.

All Modula-2 built-in procedures also return a result, described below.

ABS(n)

Returns the absolute value of n.

CAP(c)

If c is a lower case letter, it returns its upper case equivalent, otherwise it returns its argument.

CHR(i)

Returns the character whose ordinal value is i.

DEC(v)

Decrements the value in the variable v by one. Returns the new value.

DEC(v,i)

Decrements the value in the variable v by i. Returns the new value.

EXCL(m,s)

Removes the element m from the set s. Returns the new set.

FLOAT(i)

Returns the floating point equivalent of the integer i.

HIGH(a)

Returns the index of the last member of a.

INC(v)

Increments the value in the variable v by one. Returns the new value.

INC(v,i)

Increments the value in the variable v by i. Returns the new value.

INCL(m,s)

Adds the element m to the set s if it is not already there. Returns the new set.

MAX(t)

Returns the maximum value of the type t.

MIN(t)

Returns the minimum value of the type t.

ODD(i)

Returns boolean TRUE if i is an odd number.

ORD(x)

Returns the ordinal value of its argument. For example, the ordinal value of a character is its ASCII value (on machines supporting the ASCII character set). The argument x must be of an ordered type, which include integral, character and enumerated types.

SIZE(x)

Returns the size of its argument. The argument x can be a variable or a type.

TRUNC(r)

Returns the integral part of r.

TSIZE(x)

Returns the size of its argument. The argument x can be a variable or a type.

VAL(t,i)

Returns the member of the type t whose ordinal value is i.

Warning: Sets and their operations are not yet supported, so GDB treats the use of procedures INCL and EXCL as an error.


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15.4.8.3 Constants

GDB allows you to express the constants of Modula-2 in the following ways:


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15.4.8.4 Modula-2 Types

Currently GDB can print the following data types in Modula-2 syntax: array types, record types, set types, pointer types, procedure types, enumerated types, subrange types and base types. You can also print the contents of variables declared using these type. This section gives a number of simple source code examples together with sample GDB sessions.

The first example contains the following section of code:

VAR
   s: SET OF CHAR ;
   r: [20..40] ;

and you can request GDB to interrogate the type and value of r and s.

(gdb) print s
{'A'..'C', 'Z'}
(gdb) ptype s
SET OF CHAR
(gdb) print r
21
(gdb) ptype r
[20..40]

Likewise if your source code declares s as:

VAR
   s: SET ['A'..'Z'] ;

then you may query the type of s by:

(gdb) ptype s
type = SET ['A'..'Z']

Note that at present you cannot interactively manipulate set expressions using the debugger.

The following example shows how you might declare an array in Modula-2 and how you can interact with GDB to print its type and contents:

VAR
   s: ARRAY [-10..10] OF CHAR ;
(gdb) ptype s
ARRAY [-10..10] OF CHAR

Note that the array handling is not yet complete and although the type is printed correctly, expression handling still assumes that all arrays have a lower bound of zero and not -10 as in the example above.

Here are some more type related Modula-2 examples:

TYPE
   colour = (blue, red, yellow, green) ;
   t = [blue..yellow] ;
VAR
   s: t ;
BEGIN
   s := blue ;

The GDB interaction shows how you can query the data type and value of a variable.

(gdb) print s
$1 = blue
(gdb) ptype t
type = [blue..yellow]

In this example a Modula-2 array is declared and its contents displayed. Observe that the contents are written in the same way as their C counterparts.

VAR
   s: ARRAY [1..5] OF CARDINAL ;
BEGIN
   s[1] := 1 ;
(gdb) print s
$1 = {1, 0, 0, 0, 0}
(gdb) ptype s
type = ARRAY [1..5] OF CARDINAL

The Modula-2 language interface to GDB also understands pointer types as shown in this example:

VAR
   s: POINTER TO ARRAY [1..5] OF CARDINAL ;
BEGIN
   NEW(s) ;
   s^[1] := 1 ;

and you can request that GDB describes the type of s.

(gdb) ptype s
type = POINTER TO ARRAY [1..5] OF CARDINAL

GDB handles compound types as we can see in this example. Here we combine array types, record types, pointer types and subrange types:

TYPE
   foo = RECORD
            f1: CARDINAL ;
            f2: CHAR ;
            f3: myarray ;
         END ;

   myarray = ARRAY myrange OF CARDINAL ;
   myrange = [-2..2] ;
VAR
   s: POINTER TO ARRAY myrange OF foo ;

and you can ask GDB to describe the type of s as shown below.

(gdb) ptype s
type = POINTER TO ARRAY [-2..2] OF foo = RECORD
    f1 : CARDINAL;
    f2 : CHAR;
    f3 : ARRAY [-2..2] OF CARDINAL;
END 

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15.4.8.5 Modula-2 Defaults

If type and range checking are set automatically by GDB, they both default to on whenever the working language changes to Modula-2. This happens regardless of whether you or GDB selected the working language.

If you allow GDB to set the language automatically, then entering code compiled from a file whose name ends with .mod sets the working language to Modula-2. See Having GDB Infer the Source Language, for further details.


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15.4.8.6 Deviations from Standard Modula-2

A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:


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15.4.8.7 Modula-2 Type and Range Checks

Warning: in this release, GDB does not yet perform type or range checking.

GDB considers two Modula-2 variables type equivalent if:

As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error.

Range checking is done on all mathematical operations, assignment, array index bounds, and all built-in functions and procedures.


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15.4.8.8 The Scope Operators :: and .

There are a few subtle differences between the Modula-2 scope operator (.) and the GDB scope operator (::). The two have similar syntax:

module . id
scope :: id

where scope is the name of a module or a procedure, module the name of a module, and id is any declared identifier within your program, except another module.

Using the :: operator makes GDB search the scope specified by scope for the identifier id. If it is not found in the specified scope, then GDB searches all scopes enclosing the one specified by scope.

Using the . operator makes GDB search the current scope for the identifier specified by id that was imported from the definition module specified by module. With this operator, it is an error if the identifier id was not imported from definition module module, or if id is not an identifier in module.


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15.4.8.9 GDB and Modula-2

Some GDB commands have little use when debugging Modula-2 programs. Five subcommands of set print and show print apply specifically to C and C++: ‘vtbl’, ‘demangle’, ‘asm-demangle’, ‘object’, and ‘union’. The first four apply to C++, and the last to the C union type, which has no direct analogue in Modula-2.

The @ operator (see Expressions), while available with any language, is not useful with Modula-2. Its intent is to aid the debugging of dynamic arrays, which cannot be created in Modula-2 as they can in C or C++. However, because an address can be specified by an integral constant, the construct ‘{type}adrexp’ is still useful.

In GDB scripts, the Modula-2 inequality operator # is interpreted as the beginning of a comment. Use <> instead.


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15.4.9 Ada

The extensions made to GDB for Ada only support output from the GNU Ada (GNAT) compiler. Other Ada compilers are not currently supported, and attempting to debug executables produced by them is most likely to be difficult.


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15.4.9.1 Introduction

The Ada mode of GDB supports a fairly large subset of Ada expression syntax, with some extensions. The philosophy behind the design of this subset is

Thus, for brevity, the debugger acts as if all names declared in user-written packages are directly visible, even if they are not visible according to Ada rules, thus making it unnecessary to fully qualify most names with their packages, regardless of context. Where this causes ambiguity, GDB asks the user’s intent.

The debugger will start in Ada mode if it detects an Ada main program. As for other languages, it will enter Ada mode when stopped in a program that was translated from an Ada source file.

While in Ada mode, you may use ‘--’ for comments. This is useful mostly for documenting command files. The standard GDB comment (‘#’) still works at the beginning of a line in Ada mode, but not in the middle (to allow based literals).

The debugger supports limited overloading. Given a subprogram call in which the function symbol has multiple definitions, it will use the number of actual parameters and some information about their types to attempt to narrow the set of definitions. It also makes very limited use of context, preferring procedures to functions in the context of the call command, and functions to procedures elsewhere.


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15.4.9.2 Omissions from Ada

Here are the notable omissions from the subset:


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15.4.9.3 Additions to Ada

As it does for other languages, GDB makes certain generic extensions to Ada (see Expressions):

In addition, GDB provides a few other shortcuts and outright additions specific to Ada:


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15.4.9.4 Stopping at the Very Beginning

It is sometimes necessary to debug the program during elaboration, and before reaching the main procedure. As defined in the Ada Reference Manual, the elaboration code is invoked from a procedure called adainit. To run your program up to the beginning of elaboration, simply use the following two commands: tbreak adainit and run.


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15.4.9.5 Ada Exceptions

A command is provided to list all Ada exceptions:

info exceptions
info exceptions regexp

The info exceptions command allows you to list all Ada exceptions defined within the program being debugged, as well as their addresses. With a regular expression, regexp, as argument, only those exceptions whose names match regexp are listed.

Below is a small example, showing how the command can be used, first without argument, and next with a regular expression passed as an argument.

(gdb) info exceptions
All defined Ada exceptions:
constraint_error: 0x613da0
program_error: 0x613d20
storage_error: 0x613ce0
tasking_error: 0x613ca0
const.aint_global_e: 0x613b00
(gdb) info exceptions const.aint
All Ada exceptions matching regular expression "const.aint":
constraint_error: 0x613da0
const.aint_global_e: 0x613b00

It is also possible to ask GDB to stop your program’s execution when an exception is raised. For more details, see Set Catchpoints.


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15.4.9.6 Extensions for Ada Tasks

Support for Ada tasks is analogous to that for threads (see Threads). GDB provides the following task-related commands:

info tasks

This command shows a list of current Ada tasks, as in the following example:

(gdb) info tasks
  ID       TID P-ID Pri State                 Name
   1   8088000   0   15 Child Activation Wait main_task
   2   80a4000   1   15 Accept Statement      b
   3   809a800   1   15 Child Activation Wait a
*  4   80ae800   3   15 Runnable              c

In this listing, the asterisk before the last task indicates it to be the task currently being inspected.

ID

Represents GDB’s internal task number.

TID

The Ada task ID.

P-ID

The parent’s task ID (GDB’s internal task number).

Pri

The base priority of the task.

State

Current state of the task.

Unactivated

The task has been created but has not been activated. It cannot be executing.

Runnable

The task is not blocked for any reason known to Ada. (It may be waiting for a mutex, though.) It is conceptually "executing" in normal mode.

Terminated

The task is terminated, in the sense of ARM 9.3 (5). Any dependents that were waiting on terminate alternatives have been awakened and have terminated themselves.

Child Activation Wait

The task is waiting for created tasks to complete activation.

Accept Statement

The task is waiting on an accept or selective wait statement.

Waiting on entry call

The task is waiting on an entry call.

Async Select Wait

The task is waiting to start the abortable part of an asynchronous select statement.

Delay Sleep

The task is waiting on a select statement with only a delay alternative open.

Child Termination Wait

The task is sleeping having completed a master within itself, and is waiting for the tasks dependent on that master to become terminated or waiting on a terminate Phase.

Wait Child in Term Alt

The task is sleeping waiting for tasks on terminate alternatives to finish terminating.

Accepting RV with taskno

The task is accepting a rendez-vous with the task taskno.

Name

Name of the task in the program.

info task taskno

This command shows detailled informations on the specified task, as in the following example:

(gdb) info tasks
  ID       TID P-ID Pri State                  Name
   1   8077880    0  15 Child Activation Wait  main_task
*  2   807c468    1  15 Runnable               task_1
(gdb) info task 2
Ada Task: 0x807c468
Name: task_1
Thread: 0x807f378
Parent: 1 (main_task)
Base Priority: 15
State: Runnable
task

This command prints the ID of the current task.

(gdb) info tasks
  ID       TID P-ID Pri State                  Name
   1   8077870    0  15 Child Activation Wait  main_task
*  2   807c458    1  15 Runnable               t
(gdb) task
[Current task is 2]
task taskno

This command is like the thread threadno command (see Threads). It switches the context of debugging from the current task to the given task.

(gdb) info tasks
  ID       TID P-ID Pri State                  Name
   1   8077870    0  15 Child Activation Wait  main_task
*  2   807c458    1  15 Runnable               t
(gdb) task 1
[Switching to task 1]
#0  0x8067726 in pthread_cond_wait ()
(gdb) bt
#0  0x8067726 in pthread_cond_wait ()
#1  0x8056714 in system.os_interface.pthread_cond_wait ()
#2  0x805cb63 in system.task_primitives.operations.sleep ()
#3  0x806153e in system.tasking.stages.activate_tasks ()
#4  0x804aacc in un () at un.adb:5
break linespec task taskno
break linespec task taskno if …

These commands are like the break … thread … command (see Thread Stops). The linespec argument specifies source lines, as described in Specify Location.

Use the qualifier ‘task taskno’ with a breakpoint command to specify that you only want GDB to stop the program when a particular Ada task reaches this breakpoint. The taskno is one of the numeric task identifiers assigned by GDB, shown in the first column of the ‘info tasks’ display.

If you do not specify ‘task taskno’ when you set a breakpoint, the breakpoint applies to all tasks of your program.

You can use the task qualifier on conditional breakpoints as well; in this case, place ‘task taskno’ before the breakpoint condition (before the if).

For example,

(gdb) info tasks
  ID       TID P-ID Pri State                 Name
   1 140022020   0   15 Child Activation Wait main_task
   2 140045060   1   15 Accept/Select Wait    t2
   3 140044840   1   15 Runnable              t1
*  4 140056040   1   15 Runnable              t3
(gdb) b 15 task 2
Breakpoint 5 at 0x120044cb0: file test_task_debug.adb, line 15.
(gdb) cont
Continuing.
task # 1 running
task # 2 running

Breakpoint 5, test_task_debug () at test_task_debug.adb:15
15               flush;
(gdb) info tasks
  ID       TID P-ID Pri State                 Name
   1 140022020   0   15 Child Activation Wait main_task
*  2 140045060   1   15 Runnable              t2
   3 140044840   1   15 Runnable              t1
   4 140056040   1   15 Delay Sleep           t3

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15.4.9.7 Tasking Support when Debugging Core Files

When inspecting a core file, as opposed to debugging a live program, tasking support may be limited or even unavailable, depending on the platform being used. For instance, on x86-linux, the list of tasks is available, but task switching is not supported.

On certain platforms, the debugger needs to perform some memory writes in order to provide Ada tasking support. When inspecting a core file, this means that the core file must be opened with read-write privileges, using the command ‘"set write on"’ (see Patching). Under these circumstances, you should make a backup copy of the core file before inspecting it with GDB.


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15.4.9.8 Tasking Support when using the Ravenscar Profile

The Ravenscar Profile is a subset of the Ada tasking features, specifically designed for systems with safety-critical real-time requirements.

set ravenscar task-switching on

Allows task switching when debugging a program that uses the Ravenscar Profile. This is the default.

set ravenscar task-switching off

Turn off task switching when debugging a program that uses the Ravenscar Profile. This is mostly intended to disable the code that adds support for the Ravenscar Profile, in case a bug in either GDB or in the Ravenscar runtime is preventing GDB from working properly. To be effective, this command should be run before the program is started.

show ravenscar task-switching

Show whether it is possible to switch from task to task in a program using the Ravenscar Profile.


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15.4.9.9 Known Peculiarities of Ada Mode

Besides the omissions listed previously (see Omissions from Ada), we know of several problems with and limitations of Ada mode in GDB, some of which will be fixed with planned future releases of the debugger and the GNU Ada compiler.

Older versions of the compiler sometimes generate erroneous debugging information, resulting in the debugger incorrectly printing the value of affected entities. In some cases, the debugger is able to work around an issue automatically. In other cases, the debugger is able to work around the issue, but the work-around has to be specifically enabled.

set ada trust-PAD-over-XVS on

Configure GDB to strictly follow the GNAT encoding when computing the value of Ada entities, particularly when PAD and PAD___XVS types are involved (see ada/exp_dbug.ads in the GCC sources for a complete description of the encoding used by the GNAT compiler). This is the default.

set ada trust-PAD-over-XVS off

This is related to the encoding using by the GNAT compiler. If GDB sometimes prints the wrong value for certain entities, changing ada trust-PAD-over-XVS to off activates a work-around which may fix the issue. It is always safe to set ada trust-PAD-over-XVS to off, but this incurs a slight performance penalty, so it is recommended to leave this setting to on unless necessary.

Internally, the debugger also relies on the compiler following a number of conventions known as the ‘GNAT Encoding’, all documented in gcc/ada/exp_dbug.ads in the GCC sources. This encoding describes how the debugging information should be generated for certain types. In particular, this convention makes use of descriptive types, which are artificial types generated purely to help the debugger.

These encodings were defined at a time when the debugging information format used was not powerful enough to describe some of the more complex types available in Ada. Since DWARF allows us to express nearly all Ada features, the long-term goal is to slowly replace these descriptive types by their pure DWARF equivalent. To facilitate that transition, a new maintenance option is available to force the debugger to ignore those descriptive types. It allows the user to quickly evaluate how well GDB works without them.

maintenance ada set ignore-descriptive-types [on|off]

Control whether the debugger should ignore descriptive types. The default is not to ignore descriptives types (off).

maintenance ada show ignore-descriptive-types

Show if descriptive types are ignored by GDB.


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15.5 Unsupported Languages

In addition to the other fully-supported programming languages, GDB also provides a pseudo-language, called minimal. It does not represent a real programming language, but provides a set of capabilities close to what the C or assembly languages provide. This should allow most simple operations to be performed while debugging an application that uses a language currently not supported by GDB.

If the language is set to auto, GDB will automatically select this language if the current frame corresponds to an unsupported language.


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16 Examining the Symbol Table

The commands described in this chapter allow you to inquire about the symbols (names of variables, functions and types) defined in your program. This information is inherent in the text of your program and does not change as your program executes. GDB finds it in your program’s symbol table, in the file indicated when you started GDB (see Choosing Files), or by one of the file-management commands (see Commands to Specify Files).

Occasionally, you may need to refer to symbols that contain unusual characters, which GDB ordinarily treats as word delimiters. The most frequent case is in referring to static variables in other source files (see Program Variables). File names are recorded in object files as debugging symbols, but GDB would ordinarily parse a typical file name, like foo.c, as the three words ‘foo’ ‘.’ ‘c’. To allow GDB to recognize ‘foo.c’ as a single symbol, enclose it in single quotes; for example,

p 'foo.c'::x

looks up the value of x in the scope of the file foo.c.

set case-sensitive on
set case-sensitive off
set case-sensitive auto

Normally, when GDB looks up symbols, it matches their names with case sensitivity determined by the current source language. Occasionally, you may wish to control that. The command set case-sensitive lets you do that by specifying on for case-sensitive matches or off for case-insensitive ones. If you specify auto, case sensitivity is reset to the default suitable for the source language. The default is case-sensitive matches for all languages except for Fortran, for which the default is case-insensitive matches.

show case-sensitive

This command shows the current setting of case sensitivity for symbols lookups.

set print type methods
set print type methods on
set print type methods off

Normally, when GDB prints a class, it displays any methods declared in that class. You can control this behavior either by passing the appropriate flag to ptype, or using set print type methods. Specifying on will cause GDB to display the methods; this is the default. Specifying off will cause GDB to omit the methods.

show print type methods

This command shows the current setting of method display when printing classes.

set print type typedefs
set print type typedefs on
set print type typedefs off

Normally, when GDB prints a class, it displays any typedefs defined in that class. You can control this behavior either by passing the appropriate flag to ptype, or using set print type typedefs. Specifying on will cause GDB to display the typedef definitions; this is the default. Specifying off will cause GDB to omit the typedef definitions. Note that this controls whether the typedef definition itself is printed, not whether typedef names are substituted when printing other types.

show print type typedefs

This command shows the current setting of typedef display when printing classes.

info address symbol

Describe where the data for symbol is stored. For a register variable, this says which register it is kept in. For a non-register local variable, this prints the stack-frame offset at which the variable is always stored.

Note the contrast with ‘print &symbol’, which does not work at all for a register variable, and for a stack local variable prints the exact address of the current instantiation of the variable.

info symbol addr

Print the name of a symbol which is stored at the address addr. If no symbol is stored exactly at addr, GDB prints the nearest symbol and an offset from it:

(gdb) info symbol 0x54320
_initialize_vx + 396 in section .text

This is the opposite of the info address command. You can use it to find out the name of a variable or a function given its address.

For dynamically linked executables, the name of executable or shared library containing the symbol is also printed:

(gdb) info symbol 0x400225
_start + 5 in section .text of /tmp/a.out
(gdb) info symbol 0x2aaaac2811cf
__read_nocancel + 6 in section .text of /usr/lib64/libc.so.6
whatis[/flags] [arg]

Print the data type of arg, which can be either an expression or a name of a data type. With no argument, print the data type of $, the last value in the value history.

If arg is an expression (see Expressions), it is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place.

If arg is a variable or an expression, whatis prints its literal type as it is used in the source code. If the type was defined using a typedef, whatis will not print the data type underlying the typedef. If the type of the variable or the expression is a compound data type, such as struct or class, whatis never prints their fields or methods. It just prints the struct/class name (a.k.a. its tag). If you want to see the members of such a compound data type, use ptype.

If arg is a type name that was defined using typedef, whatis unrolls only one level of that typedef. Unrolling means that whatis will show the underlying type used in the typedef declaration of arg. However, if that underlying type is also a typedef, whatis will not unroll it.

For C code, the type names may also have the form ‘class class-name’, ‘struct struct-tag’, ‘union union-tag’ or ‘enum enum-tag’.

flags can be used to modify how the type is displayed. Available flags are:

r

Display in “raw” form. Normally, GDB substitutes template parameters and typedefs defined in a class when printing the class’ members. The /r flag disables this.

m

Do not print methods defined in the class.

M

Print methods defined in the class. This is the default, but the flag exists in case you change the default with set print type methods.

t

Do not print typedefs defined in the class. Note that this controls whether the typedef definition itself is printed, not whether typedef names are substituted when printing other types.

T

Print typedefs defined in the class. This is the default, but the flag exists in case you change the default with set print type typedefs.

ptype[/flags] [arg]

ptype accepts the same arguments as whatis, but prints a detailed description of the type, instead of just the name of the type. See Expressions.

Contrary to whatis, ptype always unrolls any typedefs in its argument declaration, whether the argument is a variable, expression, or a data type. This means that ptype of a variable or an expression will not print literally its type as present in the source code—use whatis for that. typedefs at the pointer or reference targets are also unrolled. Only typedefs of fields, methods and inner class typedefs of structs, classes and unions are not unrolled even with ptype.

For example, for this variable declaration:

typedef double real_t;
struct complex { real_t real; double imag; };
typedef struct complex complex_t;
complex_t var;
real_t *real_pointer_var;

the two commands give this output:

(gdb) whatis var
type = complex_t
(gdb) ptype var
type = struct complex {
    real_t real;
    double imag;
}
(gdb) whatis complex_t
type = struct complex
(gdb) whatis struct complex
type = struct complex
(gdb) ptype struct complex
type = struct complex {
    real_t real;
    double imag;
}
(gdb) whatis real_pointer_var
type = real_t *
(gdb) ptype real_pointer_var
type = double *

As with whatis, using ptype without an argument refers to the type of $, the last value in the value history.

Sometimes, programs use opaque data types or incomplete specifications of complex data structure. If the debug information included in the program does not allow GDB to display a full declaration of the data type, it will say ‘<incomplete type>’. For example, given these declarations:

    struct foo;
    struct foo *fooptr;

but no definition for struct foo itself, GDB will say:

  (gdb) ptype foo
  $1 = <incomplete type>

“Incomplete type” is C terminology for data types that are not completely specified.

info types regexp
info types

Print a brief description of all types whose names match the regular expression regexp (or all types in your program, if you supply no argument). Each complete typename is matched as though it were a complete line; thus, ‘i type value’ gives information on all types in your program whose names include the string value, but ‘i type ^value$’ gives information only on types whose complete name is value.

This command differs from ptype in two ways: first, like whatis, it does not print a detailed description; second, it lists all source files where a type is defined.

info type-printers

Versions of GDB that ship with Python scripting enabled may have “type printers” available. When using ptype or whatis, these printers are consulted when the name of a type is needed. See Type Printing API, for more information on writing type printers.

info type-printers displays all the available type printers.

enable type-printer name
disable type-printer name

These commands can be used to enable or disable type printers.

info scope location

List all the variables local to a particular scope. This command accepts a location argument—a function name, a source line, or an address preceded by a ‘*’, and prints all the variables local to the scope defined by that location. (See Specify Location, for details about supported forms of location.) For example:

(gdb) info scope command_line_handler
Scope for command_line_handler:
Symbol rl is an argument at stack/frame offset 8, length 4.
Symbol linebuffer is in static storage at address 0x150a18, length 4.
Symbol linelength is in static storage at address 0x150a1c, length 4.
Symbol p is a local variable in register $esi, length 4.
Symbol p1 is a local variable in register $ebx, length 4.
Symbol nline is a local variable in register $edx, length 4.
Symbol repeat is a local variable at frame offset -8, length 4.

This command is especially useful for determining what data to collect during a trace experiment, see collect.

info source

Show information about the current source file—that is, the source file for the function containing the current point of execution:

info sources

Print the names of all source files in your program for which there is debugging information, organized into two lists: files whose symbols have already been read, and files whose symbols will be read when needed.

info functions

Print the names and data types of all defined functions.

info functions regexp

Print the names and data types of all defined functions whose names contain a match for regular expression regexp. Thus, ‘info fun step’ finds all functions whose names include step; ‘info fun ^step’ finds those whose names start with step. If a function name contains characters that conflict with the regular expression language (e.g. ‘operator*()’), they may be quoted with a backslash.

info variables

Print the names and data types of all variables that are defined outside of functions (i.e. excluding local variables).

info variables regexp

Print the names and data types of all variables (except for local variables) whose names contain a match for regular expression regexp.

info classes
info classes regexp

Display all Objective-C classes in your program, or (with the regexp argument) all those matching a particular regular expression.

info selectors
info selectors regexp

Display all Objective-C selectors in your program, or (with the regexp argument) all those matching a particular regular expression.

set opaque-type-resolution on

Tell GDB to resolve opaque types. An opaque type is a type declared as a pointer to a struct, class, or union—for example, struct MyType *—that is used in one source file although the full declaration of struct MyType is in another source file. The default is on.

A change in the setting of this subcommand will not take effect until the next time symbols for a file are loaded.

set opaque-type-resolution off

Tell GDB not to resolve opaque types. In this case, the type is printed as follows:

{<no data fields>}
show opaque-type-resolution

Show whether opaque types are resolved or not.

set print symbol-loading
set print symbol-loading full
set print symbol-loading brief
set print symbol-loading off

The set print symbol-loading command allows you to control the printing of messages when GDB loads symbol information. By default a message is printed for the executable and one for each shared library, and normally this is what you want. However, when debugging apps with large numbers of shared libraries these messages can be annoying. When set to brief a message is printed for each executable, and when GDB loads a collection of shared libraries at once it will only print one message regardless of the number of shared libraries. When set to off no messages are printed.

show print symbol-loading

Show whether messages will be printed when a GDB command entered from the keyboard causes symbol information to be loaded.

maint print symbols filename
maint print psymbols filename
maint print msymbols filename

Write a dump of debugging symbol data into the file filename. These commands are used to debug the GDB symbol-reading code. Only symbols with debugging data are included. If you use ‘maint print symbols’, GDB includes all the symbols for which it has already collected full details: that is, filename reflects symbols for only those files whose symbols GDB has read. You can use the command info sources to find out which files these are. If you use ‘maint print psymbols’ instead, the dump shows information about symbols that GDB only knows partially—that is, symbols defined in files that GDB has skimmed, but not yet read completely. Finally, ‘maint print msymbols’ dumps just the minimal symbol information required for each object file from which GDB has read some symbols. See Commands to Specify Files, for a discussion of how GDB reads symbols (in the description of symbol-file).

maint info symtabs [ regexp ]
maint info psymtabs [ regexp ]

List the struct symtab or struct partial_symtab structures whose names match regexp. If regexp is not given, list them all. The output includes expressions which you can copy into a GDB debugging this one to examine a particular structure in more detail. For example:

(gdb) maint info psymtabs dwarf2read
{ objfile /home/gnu/build/gdb/gdb
  ((struct objfile *) 0x82e69d0)
  { psymtab /home/gnu/src/gdb/dwarf2read.c
    ((struct partial_symtab *) 0x8474b10)
    readin no
    fullname (null)
    text addresses 0x814d3c8 -- 0x8158074
    globals (* (struct partial_symbol **) 0x8507a08 @ 9)
    statics (* (struct partial_symbol **) 0x40e95b78 @ 2882)
    dependencies (none)
  }
}
(gdb) maint info symtabs
(gdb)

We see that there is one partial symbol table whose filename contains the string ‘dwarf2read’, belonging to the ‘gdb’ executable; and we see that GDB has not read in any symtabs yet at all. If we set a breakpoint on a function, that will cause GDB to read the symtab for the compilation unit containing that function:

(gdb) break dwarf2_psymtab_to_symtab
Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c,
line 1574.
(gdb) maint info symtabs
{ objfile /home/gnu/build/gdb/gdb
  ((struct objfile *) 0x82e69d0)
  { symtab /home/gnu/src/gdb/dwarf2read.c
    ((struct symtab *) 0x86c1f38)
    dirname (null)
    fullname (null)
    blockvector ((struct blockvector *) 0x86c1bd0) (primary)
    linetable ((struct linetable *) 0x8370fa0)
    debugformat DWARF 2
  }
}
(gdb)

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17 Altering Execution

Once you think you have found an error in your program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run. You can find the answer by experiment, using the GDB features for altering execution of the program.

For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function.


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17.1 Assignment to Variables

To alter the value of a variable, evaluate an assignment expression. See Expressions. For example,

print x=4

stores the value 4 into the variable x, and then prints the value of the assignment expression (which is 4). See Using GDB with Different Languages, for more information on operators in supported languages.

If you are not interested in seeing the value of the assignment, use the set command instead of the print command. set is really the same as print except that the expression’s value is not printed and is not put in the value history (see Value History). The expression is evaluated only for its effects.

If the beginning of the argument string of the set command appears identical to a set subcommand, use the set variable command instead of just set. This command is identical to set except for its lack of subcommands. For example, if your program has a variable width, you get an error if you try to set a new value with just ‘set width=13’, because GDB has the command set width:

(gdb) whatis width
type = double
(gdb) p width
$4 = 13
(gdb) set width=47
Invalid syntax in expression.

The invalid expression, of course, is ‘=47’. In order to actually set the program’s variable width, use

(gdb) set var width=47

Because the set command has many subcommands that can conflict with the names of program variables, it is a good idea to use the set variable command instead of just set. For example, if your program has a variable g, you run into problems if you try to set a new value with just ‘set g=4’, because GDB has the command set gnutarget, abbreviated set g:

(gdb) whatis g
type = double
(gdb) p g
$1 = 1
(gdb) set g=4
(gdb) p g
$2 = 1
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/smith/cc_progs/a.out
"/home/smith/cc_progs/a.out": can't open to read symbols:
                                 Invalid bfd target.
(gdb) show g
The current BFD target is "=4".

The program variable g did not change, and you silently set the gnutarget to an invalid value. In order to set the variable g, use

(gdb) set var g=4

GDB allows more implicit conversions in assignments than C; you can freely store an integer value into a pointer variable or vice versa, and you can convert any structure to any other structure that is the same length or shorter.

To store values into arbitrary places in memory, use the ‘{…}’ construct to generate a value of specified type at a specified address (see Expressions). For example, {int}0x83040 refers to memory location 0x83040 as an integer (which implies a certain size and representation in memory), and

set {int}0x83040 = 4

stores the value 4 into that memory location.


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17.2 Continuing at a Different Address

Ordinarily, when you continue your program, you do so at the place where it stopped, with the continue command. You can instead continue at an address of your own choosing, with the following commands:

jump linespec
j linespec
jump location
j location

Resume execution at line linespec or at address given by location. Execution stops again immediately if there is a breakpoint there. See Specify Location, for a description of the different forms of linespec and location. It is common practice to use the tbreak command in conjunction with jump. See Setting Breakpoints.

The jump command does not change the current stack frame, or the stack pointer, or the contents of any memory location or any register other than the program counter. If line linespec is in a different function from the one currently executing, the results may be bizarre if the two functions expect different patterns of arguments or of local variables. For this reason, the jump command requests confirmation if the specified line is not in the function currently executing. However, even bizarre results are predictable if you are well acquainted with the machine-language code of your program.

On many systems, you can get much the same effect as the jump command by storing a new value into the register $pc. The difference is that this does not start your program running; it only changes the address of where it will run when you continue. For example,

set $pc = 0x485

makes the next continue command or stepping command execute at address 0x485, rather than at the address where your program stopped. See Continuing and Stepping.

The most common occasion to use the jump command is to back up—perhaps with more breakpoints set—over a portion of a program that has already executed, in order to examine its execution in more detail.


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17.3 Giving your Program a Signal

signal signal

Resume execution where your program is stopped, but immediately give it the signal signal. The signal can be the name or the number of a signal. For example, on many systems signal 2 and signal SIGINT are both ways of sending an interrupt signal.

Alternatively, if signal is zero, continue execution without giving a signal. This is useful when your program stopped on account of a signal and would ordinarily see the signal when resumed with the continue command; ‘signal 0’ causes it to resume without a signal.

Note: When resuming a multi-threaded program, signal is delivered to the currently selected thread, not the thread that last reported a stop. This includes the situation where a thread was stopped due to a signal. So if you want to continue execution suppressing the signal that stopped a thread, you should select that same thread before issuing the ‘signal 0’ command. If you issue the ‘signal 0’ command with another thread as the selected one, GDB detects that and asks for confirmation.

Invoking the signal command is not the same as invoking the kill utility from the shell. Sending a signal with kill causes GDB to decide what to do with the signal depending on the signal handling tables (see Signals). The signal command passes the signal directly to your program.

signal does not repeat when you press RET a second time after executing the command.

queue-signal signal

Queue signal to be delivered immediately to the current thread when execution of the thread resumes. The signal can be the name or the number of a signal. For example, on many systems signal 2 and signal SIGINT are both ways of sending an interrupt signal. The handling of the signal must be set to pass the signal to the program, otherwise GDB will report an error. You can control the handling of signals from GDB with the handle command (see Signals).

Alternatively, if signal is zero, any currently queued signal for the current thread is discarded and when execution resumes no signal will be delivered. This is useful when your program stopped on account of a signal and would ordinarily see the signal when resumed with the continue command.

This command differs from the signal command in that the signal is just queued, execution is not resumed. And queue-signal cannot be used to pass a signal whose handling state has been set to nopass (see Signals).


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17.4 Returning from a Function

return
return expression

You can cancel execution of a function call with the return command. If you give an expression argument, its value is used as the function’s return value.

When you use return, GDB discards the selected stack frame (and all frames within it). You can think of this as making the discarded frame return prematurely. If you wish to specify a value to be returned, give that value as the argument to return.

This pops the selected stack frame (see Selecting a Frame), and any other frames inside of it, leaving its caller as the innermost remaining frame. That frame becomes selected. The specified value is stored in the registers used for returning values of functions.

The return command does not resume execution; it leaves the program stopped in the state that would exist if the function had just returned. In contrast, the finish command (see Continuing and Stepping) resumes execution until the selected stack frame returns naturally.

GDB needs to know how the expression argument should be set for the inferior. The concrete registers assignment depends on the OS ABI and the type being returned by the selected stack frame. For example it is common for OS ABI to return floating point values in FPU registers while integer values in CPU registers. Still some ABIs return even floating point values in CPU registers. Larger integer widths (such as long long int) also have specific placement rules. GDB already knows the OS ABI from its current target so it needs to find out also the type being returned to make the assignment into the right register(s).

Normally, the selected stack frame has debug info. GDB will always use the debug info instead of the implicit type of expression when the debug info is available. For example, if you type return -1, and the function in the current stack frame is declared to return a long long int, GDB transparently converts the implicit int value of -1 into a long long int:

Breakpoint 1, func () at gdb.base/return-nodebug.c:29
29        return 31;
(gdb) return -1
Make func return now? (y or n) y
#0  0x004004f6 in main () at gdb.base/return-nodebug.c:43
43        printf ("result=%lld\n", func ());
(gdb)

However, if the selected stack frame does not have a debug info, e.g., if the function was compiled without debug info, GDB has to find out the type to return from user. Specifying a different type by mistake may set the value in different inferior registers than the caller code expects. For example, typing return -1 with its implicit type int would set only a part of a long long int result for a debug info less function (on 32-bit architectures). Therefore the user is required to specify the return type by an appropriate cast explicitly:

Breakpoint 2, 0x0040050b in func ()
(gdb) return -1
Return value type not available for selected stack frame.
Please use an explicit cast of the value to return.
(gdb) return (long long int) -1
Make selected stack frame return now? (y or n) y
#0  0x00400526 in main ()
(gdb)

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17.5 Calling Program Functions

print expr

Evaluate the expression expr and display the resulting value. The expression may include calls to functions in the program being debugged.

call expr

Evaluate the expression expr without displaying void returned values.

You can use this variant of the print command if you want to execute a function from your program that does not return anything (a.k.a. a void function), but without cluttering the output with void returned values that GDB will otherwise print. If the result is not void, it is printed and saved in the value history.

It is possible for the function you call via the print or call command to generate a signal (e.g., if there’s a bug in the function, or if you passed it incorrect arguments). What happens in that case is controlled by the set unwindonsignal command.

Similarly, with a C++ program it is possible for the function you call via the print or call command to generate an exception that is not handled due to the constraints of the dummy frame. In this case, any exception that is raised in the frame, but has an out-of-frame exception handler will not be found. GDB builds a dummy-frame for the inferior function call, and the unwinder cannot seek for exception handlers outside of this dummy-frame. What happens in that case is controlled by the set unwind-on-terminating-exception command.

set unwindonsignal

Set unwinding of the stack if a signal is received while in a function that GDB called in the program being debugged. If set to on, GDB unwinds the stack it created for the call and restores the context to what it was before the call. If set to off (the default), GDB stops in the frame where the signal was received.

show unwindonsignal

Show the current setting of stack unwinding in the functions called by GDB.

set unwind-on-terminating-exception

Set unwinding of the stack if a C++ exception is raised, but left unhandled while in a function that GDB called in the program being debugged. If set to on (the default), GDB unwinds the stack it created for the call and restores the context to what it was before the call. If set to off, GDB the exception is delivered to the default C++ exception handler and the inferior terminated.

show unwind-on-terminating-exception

Show the current setting of stack unwinding in the functions called by GDB.

Sometimes, a function you wish to call is actually a weak alias for another function. In such case, GDB might not pick up the type information, including the types of the function arguments, which causes GDB to call the inferior function incorrectly. As a result, the called function will function erroneously and may even crash. A solution to that is to use the name of the aliased function instead.


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17.6 Patching Programs

By default, GDB opens the file containing your program’s executable code (or the corefile) read-only. This prevents accidental alterations to machine code; but it also prevents you from intentionally patching your program’s binary.

If you’d like to be able to patch the binary, you can specify that explicitly with the set write command. For example, you might want to turn on internal debugging flags, or even to make emergency repairs.

set write on
set write off

If you specify ‘set write on’, GDB opens executable and core files for both reading and writing; if you specify set write off (the default), GDB opens them read-only.

If you have already loaded a file, you must load it again (using the exec-file or core-file command) after changing set write, for your new setting to take effect.

show write

Display whether executable files and core files are opened for writing as well as reading.


Next: , Previous: , Up: Top   [Contents][Index]

18 GDB Files

GDB needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start your program. To debug a core dump of a previous run, you must also tell GDB the name of the core dump file.


Next: , Up: GDB Files   [Contents][Index]

18.1 Commands to Specify Files

You may want to specify executable and core dump file names. The usual way to do this is at start-up time, using the arguments to GDB’s start-up commands (see Getting In and Out of GDB).

Occasionally it is necessary to change to a different file during a GDB session. Or you may run GDB and forget to specify a file you want to use. Or you are debugging a remote target via gdbserver (see Using the gdbserver Program). In these situations the GDB commands to specify new files are useful.

file filename

Use filename as the program to be debugged. It is read for its symbols and for the contents of pure memory. It is also the program executed when you use the run command. If you do not specify a directory and the file is not found in the GDB working directory, GDB uses the environment variable PATH as a list of directories to search, just as the shell does when looking for a program to run. You can change the value of this variable, for both GDB and your program, using the path command.

You can load unlinked object .o files into GDB using the file command. You will not be able to “run” an object file, but you can disassemble functions and inspect variables. Also, if the underlying BFD functionality supports it, you could use gdb -write to patch object files using this technique. Note that GDB can neither interpret nor modify relocations in this case, so branches and some initialized variables will appear to go to the wrong place. But this feature is still handy from time to time.

file

file with no argument makes GDB discard any information it has on both executable file and the symbol table.

exec-file [ filename ]

Specify that the program to be run (but not the symbol table) is found in filename. GDB searches the environment variable PATH if necessary to locate your program. Omitting filename means to discard information on the executable file.

symbol-file [ filename ]

Read symbol table information from file filename. PATH is searched when necessary. Use the file command to get both symbol table and program to run from the same file.

symbol-file with no argument clears out GDB information on your program’s symbol table.

The symbol-file command causes GDB to forget the contents of some breakpoints and auto-display expressions. This is because they may contain pointers to the internal data recording symbols and data types, which are part of the old symbol table data being discarded inside GDB.

symbol-file does not repeat if you press RET again after executing it once.

When GDB is configured for a particular environment, it understands debugging information in whatever format is the standard generated for that environment; you may use either a GNU compiler, or other compilers that adhere to the local conventions. Best results are usually obtained from GNU compilers; for example, using GCC you can generate debugging information for optimized code.

For most kinds of object files, with the exception of old SVR3 systems using COFF, the symbol-file command does not normally read the symbol table in full right away. Instead, it scans the symbol table quickly to find which source files and which symbols are present. The details are read later, one source file at a time, as they are needed.

The purpose of this two-stage reading strategy is to make GDB start up faster. For the most part, it is invisible except for occasional pauses while the symbol table details for a particular source file are being read. (The set verbose command can turn these pauses into messages if desired. See Optional Warnings and Messages.)

We have not implemented the two-stage strategy for COFF yet. When the symbol table is stored in COFF format, symbol-file reads the symbol table data in full right away. Note that “stabs-in-COFF” still does the two-stage strategy, since the debug info is actually in stabs format.

symbol-file [ -readnow ] filename
file [ -readnow ] filename

You can override the GDB two-stage strategy for reading symbol tables by using the ‘-readnow’ option with any of the commands that load symbol table information, if you want to be sure GDB has the entire symbol table available.

core-file [filename]
core

Specify the whereabouts of a core dump file to be used as the “contents of memory”. Traditionally, core files contain only some parts of the address space of the process that generated them; GDB can access the executable file itself for other parts.

core-file with no argument specifies that no core file is to be used.

Note that the core file is ignored when your program is actually running under GDB. So, if you have been running your program and you wish to debug a core file instead, you must kill the subprocess in which the program is running. To do this, use the kill command (see Killing the Child Process).

add-symbol-file filename address
add-symbol-file filename address [ -readnow ]
add-symbol-file filename address -s section address

The add-symbol-file command reads additional symbol table information from the file filename. You would use this command when filename has been dynamically loaded (by some other means) into the program that is running. The address should give the memory address at which the file has been loaded; GDB cannot figure this out for itself. You can additionally specify an arbitrary number of ‘-s section address’ pairs, to give an explicit section name and base address for that section. You can specify any address as an expression.

The symbol table of the file filename is added to the symbol table originally read with the symbol-file command. You can use the add-symbol-file command any number of times; the new symbol data thus read is kept in addition to the old.

Changes can be reverted using the command remove-symbol-file.

Although filename is typically a shared library file, an executable file, or some other object file which has been fully relocated for loading into a process, you can also load symbolic information from relocatable .o files, as long as:

Some embedded operating systems, like Sun Chorus and VxWorks, can load relocatable files into an already running program; such systems typically make the requirements above easy to meet. However, it’s important to recognize that many native systems use complex link procedures (.linkonce section factoring and C++ constructor table assembly, for example) that make the requirements difficult to meet. In general, one cannot assume that using add-symbol-file to read a relocatable object file’s symbolic information will have the same effect as linking the relocatable object file into the program in the normal way.

add-symbol-file does not repeat if you press RET after using it.

remove-symbol-file filename
remove-symbol-file -a address

Remove a symbol file added via the add-symbol-file command. The file to remove can be identified by its filename or by an address that lies within the boundaries of this symbol file in memory. Example:

(gdb) add-symbol-file /home/user/gdb/mylib.so 0x7ffff7ff9480
add symbol table from file "/home/user/gdb/mylib.so" at
    .text_addr = 0x7ffff7ff9480
(y or n) y
Reading symbols from /home/user/gdb/mylib.so...done.
(gdb) remove-symbol-file -a 0x7ffff7ff9480
Remove symbol table from file "/home/user/gdb/mylib.so"? (y or n) y
(gdb)

remove-symbol-file does not repeat if you press RET after using it.

add-symbol-file-from-memory address

Load symbols from the given address in a dynamically loaded object file whose image is mapped directly into the inferior’s memory. For example, the Linux kernel maps a syscall DSO into each process’s address space; this DSO provides kernel-specific code for some system calls. The argument can be any expression whose evaluation yields the address of the file’s shared object file header. For this command to work, you must have used symbol-file or exec-file commands in advance.

add-shared-symbol-files library-file
assf library-file

This command is deprecated and will be removed in future versions of GDB. Use the sharedlibrary command instead.

The add-shared-symbol-files command can currently be used only in the Cygwin build of GDB on MS-Windows OS, where it is an alias for the dll-symbols command (see Cygwin Native). GDB automatically looks for shared libraries, however if GDB does not find yours, you can invoke add-shared-symbol-files. It takes one argument: the shared library’s file name. assf is a shorthand alias for add-shared-symbol-files.

section section addr

The section command changes the base address of the named section of the exec file to addr. This can be used if the exec file does not contain section addresses, (such as in the a.out format), or when the addresses specified in the file itself are wrong. Each section must be changed separately. The info files command, described below, lists all the sections and their addresses.

info files
info target

info files and info target are synonymous; both print the current target (see Specifying a Debugging Target), including the names of the executable and core dump files currently in use by GDB, and the files from which symbols were loaded. The command help target lists all possible targets rather than current ones.

maint info sections

Another command that can give you extra information about program sections is maint info sections. In addition to the section information displayed by info files, this command displays the flags and file offset of each section in the executable and core dump files. In addition, maint info sections provides the following command options (which may be arbitrarily combined):

ALLOBJ

Display sections for all loaded object files, including shared libraries.

sections

Display info only for named sections.

section-flags

Display info only for sections for which section-flags are true. The section flags that GDB currently knows about are:

ALLOC

Section will have space allocated in the process when loaded. Set for all sections except those containing debug information.

LOAD

Section will be loaded from the file into the child process memory. Set for pre-initialized code and data, clear for .bss sections.

RELOC

Section needs to be relocated before loading.

READONLY

Section cannot be modified by the child process.

CODE

Section contains executable code only.

DATA

Section contains data only (no executable code).

ROM

Section will reside in ROM.

CONSTRUCTOR

Section contains data for constructor/destructor lists.

HAS_CONTENTS

Section is not empty.

NEVER_LOAD

An instruction to the linker to not output the section.

COFF_SHARED_LIBRARY

A notification to the linker that the section contains COFF shared library information.

IS_COMMON

Section contains common symbols.

set trust-readonly-sections on

Tell GDB that readonly sections in your object file really are read-only (i.e. that their contents will not change). In that case, GDB can fetch values from these sections out of the object file, rather than from the target program. For some targets (notably embedded ones), this can be a significant enhancement to debugging performance.

The default is off.

set trust-readonly-sections off

Tell GDB not to trust readonly sections. This means that the contents of the section might change while the program is running, and must therefore be fetched from the target when needed.

show trust-readonly-sections

Show the current setting of trusting readonly sections.

All file-specifying commands allow both absolute and relative file names as arguments. GDB always converts the file name to an absolute file name and remembers it that way.

GDB supports GNU/Linux, MS-Windows, HP-UX, SunOS, SVr4, Irix, and IBM RS/6000 AIX shared libraries.

On MS-Windows GDB must be linked with the Expat library to support shared libraries. See Expat.

GDB automatically loads symbol definitions from shared libraries when you use the run command, or when you examine a core file. (Before you issue the run command, GDB does not understand references to a function in a shared library, however—unless you are debugging a core file).

On HP-UX, if the program loads a library explicitly, GDB automatically loads the symbols at the time of the shl_load call.

There are times, however, when you may wish to not automatically load symbol definitions from shared libraries, such as when they are particularly large or there are many of them.

To control the automatic loading of shared library symbols, use the commands:

set auto-solib-add mode

If mode is on, symbols from all shared object libraries will be loaded automatically when the inferior begins execution, you attach to an independently started inferior, or when the dynamic linker informs GDB that a new library has been loaded. If mode is off, symbols must be loaded manually, using the sharedlibrary command. The default value is on.

If your program uses lots of shared libraries with debug info that takes large amounts of memory, you can decrease the GDB memory footprint by preventing it from automatically loading the symbols from shared libraries. To that end, type set auto-solib-add off before running the inferior, then load each library whose debug symbols you do need with sharedlibrary regexp, where regexp is a regular expression that matches the libraries whose symbols you want to be loaded.

show auto-solib-add

Display the current autoloading mode.

To explicitly load shared library symbols, use the sharedlibrary command:

info share regex
info sharedlibrary regex

Print the names of the shared libraries which are currently loaded that match regex. If regex is omitted then print all shared libraries that are loaded.

sharedlibrary regex
share regex

Load shared object library symbols for files matching a Unix regular expression. As with files loaded automatically, it only loads shared libraries required by your program for a core file or after typing run. If regex is omitted all shared libraries required by your program are loaded.

nosharedlibrary

Unload all shared object library symbols. This discards all symbols that have been loaded from all shared libraries. Symbols from shared libraries that were loaded by explicit user requests are not discarded.

Sometimes you may wish that GDB stops and gives you control when any of shared library events happen. The best way to do this is to use catch load and catch unload (see Set Catchpoints).

GDB also supports the the set stop-on-solib-events command for this. This command exists for historical reasons. It is less useful than setting a catchpoint, because it does not allow for conditions or commands as a catchpoint does.

set stop-on-solib-events

This command controls whether GDB should give you control when the dynamic linker notifies it about some shared library event. The most common event of interest is loading or unloading of a new shared library.

show stop-on-solib-events

Show whether GDB stops and gives you control when shared library events happen.

Shared libraries are also supported in many cross or remote debugging configurations. GDB needs to have access to the target’s libraries; this can be accomplished either by providing copies of the libraries on the host system, or by asking GDB to automatically retrieve the libraries from the target. If copies of the target libraries are provided, they need to be the same as the target libraries, although the copies on the target can be stripped as long as the copies on the host are not.

For remote debugging, you need to tell GDB where the target libraries are, so that it can load the correct copies—otherwise, it may try to load the host’s libraries. GDB has two variables to specify the search directories for target libraries.

set sysroot path

Use path as the system root for the program being debugged. Any absolute shared library paths will be prefixed with path; many runtime loaders store the absolute paths to the shared library in the target program’s memory. If you use set sysroot to find shared libraries, they need to be laid out in the same way that they are on the target, with e.g. a /lib and /usr/lib hierarchy under path.

If path starts with the sequence remote:, GDB will retrieve the target libraries from the remote system. This is only supported when using a remote target that supports the remote get command (see Sending files to a remote system). The part of path following the initial remote: (if present) is used as system root prefix on the remote file system. 12

For targets with an MS-DOS based filesystem, such as MS-Windows and SymbianOS, GDB tries prefixing a few variants of the target absolute file name with path. But first, on Unix hosts, GDB converts all backslash directory separators into forward slashes, because the backslash is not a directory separator on Unix:

  c:\foo\bar.dll ⇒ c:/foo/bar.dll

Then, GDB attempts prefixing the target file name with path, and looks for the resulting file name in the host file system:

  c:/foo/bar.dll ⇒ /path/to/sysroot/c:/foo/bar.dll

If that does not find the shared library, GDB tries removing the ‘:’ character from the drive spec, both for convenience, and, for the case of the host file system not supporting file names with colons:

  c:/foo/bar.dll ⇒ /path/to/sysroot/c/foo/bar.dll

This makes it possible to have a system root that mirrors a target with more than one drive. E.g., you may want to setup your local copies of the target system shared libraries like so (note ‘c’ vs ‘z’):

 /path/to/sysroot/c/sys/bin/foo.dll
 /path/to/sysroot/c/sys/bin/bar.dll
 /path/to/sysroot/z/sys/bin/bar.dll

and point the system root at /path/to/sysroot, so that GDB can find the correct copies of both c:\sys\bin\foo.dll, and z:\sys\bin\bar.dll.

If that still does not find the shared library, GDB tries removing the whole drive spec from the target file name:

  c:/foo/bar.dll ⇒ /path/to/sysroot/foo/bar.dll

This last lookup makes it possible to not care about the drive name, if you don’t want or need to.

The set solib-absolute-prefix command is an alias for set sysroot.

You can set the default system root by using the configure-time ‘--with-sysroot’ option. If the system root is inside GDB’s configured binary prefix (set with ‘--prefix’ or ‘--exec-prefix’), then the default system root will be updated automatically if the installed GDB is moved to a new location.

show sysroot

Display the current shared library prefix.

set solib-search-path path

If this variable is set, path is a colon-separated list of directories to search for shared libraries. ‘solib-search-path’ is used after ‘sysroot’ fails to locate the library, or if the path to the library is relative instead of absolute. If you want to use ‘solib-search-path’ instead of ‘sysroot’, be sure to set ‘sysroot’ to a nonexistent directory to prevent GDB from finding your host’s libraries. ‘sysroot’ is preferred; setting it to a nonexistent directory may interfere with automatic loading of shared library symbols.

show solib-search-path

Display the current shared library search path.

set target-file-system-kind kind

Set assumed file system kind for target reported file names.

Shared library file names as reported by the target system may not make sense as is on the system GDB is running on. For example, when remote debugging a target that has MS-DOS based file system semantics, from a Unix host, the target may be reporting to GDB a list of loaded shared libraries with file names such as c:\Windows\kernel32.dll. On Unix hosts, there’s no concept of drive letters, so the ‘c:\’ prefix is not normally understood as indicating an absolute file name, and neither is the backslash normally considered a directory separator character. In that case, the native file system would interpret this whole absolute file name as a relative file name with no directory components. This would make it impossible to point GDB at a copy of the remote target’s shared libraries on the host using set sysroot, and impractical with set solib-search-path. Setting target-file-system-kind to dos-based tells GDB to interpret such file names similarly to how the target would, and to map them to file names valid on GDB’s native file system semantics. The value of kind can be "auto", in addition to one of the supported file system kinds. In that case, GDB tries to determine the appropriate file system variant based on the current target’s operating system (see Configuring the Current ABI). The supported file system settings are:

unix

Instruct GDB to assume the target file system is of Unix kind. Only file names starting the forward slash (‘/’) character are considered absolute, and the directory separator character is also the forward slash.

dos-based

Instruct GDB to assume the target file system is DOS based. File names starting with either a forward slash, or a drive letter followed by a colon (e.g., ‘c:’), are considered absolute, and both the slash (‘/’) and the backslash (‘\\’) characters are considered directory separators.

auto

Instruct GDB to use the file system kind associated with the target operating system (see Configuring the Current ABI). This is the default.

When processing file names provided by the user, GDB frequently needs to compare them to the file names recorded in the program’s debug info. Normally, GDB compares just the base names of the files as strings, which is reasonably fast even for very large programs. (The base name of a file is the last portion of its name, after stripping all the leading directories.) This shortcut in comparison is based upon the assumption that files cannot have more than one base name. This is usually true, but references to files that use symlinks or similar filesystem facilities violate that assumption. If your program records files using such facilities, or if you provide file names to GDB using symlinks etc., you can set basenames-may-differ to true to instruct GDB to completely canonicalize each pair of file names it needs to compare. This will make file-name comparisons accurate, but at a price of a significant slowdown.

set basenames-may-differ

Set whether a source file may have multiple base names.

show basenames-may-differ

Show whether a source file may have multiple base names.


Next: , Previous: , Up: GDB Files   [Contents][Index]

18.2 Debugging Information in Separate Files

GDB allows you to put a program’s debugging information in a file separate from the executable itself, in a way that allows GDB to find and load the debugging information automatically. Since debugging information can be very large—sometimes larger than the executable code itself—some systems distribute debugging information for their executables in separate files, which users can install only when they need to debug a problem.

GDB supports two ways of specifying the separate debug info file:

Depending on the way the debug info file is specified, GDB uses two different methods of looking for the debug file:

So, for example, suppose you ask GDB to debug /usr/bin/ls, which has a debug link that specifies the file ls.debug, and a build ID whose value in hex is abcdef1234. If the list of the global debug directories includes /usr/lib/debug, then GDB will look for the following debug information files, in the indicated order:

Global debugging info directories default to what is set by GDB configure option --with-separate-debug-dir. During GDB run you can also set the global debugging info directories, and view the list GDB is currently using.

set debug-file-directory directories

Set the directories which GDB searches for separate debugging information files to directory. Multiple path components can be set concatenating them by a path separator.

show debug-file-directory

Show the directories GDB searches for separate debugging information files.

A debug link is a special section of the executable file named .gnu_debuglink. The section must contain:

Any executable file format can carry a debug link, as long as it can contain a section named .gnu_debuglink with the contents described above.

The build ID is a special section in the executable file (and in other ELF binary files that GDB may consider). This section is often named .note.gnu.build-id, but that name is not mandatory. It contains unique identification for the built files—the ID remains the same across multiple builds of the same build tree. The default algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the content for the build ID string. The same section with an identical value is present in the original built binary with symbols, in its stripped variant, and in the separate debugging information file.

The debugging information file itself should be an ordinary executable, containing a full set of linker symbols, sections, and debugging information. The sections of the debugging information file should have the same names, addresses, and sizes as the original file, but they need not contain any data—much like a .bss section in an ordinary executable.

The GNU binary utilities (Binutils) package includes the ‘objcopy’ utility that can produce the separated executable / debugging information file pairs using the following commands:

objcopy --only-keep-debug foo foo.debug
strip -g foo

These commands remove the debugging information from the executable file foo and place it in the file foo.debug. You can use the first, second or both methods to link the two files:

The CRC used in .gnu_debuglink is the CRC-32 defined in IEEE 802.3 using the polynomial:

 x32 + x26 + x23 + x22 + x16 + x12 + x11
 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1

The function is computed byte at a time, taking the least significant bit of each byte first. The initial pattern 0xffffffff is used, to ensure leading zeros affect the CRC and the final result is inverted to ensure trailing zeros also affect the CRC.

Note: This is the same CRC polynomial as used in handling the Remote Serial Protocol qCRC packet (see qCRC packet). However in the case of the Remote Serial Protocol, the CRC is computed most significant bit first, and the result is not inverted, so trailing zeros have no effect on the CRC value.

To complete the description, we show below the code of the function which produces the CRC used in .gnu_debuglink. Inverting the initially supplied crc argument means that an initial call to this function passing in zero will start computing the CRC using 0xffffffff.

unsigned long
gnu_debuglink_crc32 (unsigned long crc,
                     unsigned char *buf, size_t len)
{
  static const unsigned long crc32_table[256] =
    {
      0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
      0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
      0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
      0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
      0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
      0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
      0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
      0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
      0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
      0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
      0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
      0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
      0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
      0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
      0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
      0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
      0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
      0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
      0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
      0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
      0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
      0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
      0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
      0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
      0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
      0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
      0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
      0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
      0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
      0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
      0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
      0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
      0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
      0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
      0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
      0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
      0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
      0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
      0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
      0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
      0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
      0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
      0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
      0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
      0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
      0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
      0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
      0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
      0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
      0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
      0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
      0x2d02ef8d
    };
  unsigned char *end;

  crc = ~crc & 0xffffffff;
  for (end = buf + len; buf < end; ++buf)
    crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
  return ~crc & 0xffffffff;
}

This computation does not apply to the “build ID” method.


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18.3 Debugging information in a special section

Some systems ship pre-built executables and libraries that have a special ‘.gnu_debugdata’ section. This feature is called MiniDebugInfo. This section holds an LZMA-compressed object and is used to supply extra symbols for backtraces.

The intent of this section is to provide extra minimal debugging information for use in simple backtraces. It is not intended to be a replacement for full separate debugging information (see Separate Debug Files). The example below shows the intended use; however, GDB does not currently put restrictions on what sort of debugging information might be included in the section.

GDB has support for this extension. If the section exists, then it is used provided that no other source of debugging information can be found, and that GDB was configured with LZMA support.

This section can be easily created using objcopy and other standard utilities:

# Extract the dynamic symbols from the main binary, there is no need
# to also have these in the normal symbol table.
nm -D binary --format=posix --defined-only \
  | awk '{ print $1 }' | sort > dynsyms

# Extract all the text (i.e. function) symbols from the debuginfo.
# (Note that we actually also accept "D" symbols, for the benefit
# of platforms like PowerPC64 that use function descriptors.)
nm binary --format=posix --defined-only \
  | awk '{ if ($2 == "T" || $2 == "t" || $2 == "D") print $1 }' \
  | sort > funcsyms

# Keep all the function symbols not already in the dynamic symbol
# table.
comm -13 dynsyms funcsyms > keep_symbols

# Separate full debug info into debug binary.
objcopy --only-keep-debug binary debug

# Copy the full debuginfo, keeping only a minimal set of symbols and
# removing some unnecessary sections.
objcopy -S --remove-section .gdb_index --remove-section .comment \
  --keep-symbols=keep_symbols debug mini_debuginfo

# Drop the full debug info from the original binary.
strip --strip-all -R .comment binary

# Inject the compressed data into the .gnu_debugdata section of the
# original binary.
xz mini_debuginfo
objcopy --add-section .gnu_debugdata=mini_debuginfo.xz binary

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18.4 Index Files Speed Up GDB

When GDB finds a symbol file, it scans the symbols in the file in order to construct an internal symbol table. This lets most GDB operations work quickly—at the cost of a delay early on. For large programs, this delay can be quite lengthy, so GDB provides a way to build an index, which speeds up startup.

The index is stored as a section in the symbol file. GDB can write the index to a file, then you can put it into the symbol file using objcopy.

To create an index file, use the save gdb-index command:

save gdb-index directory

Create an index file for each symbol file currently known by GDB. Each file is named after its corresponding symbol file, with ‘.gdb-index’ appended, and is written into the given directory.

Once you have created an index file you can merge it into your symbol file, here named symfile, using objcopy:

$ objcopy --add-section .gdb_index=symfile.gdb-index \
    --set-section-flags .gdb_index=readonly symfile symfile

GDB will normally ignore older versions of .gdb_index sections that have been deprecated. Usually they are deprecated because they are missing a new feature or have performance issues. To tell GDB to use a deprecated index section anyway specify set use-deprecated-index-sections on. The default is off. This can speed up startup, but may result in some functionality being lost. See Index Section Format.

Warning: Setting use-deprecated-index-sections to on must be done before gdb reads the file. The following will not work:

$ gdb -ex "set use-deprecated-index-sections on" <program>

Instead you must do, for example,

$ gdb -iex "set use-deprecated-index-sections on" <program>

There are currently some limitation on indices. They only work when for DWARF debugging information, not stabs. And, they do not currently work for programs using Ada.


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18.5 Errors Reading Symbol Files

While reading a symbol file, GDB occasionally encounters problems, such as symbol types it does not recognize, or known bugs in compiler output. By default, GDB does not notify you of such problems, since they are relatively common and primarily of interest to people debugging compilers. If you are interested in seeing information about ill-constructed symbol tables, you can either ask GDB to print only one message about each such type of problem, no matter how many times the problem occurs; or you can ask GDB to print more messages, to see how many times the problems occur, with the set complaints command (see Optional Warnings and Messages).

The messages currently printed, and their meanings, include:

inner block not inside outer block in symbol

The symbol information shows where symbol scopes begin and end (such as at the start of a function or a block of statements). This error indicates that an inner scope block is not fully contained in its outer scope blocks.

GDB circumvents the problem by treating the inner block as if it had the same scope as the outer block. In the error message, symbol may be shown as “(don't know)” if the outer block is not a function.

block at address out of order

The symbol information for symbol scope blocks should occur in order of increasing addresses. This error indicates that it does not do so.

GDB does not circumvent this problem, and has trouble locating symbols in the source file whose symbols it is reading. (You can often determine what source file is affected by specifying set verbose on. See Optional Warnings and Messages.)

bad block start address patched

The symbol information for a symbol scope block has a start address smaller than the address of the preceding source line. This is known to occur in the SunOS 4.1.1 (and earlier) C compiler.

GDB circumvents the problem by treating the symbol scope block as starting on the previous source line.

bad string table offset in symbol n

Symbol number n contains a pointer into the string table which is larger than the size of the string table.

GDB circumvents the problem by considering the symbol to have the name foo, which may cause other problems if many symbols end up with this name.

unknown symbol type 0xnn

The symbol information contains new data types that GDB does not yet know how to read. 0xnn is the symbol type of the uncomprehended information, in hexadecimal.

GDB circumvents the error by ignoring this symbol information. This usually allows you to debug your program, though certain symbols are not accessible. If you encounter such a problem and feel like debugging it, you can debug gdb with itself, breakpoint on complain, then go up to the function read_dbx_symtab and examine *bufp to see the symbol.

stub type has NULL name

GDB could not find the full definition for a struct or class.

const/volatile indicator missing (ok if using g++ v1.x), got…

The symbol information for a C++ member function is missing some information that recent versions of the compiler should have output for it.

info mismatch between compiler and debugger

GDB could not parse a type specification output by the compiler.


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18.6 GDB Data Files

GDB will sometimes read an auxiliary data file. These files are kept in a directory known as the data directory.

You can set the data directory’s name, and view the name GDB is currently using.

set data-directory directory

Set the directory which GDB searches for auxiliary data files to directory.

show data-directory

Show the directory GDB searches for auxiliary data files.

You can set the default data directory by using the configure-time ‘--with-gdb-datadir’ option. If the data directory is inside GDB’s configured binary prefix (set with ‘--prefix’ or ‘--exec-prefix’), then the default data directory will be updated automatically if the installed GDB is moved to a new location.

The data directory may also be specified with the --data-directory command line option. See Mode Options.


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19 Specifying a Debugging Target

A target is the execution environment occupied by your program.

Often, GDB runs in the same host environment as your program; in that case, the debugging target is specified as a side effect when you use the file or core commands. When you need more flexibility—for example, running GDB on a physically separate host, or controlling a standalone system over a serial port or a realtime system over a TCP/IP connection—you can use the target command to specify one of the target types configured for GDB (see Commands for Managing Targets).

It is possible to build GDB for several different target architectures. When GDB is built like that, you can choose one of the available architectures with the set architecture command.

set architecture arch

This command sets the current target architecture to arch. The value of arch can be "auto", in addition to one of the supported architectures.

show architecture

Show the current target architecture.

set processor
processor

These are alias commands for, respectively, set architecture and show architecture.


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19.1 Active Targets

There are multiple classes of targets such as: processes, executable files or recording sessions. Core files belong to the process class, making core file and process mutually exclusive. Otherwise, GDB can work concurrently on multiple active targets, one in each class. This allows you to (for example) start a process and inspect its activity, while still having access to the executable file after the process finishes. Or if you start process recording (see Reverse Execution) and reverse-step there, you are presented a virtual layer of the recording target, while the process target remains stopped at the chronologically last point of the process execution.

Use the core-file and exec-file commands to select a new core file or executable target (see Commands to Specify Files). To specify as a target a process that is already running, use the attach command (see Debugging an Already-running Process).


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19.2 Commands for Managing Targets

target type parameters

Connects the GDB host environment to a target machine or process. A target is typically a protocol for talking to debugging facilities. You use the argument type to specify the type or protocol of the target machine.

Further parameters are interpreted by the target protocol, but typically include things like device names or host names to connect with, process numbers, and baud rates.

The target command does not repeat if you press RET again after executing the command.

help target

Displays the names of all targets available. To display targets currently selected, use either info target or info files (see Commands to Specify Files).

help target name

Describe a particular target, including any parameters necessary to select it.

set gnutarget args

GDB uses its own library BFD to read your files. GDB knows whether it is reading an executable, a core, or a .o file; however, you can specify the file format with the set gnutarget command. Unlike most target commands, with gnutarget the target refers to a program, not a machine.

Warning: To specify a file format with set gnutarget, you must know the actual BFD name.

See Commands to Specify Files.

show gnutarget

Use the show gnutarget command to display what file format gnutarget is set to read. If you have not set gnutarget, GDB will determine the file format for each file automatically, and show gnutarget displays ‘The current BFD target is "auto"’.

Here are some common targets (available, or not, depending on the GDB configuration):

target exec program

An executable file. ‘target exec program’ is the same as ‘exec-file program’.

target core filename

A core dump file. ‘target core filename’ is the same as ‘core-file filename’.

target remote medium

A remote system connected to GDB via a serial line or network connection. This command tells GDB to use its own remote protocol over medium for debugging. See Remote Debugging.

For example, if you have a board connected to /dev/ttya on the machine running GDB, you could say:

target remote /dev/ttya

target remote supports the load command. This is only useful if you have some other way of getting the stub to the target system, and you can put it somewhere in memory where it won’t get clobbered by the download.

target sim [simargs]

Builtin CPU simulator. GDB includes simulators for most architectures. In general,

        target sim
        load
        run

works; however, you cannot assume that a specific memory map, device drivers, or even basic I/O is available, although some simulators do provide these. For info about any processor-specific simulator details, see the appropriate section in Embedded Processors.

target native

Setup for local/native process debugging. Useful to make the run command spawn native processes (likewise attach, etc.) even when set auto-connect-native-target is off (see set auto-connect-native-target).

Different targets are available on different configurations of GDB; your configuration may have more or fewer targets.

Many remote targets require you to download the executable’s code once you’ve successfully established a connection. You may wish to control various aspects of this process.

set hash

This command controls whether a hash mark ‘#’ is displayed while downloading a file to the remote monitor. If on, a hash mark is displayed after each S-record is successfully downloaded to the monitor.

show hash

Show the current status of displaying the hash mark.

set debug monitor

Enable or disable display of communications messages between GDB and the remote monitor.

show debug monitor

Show the current status of displaying communications between GDB and the remote monitor.

load filename

Depending on what remote debugging facilities are configured into GDB, the load command may be available. Where it exists, it is meant to make filename (an executable) available for debugging on the remote system—by downloading, or dynamic linking, for example. load also records the filename symbol table in GDB, like the add-symbol-file command.

If your GDB does not have a load command, attempting to execute it gets the error message “You can't do that when your target is …

The file is loaded at whatever address is specified in the executable. For some object file formats, you can specify the load address when you link the program; for other formats, like a.out, the object file format specifies a fixed address.

Depending on the remote side capabilities, GDB may be able to load programs into flash memory.

load does not repeat if you press RET again after using it.


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19.3 Choosing Target Byte Order

Some types of processors, such as the MIPS, PowerPC, and Renesas SH, offer the ability to run either big-endian or little-endian byte orders. Usually the executable or symbol will include a bit to designate the endian-ness, and you will not need to worry about which to use. However, you may still find it useful to adjust GDB’s idea of processor endian-ness manually.

set endian big

Instruct GDB to assume the target is big-endian.

set endian little

Instruct GDB to assume the target is little-endian.

set endian auto

Instruct GDB to use the byte order associated with the executable.

show endian

Display GDB’s current idea of the target byte order.

Note that these commands merely adjust interpretation of symbolic data on the host, and that they have absolutely no effect on the target system.


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20 Debugging Remote Programs

If you are trying to debug a program running on a machine that cannot run GDB in the usual way, it is often useful to use remote debugging. For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger.

Some configurations of GDB have special serial or TCP/IP interfaces to make this work with particular debugging targets. In addition, GDB comes with a generic serial protocol (specific to GDB, but not specific to any particular target system) which you can use if you write the remote stubs—the code that runs on the remote system to communicate with GDB.

Other remote targets may be available in your configuration of GDB; use help target to list them.


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20.1 Connecting to a Remote Target

On the GDB host machine, you will need an unstripped copy of your program, since GDB needs symbol and debugging information. Start up GDB as usual, using the name of the local copy of your program as the first argument.

GDB can communicate with the target over a serial line, or over an IP network using TCP or UDP. In each case, GDB uses the same protocol for debugging your program; only the medium carrying the debugging packets varies. The target remote command establishes a connection to the target. Its arguments indicate which medium to use:

target remote serial-device

Use serial-device to communicate with the target. For example, to use a serial line connected to the device named /dev/ttyb:

target remote /dev/ttyb

If you’re using a serial line, you may want to give GDB the ‘--baud’ option, or use the set serial baud command (see set serial baud) before the target command.

target remote host:port
target remote tcp:host:port

Debug using a TCP connection to port on host. The host may be either a host name or a numeric IP address; port must be a decimal number. The host could be the target machine itself, if it is directly connected to the net, or it might be a terminal server which in turn has a serial line to the target.

For example, to connect to port 2828 on a terminal server named manyfarms:

target remote manyfarms:2828

If your remote target is actually running on the same machine as your debugger session (e.g. a simulator for your target running on the same host), you can omit the hostname. For example, to connect to port 1234 on your local machine:

target remote :1234

Note that the colon is still required here.

target remote udp:host:port

Debug using UDP packets to port on host. For example, to connect to UDP port 2828 on a terminal server named manyfarms:

target remote udp:manyfarms:2828

When using a UDP connection for remote debugging, you should keep in mind that the ‘U’ stands for “Unreliable”. UDP can silently drop packets on busy or unreliable networks, which will cause havoc with your debugging session.

target remote | command

Run command in the background and communicate with it using a pipe. The command is a shell command, to be parsed and expanded by the system’s command shell, /bin/sh; it should expect remote protocol packets on its standard input, and send replies on its standard output. You could use this to run a stand-alone simulator that speaks the remote debugging protocol, to make net connections using programs like ssh, or for other similar tricks.

If command closes its standard output (perhaps by exiting), GDB will try to send it a SIGTERM signal. (If the program has already exited, this will have no effect.)

Once the connection has been established, you can use all the usual commands to examine and change data. The remote program is already running; you can use step and continue, and you do not need to use run.

Whenever GDB is waiting for the remote program, if you type the interrupt character (often Ctrl-c), GDB attempts to stop the program. This may or may not succeed, depending in part on the hardware and the serial drivers the remote system uses. If you type the interrupt character once again, GDB displays this prompt:

Interrupted while waiting for the program.
Give up (and stop debugging it)?  (y or n)

If you type y, GDB abandons the remote debugging session. (If you decide you want to try again later, you can use ‘target remote’ again to connect once more.) If you type n, GDB goes back to waiting.

detach

When you have finished debugging the remote program, you can use the detach command to release it from GDB control. Detaching from the target normally resumes its execution, but the results will depend on your particular remote stub. After the detach command, GDB is free to connect to another target.

disconnect

The disconnect command behaves like detach, except that the target is generally not resumed. It will wait for GDB (this instance or another one) to connect and continue debugging. After the disconnect command, GDB is again free to connect to another target.

monitor cmd

This command allows you to send arbitrary commands directly to the remote monitor. Since GDB doesn’t care about the commands it sends like this, this command is the way to extend GDB—you can add new commands that only the external monitor will understand and implement.


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20.2 Sending files to a remote system

Some remote targets offer the ability to transfer files over the same connection used to communicate with GDB. This is convenient for targets accessible through other means, e.g. GNU/Linux systems running gdbserver over a network interface. For other targets, e.g. embedded devices with only a single serial port, this may be the only way to upload or download files.

Not all remote targets support these commands.

remote put hostfile targetfile

Copy file hostfile from the host system (the machine running GDB) to targetfile on the target system.

remote get targetfile hostfile

Copy file targetfile from the target system to hostfile on the host system.

remote delete targetfile

Delete targetfile from the target system.


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20.3 Using the gdbserver Program

gdbserver is a control program for Unix-like systems, which allows you to connect your program with a remote GDB via target remote—but without linking in the usual debugging stub.

gdbserver is not a complete replacement for the debugging stubs, because it requires essentially the same operating-system facilities that GDB itself does. In fact, a system that can run gdbserver to connect to a remote GDB could also run GDB locally! gdbserver is sometimes useful nevertheless, because it is a much smaller program than GDB itself. It is also easier to port than all of GDB, so you may be able to get started more quickly on a new system by using gdbserver. Finally, if you develop code for real-time systems, you may find that the tradeoffs involved in real-time operation make it more convenient to do as much development work as possible on another system, for example by cross-compiling. You can use gdbserver to make a similar choice for debugging.

GDB and gdbserver communicate via either a serial line or a TCP connection, using the standard GDB remote serial protocol.

Warning: gdbserver does not have any built-in security. Do not run gdbserver connected to any public network; a GDB connection to gdbserver provides access to the target system with the same privileges as the user running gdbserver.

20.3.1 Running gdbserver

Run gdbserver on the target system. You need a copy of the program you want to debug, including any libraries it requires. gdbserver does not need your program’s symbol table, so you can strip the program if necessary to save space. GDB on the host system does all the symbol handling.

To use the server, you must tell it how to communicate with GDB; the name of your program; and the arguments for your program. The usual syntax is:

target> gdbserver comm program [ args … ]

comm is either a device name (to use a serial line), or a TCP hostname and portnumber, or - or stdio to use stdin/stdout of gdbserver. For example, to debug Emacs with the argument ‘foo.txt’ and communicate with GDB over the serial port /dev/com1:

target> gdbserver /dev/com1 emacs foo.txt

gdbserver waits passively for the host GDB to communicate with it.

To use a TCP connection instead of a serial line:

target> gdbserver host:2345 emacs foo.txt

The only difference from the previous example is the first argument, specifying that you are communicating with the host GDB via TCP. The ‘host:2345’ argument means that gdbserver is to expect a TCP connection from machine ‘host’ to local TCP port 2345. (Currently, the ‘host’ part is ignored.) You can choose any number you want for the port number as long as it does not conflict with any TCP ports already in use on the target system (for example, 23 is reserved for telnet).13 You must use the same port number with the host GDB target remote command.

The stdio connection is useful when starting gdbserver with ssh:

(gdb) target remote | ssh -T hostname gdbserver - hello

The ‘-T’ option to ssh is provided because we don’t need a remote pty, and we don’t want escape-character handling. Ssh does this by default when a command is provided, the flag is provided to make it explicit. You could elide it if you want to.

Programs started with stdio-connected gdbserver have /dev/null for stdin, and stdout,stderr are sent back to gdb for display through a pipe connected to gdbserver. Both stdout and stderr use the same pipe.

20.3.1.1 Attaching to a Running Program

On some targets, gdbserver can also attach to running programs. This is accomplished via the --attach argument. The syntax is:

target> gdbserver --attach comm pid

pid is the process ID of a currently running process. It isn’t necessary to point gdbserver at a binary for the running process.

You can debug processes by name instead of process ID if your target has the pidof utility:

target> gdbserver --attach comm `pidof program`

In case more than one copy of program is running, or program has multiple threads, most versions of pidof support the -s option to only return the first process ID.

20.3.1.2 Multi-Process Mode for gdbserver

When you connect to gdbserver using target remote, gdbserver debugs the specified program only once. When the program exits, or you detach from it, GDB closes the connection and gdbserver exits.

If you connect using target extended-remote, gdbserver enters multi-process mode. When the debugged program exits, or you detach from it, GDB stays connected to gdbserver even though no program is running. The run and attach commands instruct gdbserver to run or attach to a new program. The run command uses set remote exec-file (see set remote exec-file) to select the program to run. Command line arguments are supported, except for wildcard expansion and I/O redirection (see Arguments).

To start gdbserver without supplying an initial command to run or process ID to attach, use the --multi command line option. Then you can connect using target extended-remote and start the program you want to debug.

In multi-process mode gdbserver does not automatically exit unless you use the option --once. You can terminate it by using monitor exit (see Monitor Commands for gdbserver). Note that the conditions under which gdbserver terminates depend on how GDB connects to it (target remote or target extended-remote). The --multi option to gdbserver has no influence on that.

20.3.1.3 TCP port allocation lifecycle of gdbserver

This section applies only when gdbserver is run to listen on a TCP port.

gdbserver normally terminates after all of its debugged processes have terminated in target remote mode. On the other hand, for target extended-remote, gdbserver stays running even with no processes left. GDB normally terminates the spawned debugged process on its exit, which normally also terminates gdbserver in the target remote mode. Therefore, when the connection drops unexpectedly, and GDB cannot ask gdbserver to kill its debugged processes, gdbserver stays running even in the target remote mode.

When gdbserver stays running, GDB can connect to it again later. Such reconnecting is useful for features like disconnected tracing. For completeness, at most one GDB can be connected at a time.

By default, gdbserver keeps the listening TCP port open, so that subsequent connections are possible. However, if you start gdbserver with the --once option, it will stop listening for any further connection attempts after connecting to the first GDB session. This means no further connections to gdbserver will be possible after the first one. It also means gdbserver will terminate after the first connection with remote GDB has closed, even for unexpectedly closed connections and even in the target extended-remote mode. The --once option allows reusing the same port number for connecting to multiple instances of gdbserver running on the same host, since each instance closes its port after the first connection.

20.3.1.4 Other Command-Line Arguments for gdbserver

The --debug option tells gdbserver to display extra status information about the debugging process. The --remote-debug option tells gdbserver to display remote protocol debug output. These options are intended for gdbserver development and for bug reports to the developers.

The --debug-format=option1[,option2,...] option tells gdbserver to include additional information in each output. Possible options are:

none

Turn off all extra information in debugging output.

all

Turn on all extra information in debugging output.

timestamps

Include a timestamp in each line of debugging output.

Options are processed in order. Thus, for example, if none appears last then no additional information is added to debugging output.

The --wrapper option specifies a wrapper to launch programs for debugging. The option should be followed by the name of the wrapper, then any command-line arguments to pass to the wrapper, then -- indicating the end of the wrapper arguments.

gdbserver runs the specified wrapper program with a combined command line including the wrapper arguments, then the name of the program to debug, then any arguments to the program. The wrapper runs until it executes your program, and then GDB gains control.

You can use any program that eventually calls execve with its arguments as a wrapper. Several standard Unix utilities do this, e.g. env and nohup. Any Unix shell script ending with exec "$@" will also work.

For example, you can use env to pass an environment variable to the debugged program, without setting the variable in gdbserver’s environment:

$ gdbserver --wrapper env LD_PRELOAD=libtest.so -- :2222 ./testprog

20.3.2 Connecting to gdbserver

Run GDB on the host system.

First make sure you have the necessary symbol files. Load symbols for your application using the file command before you connect. Use set sysroot to locate target libraries (unless your GDB was compiled with the correct sysroot using --with-sysroot).

The symbol file and target libraries must exactly match the executable and libraries on the target, with one exception: the files on the host system should not be stripped, even if the files on the target system are. Mismatched or missing files will lead to confusing results during debugging. On GNU/Linux targets, mismatched or missing files may also prevent gdbserver from debugging multi-threaded programs.

Connect to your target (see Connecting to a Remote Target). For TCP connections, you must start up gdbserver prior to using the target remote command. Otherwise you may get an error whose text depends on the host system, but which usually looks something like ‘Connection refused’. Don’t use the load command in GDB when using gdbserver, since the program is already on the target.

20.3.3 Monitor Commands for gdbserver

During a GDB session using gdbserver, you can use the monitor command to send special requests to gdbserver. Here are the available commands.

monitor help

List the available monitor commands.

monitor set debug 0
monitor set debug 1

Disable or enable general debugging messages.

monitor set remote-debug 0
monitor set remote-debug 1

Disable or enable specific debugging messages associated with the remote protocol (see Remote Protocol).

monitor set debug-format option1[,option2,...]

Specify additional text to add to debugging messages. Possible options are:

none

Turn off all extra information in debugging output.

all

Turn on all extra information in debugging output.

timestamps

Include a timestamp in each line of debugging output.

Options are processed in order. Thus, for example, if none appears last then no additional information is added to debugging output.

monitor set libthread-db-search-path [PATH]

When this command is issued, path is a colon-separated list of directories to search for libthread_db (see set libthread-db-search-path). If you omit path, ‘libthread-db-search-path’ will be reset to its default value.

The special entry ‘$pdir’ for ‘libthread-db-search-path’ is not supported in gdbserver.

monitor exit

Tell gdbserver to exit immediately. This command should be followed by disconnect to close the debugging session. gdbserver will detach from any attached processes and kill any processes it created. Use monitor exit to terminate gdbserver at the end of a multi-process mode debug session.

20.3.4 Tracepoints support in gdbserver

On some targets, gdbserver supports tracepoints, fast tracepoints and static tracepoints.

For fast or static tracepoints to work, a special library called the in-process agent (IPA), must be loaded in the inferior process. This library is built and distributed as an integral part of gdbserver. In addition, support for static tracepoints requires building the in-process agent library with static tracepoints support. At present, the UST (LTTng Userspace Tracer, http://lttng.org/ust) tracing engine is supported. This support is automatically available if UST development headers are found in the standard include path when gdbserver is built, or if gdbserver was explicitly configured using --with-ust to point at such headers. You can explicitly disable the support using --with-ust=no.

There are several ways to load the in-process agent in your program:

Specifying it as dependency at link time

You can link your program dynamically with the in-process agent library. On most systems, this is accomplished by adding -linproctrace to the link command.

Using the system's preloading mechanisms

You can force loading the in-process agent at startup time by using your system’s support for preloading shared libraries. Many Unixes support the concept of preloading user defined libraries. In most cases, you do that by specifying LD_PRELOAD=libinproctrace.so in the environment. See also the description of gdbserver’s --wrapper command line option.

Using GDB to force loading the agent at run time

On some systems, you can force the inferior to load a shared library, by calling a dynamic loader function in the inferior that takes care of dynamically looking up and loading a shared library. On most Unix systems, the function is dlopen. You’ll use the call command for that. For example:

(gdb) call dlopen ("libinproctrace.so", ...)

Note that on most Unix systems, for the dlopen function to be available, the program needs to be linked with -ldl.

On systems that have a userspace dynamic loader, like most Unix systems, when you connect to gdbserver using target remote, you’ll find that the program is stopped at the dynamic loader’s entry point, and no shared library has been loaded in the program’s address space yet, including the in-process agent. In that case, before being able to use any of the fast or static tracepoints features, you need to let the loader run and load the shared libraries. The simplest way to do that is to run the program to the main procedure. E.g., if debugging a C or C++ program, start gdbserver like so:

$ gdbserver :9999 myprogram

Start GDB and connect to gdbserver like so, and run to main:

$ gdb myprogram
(gdb) target remote myhost:9999
0x00007f215893ba60 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) b main
(gdb) continue

The in-process tracing agent library should now be loaded into the process; you can confirm it with the info sharedlibrary command, which will list libinproctrace.so as loaded in the process. You are now ready to install fast tracepoints, list static tracepoint markers, probe static tracepoints markers, and start tracing.


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20.4 Remote Configuration

This section documents the configuration options available when debugging remote programs. For the options related to the File I/O extensions of the remote protocol, see system-call-allowed.

set remoteaddresssize bits

Set the maximum size of address in a memory packet to the specified number of bits. GDB will mask off the address bits above that number, when it passes addresses to the remote target. The default value is the number of bits in the target’s address.

show remoteaddresssize

Show the current value of remote address size in bits.

set serial baud n

Set the baud rate for the remote serial I/O to n baud. The value is used to set the speed of the serial port used for debugging remote targets.

show serial baud

Show the current speed of the remote connection.

set remotebreak

If set to on, GDB sends a BREAK signal to the remote when you type Ctrl-c to interrupt the program running on the remote. If set to off, GDB sends the ‘Ctrl-C’ character instead. The default is off, since most remote systems expect to see ‘Ctrl-C’ as the interrupt signal.

show remotebreak

Show whether GDB sends BREAK or ‘Ctrl-C’ to interrupt the remote program.

set remoteflow on
set remoteflow off

Enable or disable hardware flow control (RTS/CTS) on the serial port used to communicate to the remote target.

show remoteflow

Show the current setting of hardware flow control.

set remotelogbase base

Set the base (a.k.a. radix) of logging serial protocol communications to base. Supported values of base are: ascii, octal, and hex. The default is ascii.

show remotelogbase

Show the current setting of the radix for logging remote serial protocol.

set remotelogfile file

Record remote serial communications on the named file. The default is not to record at all.

show remotelogfile.

Show the current setting of the file name on which to record the serial communications.

set remotetimeout num

Set the timeout limit to wait for the remote target to respond to num seconds. The default is 2 seconds.

show remotetimeout

Show the current number of seconds to wait for the remote target responses.

set remote hardware-watchpoint-limit limit
set remote hardware-breakpoint-limit limit

Restrict GDB to using limit remote hardware breakpoint or watchpoints. A limit of -1, the default, is treated as unlimited.

set remote hardware-watchpoint-length-limit limit

Restrict GDB to using limit bytes for the maximum length of a remote hardware watchpoint. A limit of -1, the default, is treated as unlimited.

show remote hardware-watchpoint-length-limit

Show the current limit (in bytes) of the maximum length of a remote hardware watchpoint.

set remote exec-file filename
show remote exec-file

Select the file used for run with target extended-remote. This should be set to a filename valid on the target system. If it is not set, the target will use a default filename (e.g. the last program run).

set remote interrupt-sequence

Allow the user to select one of ‘Ctrl-C’, a BREAK or ‘BREAK-g’ as the sequence to the remote target in order to interrupt the execution. ‘Ctrl-C’ is a default. Some system prefers BREAK which is high level of serial line for some certain time. Linux kernel prefers ‘BREAK-g