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This file describes GDB, the GNU symbolic debugger.
This is the Ninth Edition, for GDB (GDB) Version 6.8.50.20090704.
Copyright (C) 1988-2009 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.
Summary of GDB 1. A Sample GDB Session A sample GDB session
2. Getting In and Out of GDB Getting in and out of GDB 3. GDB Commands GDB commands 4. Running Programs Under GDB Running programs under GDB 5. Stopping and Continuing Stopping and continuing 6. Running programs backward 7. Recording Inferior's Execution and Replaying It Recording inferior's execution and replaying it 8. Examining the Stack Examining the stack 9. Examining Source Files Examining source files 10. Examining Data Examining data 11. Debugging Optimized Code Debugging optimized code 12. C Preprocessor Macros Preprocessor Macros 13. Tracepoints Debugging remote targets non-intrusively 14. Debugging Programs That Use Overlays Debugging programs that use overlays
15. Using GDB with Different Languages Using GDB with different languages
16. Examining the Symbol Table Examining the symbol table 17. Altering Execution Altering execution 18. GDB Files GDB files 19. Specifying a Debugging Target Specifying a debugging target 20. Debugging Remote Programs Debugging remote programs 21. Configuration-Specific Information Configuration-specific information 22. Controlling GDB 23. Extending GDB 24. Command Interpreters 25. GDB Text User Interface 26. Using GDB under GNU Emacs 27. The GDB/MI Interface GDB's Machine Interface. 28. GDB Annotations GDB's annotation interface.
29. Reporting Bugs in GDB Reporting bugs in GDB
GDB
30. Command Line Editing 31. Using History Interactively A. Formatting Documentation How to format and print GDB documentation B. Installing GDB C. Maintenance Commands D. GDB Remote Serial Protocol E. The GDB Agent Expression Mechanism F. Target Descriptions How targets can describe themselves to
G. Operating System Information Getting additional information from the operating system H. GNU GENERAL PUBLIC LICENSE GNU General Public License says how you can copy and share GDB I. GNU Free Documentation License The license for this documentation Index
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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 Modula-2 is partial. For information on Modula-2, see Modula-2.
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.
Free Software Freely redistributable software Contributors to GDB
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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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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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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.
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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 6.8.50.20090704, 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 " |
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 " |
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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This chapter discusses how to start GDB, and how to get out of it. The essentials are:
2.1 Invoking GDB How to start GDB 2.2 Quitting GDB How to quit GDB 2.3 Shell Commands How to use shell commands inside GDB 2.4 Logging Output How to log GDB's output to a file
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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 |
gdb to debug gcc, and to set
gcc's command-line arguments (see section 4.3 Your Program's Arguments) to `-O2 -c foo.c'.
You can run gdb without printing the front material, which describes
GDB's non-warranty, by specifying -silent:
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.
2.1.1 Choosing Files Choosing files 2.1.2 Choosing Modes Choosing modes 2.1.3 What GDB Does During Startup What GDB does during startup
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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
-exec file
-e file
-se file
-core file
-c file
-pid number
-p number
attach command.
-command file
-x file
-eval-command command
-ex 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 |
-directory directory
-d directory
-r
-readnow
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You can run GDB in various alternative modes--for example, in batch mode or quiet mode.
-nx
-n
-quiet
-silent
-q
-batch
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 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
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
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
-windows
-w
-cd directory
-fullname
-f
-epoch
-annotate level
The annotation mechanism has largely been superseded by GDB/MI (see section 27. The GDB/MI Interface).
--args
-baud bps
-b bps
-l timeout
-tty device
-t device
-tui
-interpreter interp
`--interpreter=mi' (or `--interpreter=mi2') causes GDB to use the GDB/MI interface (see section 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
-statistics
-version
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Here's the description of what GDB does during session startup:
Init files use the same syntax as command files (see section 23.1.3 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 section 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 ports of GDB use the standard name, but if they find a `gdb.ini' file, they warn you about that and suggest to rename the file to the standard name.
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quit [expression]
q
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 section Debugging an Already-running Process).
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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
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
make program with the specified
arguments. This is equivalent to `shell make make-args'.
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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
set logging off
set logging file file
set logging overwrite [on|off]
overwrite if
you want set logging on to overwrite the logfile instead.
set logging redirect [on|off]
redirect if you want output to go only to the log file.
show logging
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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).
3.1 Command Syntax How to give commands to GDB 3.2 Command Completion Command completion 3.3 Getting Help How to ask GDB for help
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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 section 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 section 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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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 tries(2) to limit completions to the field names available in the type of the left-hand-side:
(gdb) p gdb_stdout.M-? magic to_delete to_fputs to_put to_rewind to_data to_flush to_isatty to_read to_write |
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_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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You can always ask GDB itself for information on its commands,
using the command help.
help
h
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
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
help argument, GDB displays a
short paragraph on how to use that command.
apropos args
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 reload |
results in:
set symbol-reloading -- Set dynamic symbol table reloading
multiple times in one run
show symbol-reloading -- Show dynamic symbol table reloading
multiple times in one run
|
complete args
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 Index point to
all the sub-commands. See section Index.
info
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
set. For example, you can set the GDB prompt to a $-sign with
set prompt $.
show
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 three miscellaneous show subcommands, all of which are
exceptional in lacking corresponding set commands:
show version
show copying
info copying
show warranty
info warranty
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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.
4.1 Compiling for Debugging Compiling for debugging 4.2 Starting your Program Starting your program 4.3 Your Program's Arguments Your program's arguments 4.4 Your Program's Environment Your program's environment
4.5 Your Program's Working Directory Your program's working directory 4.6 Your Program's Input and Output Your program's input and output 4.7 Debugging an Already-running Process Debugging an already-running process 4.8 Killing the Child Process Killing the child process
4.9 Debugging Multiple Inferiors Debugging multiple inferiors 4.10 Debugging Programs with Multiple Threads Debugging programs with multiple threads 4.11 Debugging Programs with Multiple Processes Debugging programs with multiple processes 4.12 Setting a Bookmark to Return to Later Setting a bookmark to return to later
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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 11. Debugging 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 section 12. C Preprocessor Macros). Most compilers do not include information about preprocessor macros in the debugging information if you specify the `-g' flag alone, because this information is rather large. Version 3.1 and later of GCC, the GNU C compiler, provides macro information if you specify the options `-gdwarf-2' and `-g3'; the former option requests debugging information in the Dwarf 2 format, and the latter requests "extra information". In the future, we hope to find more compact ways to represent macro information, so that it can be included with `-g' alone.
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run
r
run command to start your program under GDB.
You must first specify the program name (except on VxWorks) with an
argument to GDB (see section Getting In and Out of GDB), or by using the file or exec-file command
(see section 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:
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.
See section Your Program's Arguments.
set environment and unset
environment to change parts of the environment that affect
your program. See section Your Program's Environment.
cd command in GDB.
See section Your Program's Working Directory.
run command line, or you can use the tty command to
set a different device for your program.
See section 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 section 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 section 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
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
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 disable-randomization
set disable-randomization on
This feature is implemented only on GNU/Linux. You can get the same behavior using
(gdb) set exec-wrapper setarch `uname -m` -R |
set disable-randomization off
The virtual address space randomization is implemented only on GNU/Linux. It protects the programs against some 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
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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
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
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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
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
PATH
environment variable).
show environment [varname]
environment as env.
set environment varname [=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.)
unset environment 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--such as `.cshrc' for C-shell, or
`.bashrc' 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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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 section Commands to Specify Files.
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 section 21.1.3 SVR4 Process Information) to find out the
current working directory of the debuggee.
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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
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
show inferior-tty
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attach process-id
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 section Specifying Source Directories). You can also use
the file command to load the program. See section 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
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 section Optional Warnings and Messages).
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kill
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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Some GDB targets are able to run multiple processes created from a single executable. This can happen, for instance, with an embedded system reporting back several processes via the remote protocol.
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 (in future) be retained after a process exits. Each run of an executable creates a new inferior, as does each attachment to an existing process. Inferiors have unique identifiers that are different from process ids, and may optionally be named as well. 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 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
To switch focus between inferiors, use the inferior command:
inferior inferior-id
To quit debugging one of the inferiors, you can either detach from it
by using the detach inferior command (allowing it to run
independently), or kill it using the kill inferior command:
detach inferior inferior-id
kill inferior inferior-id
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
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
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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:
libthread_db to use if the default choice
isn't compatible with the program.
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 thethreadcommand, like this:
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]'. systag is a thread identifier whose form varies depending on the particular system. For example, on GNU/Linux, you might see
[New Thread 46912507313328 (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
An asterisk `*' to the left of the GDB thread number indicates the current thread.
For example,
(gdb) info threads
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 HP-UX systems:
For debugging purposes, GDB associates its own thread number--a small integer assigned in thread-creation order--with each thread in your program.
Whenever GDB detects a new thread in your program, it displays both GDB's thread number and the target system's identification for the thread with a message in the form `[New systag]'. systag is a thread identifier whose form varies depending on the particular system. For example, on HP-UX, you see
[New thread 2 (system thread 26594)] |
when GDB notices a new thread.
info threads
An asterisk `*' to the left of the GDB thread number indicates the current thread.
For example,
(gdb) info threads
* 3 system thread 26607 worker (wptr=0x7b09c318 "@") \ |
On Solaris, you can display more information about user threads with a Solaris-specific command:
maint info sol-threads
thread threadno
(gdb) thread 2 [Switching to process 35 thread 23] 0x34e5 in sigpause () |
As with the `[New ...]' message, the form of the text after `Switching to' depends on your system's conventions for identifying threads.
thread apply [threadno] [all] command
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.
set print thread-events
set print thread-events on
set print thread-events off
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
See section Stopping and Starting Multi-thread Programs, for more information about how GDB behaves when you stop and start programs with multiple threads.
See section Setting Watchpoints, for information about watchpoints in programs with multiple threads.
set libthread-db-search-path [path]
libthread_db.
If you omit path, `libthread-db-search-path' will be reset to
an empty list.
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. If that fails, GDB will continue
with default system shared library directories, and finally 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
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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 section 4.7 Debugging an Already-running Process). 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
fork or
vfork. A call to fork or vfork creates a new
process. The mode argument can be:
parent
child
show follow-fork-mode
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
on
follow-fork-mode) will be detached and allowed to run
independently. This is the default.
off
follow-fork-mode) is debugged as usual, while the other
is held suspended.
show detach-on-fork
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 section Debugging Multiple Inferiors).
To quit debugging one of the forked processes, you can either detach
from it by using the detach inferior command (allowing it
to run independently), or kill it using the kill inferior
command. See section Debugging Multiple Inferiors.
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.
You can use the catch command to make GDB stop whenever
a fork, vfork, or exec call is made. See section Setting Catchpoints.
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On certain operating systems(3), 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
checkpoint command takes no arguments, but each checkpoint
is assigned a small integer id, similar to a breakpoint id.
info checkpoints
Checkpoint ID
Process ID
Code Address
Source line, or label
restart checkpoint-id
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
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.
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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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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
5.1 Breakpoints, Watchpoints, and Catchpoints Breakpoints, watchpoints, and catchpoints 5.2 Continuing and Stepping Resuming execution 5.3 Signals 5.4 Stopping and Starting Multi-thread Programs Stopping and starting multi-thread programs
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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 section 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 section 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 section 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 section 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.
5.1.1 Setting Breakpoints Setting breakpoints 5.1.2 Setting Watchpoints Setting watchpoints 5.1.3 Setting Catchpoints Setting catchpoints 5.1.4 Deleting Breakpoints Deleting breakpoints 5.1.5 Disabling Breakpoints Disabling breakpoints 5.1.6 Break Conditions Break conditions 5.1.7 Breakpoint Command Lists Breakpoint command lists 5.1.8 "Cannot insert breakpoints" 5.1.9 "Breakpoint address adjusted..."
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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
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 section 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 section 5.4.4 Thread-Specific Breakpoints) or a specific task (see section 15.4.6.5 Extensions for Ada Tasks) hits that breakpoint.
break
break sets a breakpoint at
the next instruction to be executed in the selected stack frame
(see section 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
tbreak args
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 section Disabling Breakpoints.
hbreak args
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 section Disabling Breakpoints).
See section Break Conditions.
For remote targets, you can restrict the number of hardware
breakpoints GDB will use, see set remote hardware-breakpoint-limit.
thbreak args
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 section Disabling Breakpoints.
See also Break Conditions.
rbreak regex
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 . |
info breakpoints [n]
info break [n]
info watchpoints [n]
If a breakpoint is conditional, info break shows the condition on
the line following the affected breakpoint; 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 section 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.
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 section 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(4).
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 section 5.1.5 Disabling Breakpoints) 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
set breakpoint pending on
set breakpoint pending off
show breakpoint pending
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
set breakpoint auto-hw off
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
set breakpoint always-inserted on
set breakpoint always-inserted auto
breakpoint always-inserted mode is on. If GDB is
controlling the inferior in all-stop mode, GDB behaves as if
breakpoint always-inserted mode is off.
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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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:
int occupies 4 bytes).
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 expr [thread threadnum]
(gdb) watch foo |
If the command includes a [thread threadnum]
clause, 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.
rwatch expr [thread threadnum]
awatch expr [thread threadnum]
info watchpoints
info break (see section 5.1.1 Setting Breakpoints).
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
show can-use-hw-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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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
throw
catch
exception
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
assert
exec
exec. This is currently only available for HP-UX
and GNU/Linux.
fork
fork. This is currently only available for HP-UX
and GNU/Linux.
vfork
vfork. This is currently only available for HP-UX
and GNU/Linux.
tcatch event
Use the info break command to list the current catchpoints.
There are currently some limitations to C++ exception handling
(catch throw and catch catch) in GDB:
Sometimes catch is not the best way to debug exception handling:
if you need to know exactly where an exception is raised, it is better to
stop before the exception handler is called, since that way you
can see the stack before any unwinding takes place. If you set a
breakpoint in an exception handler instead, it may not be easy to find
out where the exception was raised.
To stop just before an exception handler is called, you need some
knowledge of the implementation. In the case of GNU C++, exceptions are
raised by calling a library function named __raise_exception
which has the following ANSI C interface:
/* addr is where the exception identifier is stored.
id is the exception identifier. */
void __raise_exception (void **addr, void *id);
|
To make the debugger catch all exceptions before any stack
unwinding takes place, set a breakpoint on __raise_exception
(see section Breakpoints; Watchpoints; and Exceptions).
With a conditional breakpoint (see section Break Conditions) that depends on the value of id, you can stop your program when a specific exception is raised. You can use multiple conditional breakpoints to stop your program when any of a number of exceptions are raised.
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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
clear location
clear function
clear filename:function
clear linenum
clear filename:linenum
delete [breakpoints] [range...]
set
confirm off). You can abbreviate this command as d.
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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 or
info watch to print a list of 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 four different states of enablement:
break command starts out in this state.
tbreak command starts out in this state.
You can use the following commands to enable or disable breakpoints, watchpoints, and catchpoints:
disable [breakpoints] [range...]
disable as dis.
enable [breakpoints] [range...]
enable [breakpoints] once range...
enable [breakpoints] delete range...
tbreak command start out in this state.
Except for a breakpoint set with tbreak (see section 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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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 section 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 section Breakpoint Command Lists).
Break conditions can be specified when a breakpoint is set, by using
`if' in the arguments to the break command. See section 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
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 section Expressions.
condition bnum
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
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 section 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 section Convenience Variables.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
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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 [bnum]
... command-list ...
end
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 bnum argument, commands refers to the last
breakpoint, watchpoint, or catchpoint set (not to the breakpoint most
recently encountered).
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 section 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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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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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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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 section Signals.)
continue [ignore-count]
c [ignore-count]
fg [ignore-count]
ignore (see section 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 section Returning from a Function) to go back to the
calling function; or jump (see section 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 section 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
s.
Warning: If you use thestepcommand 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 thestepicommand, 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
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]
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
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
step command to step over any functions which contains no
debug information. This is the default.
show step-mode
finish
fin.
Contrast this with the return command (see section Returning from a Function).
until
u
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
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
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
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 section Automatic Display.
An argument is a repeat count, as in step.
nexti
nexti arg
ni
An argument is a repeat count, as in next.
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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
info signals sig
info handle is an alias for info signals.
handle signal [keywords...]
The keywords allowed by the handle command can be abbreviated.
Their full names are:
nostop
stop
print keyword as well.
print
noprint
nostop keyword as well.
pass
noignore
pass and noignore are synonyms.
nopass
ignore
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 section 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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GDB supports debugging programs with multiple threads (see section 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.
5.4.1 All-Stop Mode All threads stop when GDB takes control 5.4.2 Non-Stop Mode Other threads continue to execute 5.4.3 Background Execution Running your program asynchronously 5.4.4 Thread-Specific Breakpoints Controlling breakpoints 5.4.5 Interrupted System Calls GDB may interfere with system calls
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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
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
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
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
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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:
# Enable the async interface. set target-async 1 # 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
set non-stop off
show non-stop
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 section 5.4.3 Background Execution) to run some threads in the background while you continue to examine or step others from GDB. The MI execution commands (see section 27.11 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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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.
You need to explicitly enable asynchronous mode before you can use background execution commands. You can use these commands to manipulate the asynchronous mode setting:
set target-async on
set target-async off
show target-async
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
attach
step
stepi
next
nexti
continue
finish
until
Background execution is especially useful in conjunction with non-stop
mode for debugging programs with multiple threads; see 5.4.2 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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When your program has multiple threads (see section 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 ...
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. threadno 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 the
breakpoint condition, like this:
(gdb) break frik.c:13 thread 28 if bartab > lim |
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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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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 values(5).
If you are debugging in a target environment that supports reverse execution, GDB provides the following commands.
reverse-continue [ignore-count]
rc [ignore-count]
reverse-step [count]
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-stepi will take you
back from the destination of the jump to the jump instruction itself.
reverse-next [count]
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
(6).
reverse-nexti [count]
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 instruction, reverse-nexti will continue to execute
in reverse until the call to that function (from the current stack
frame) is reached.
reverse-finish
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 exec-direction reverse
step, stepi, next, nexti, continue, and finish. The return
command cannot be used in reverse mode.
set exec-direction forward
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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 section 6. Running programs backward), 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:
target record
Both record and rec are aliases of target record.
Displaced stepping (see section 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 section 5.4.2 Non-Stop Mode) or in the asynchronous execution mode (see section 5.4.3 Background Execution), the process record and replay target cannot be started because it doesn't support these two modes.
record stop
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.
set record insn-number-max limit
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 zero, GDB will never delete recorded instructions from the execution log. The number of recorded instructions is unlimited in this case.
show record insn-number-max
set record stop-at-limit
If this option is OFF, GDB will automatically delete the oldest record to make room for each new one, without asking.
show record stop-at-limit
stop-at-limit.
info record insn-number
record delete
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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 section Selecting a Frame.
When your program stops, GDB automatically selects the
currently executing frame and describes it briefly, similar to the
frame command (see section Information about a Frame).
8.1 Stack Frames Stack frames 8.2 Backtraces 8.3 Selecting a Frame Selecting a frame 8.4 Information About a Frame Information on a frame
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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 section $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' |
frame args
frame command allows you to move from one stack frame to another,
and to print the stack frame you select. args may be either the
address of the frame or the stack frame number. Without an argument,
frame prints the current stack frame.
select-frame
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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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
You can stop the backtrace at any time by typing the system interrupt character, normally Ctrl-c.
backtrace n
bt n
backtrace -n
bt -n
backtrace full
bt full
bt full n
bt full -n
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 section 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 10.8 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=<value optimized out>) at macro.c:242
#2 0x6840 in expand_token (obs=0x0, t=<value 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 `<value 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
main(7).
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
set backtrace past-main off
show backtrace past-main
set backtrace past-entry
set backtrace past-entry on
main (or equivalent) is called.
set backtrace past-entry off
show backtrace past-entry
set backtrace limit n
set backtrace limit 0
show backtrace limit
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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
main.
frame addr
f addr
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
down n
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 section Printing Source Lines,
for details.
up-silently n
down-silently n
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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There are several other commands to print information about the selected stack frame.
frame
f
f. With an
argument, this command is used to select a stack frame.
See section Selecting a Frame.
info frame
info f
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
frame command.
See section Selecting a Frame.
info args
info locals
info catch
up,
down, or frame commands); then type info catch.
See section Setting Catchpoints.
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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 section 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.
9.1 Printing Source Lines Printing source lines 9.2 Specifying a Location How to specify code locations 9.3 Editing Source Files Editing source files 9.4 Searching Source Files Searching source files 9.5 Specifying Source Directories Specifying source directories 9.6 Source and Machine Code Source and machine code
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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 9.2 Specifying a Location, for the full list.
Here are the forms of the list command most commonly used:
list linenum
list function
list
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 section Examining the Stack), this prints lines centered around that line.
list -
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
list command display count source lines (unless
the list argument explicitly specifies some other number).
show listsize
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 section 9.2 Specifying a Location), but the effect is always
to specify some source line.
Here is a complete description of the possible arguments for list:
list linespec
list first,last
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
list first,
list +
list -
list
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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
-offset
+offset
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 section 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
function
filename:function
*address
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 section 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
funcaddr
&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
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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
location. Editing starts at
that location, e.g., at the specified source line of the
specified file. See section 9.2 Specifying a Location, for all the possible forms
of the location argument; here are the forms of the edit
command most commonly used:
edit number
edit function
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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 ... |
csh shell,
setenv EDITOR /usr/bin/vi gdb ... |
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There are two commands for searching through the current source file for a regular expression.
forward-search regexp
search regexp
fo.
reverse-search regexp
rev.
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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 ...
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
show directories
set substitute-path from to
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 no path is specified, then all substitution rules are deleted.
show substitute-path [path]
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:
directory with no argument to reset the source path to its default value.
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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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
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 section Examining Memory). Also, this address is saved as the value of the
convenience variable $_ (see section Convenience Variables).
disassemble
disassemble /m
/m modifier.
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. Two arguments specify a range of addresses
(first inclusive, second exclusive) to dump.
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:
(gdb) disas /m main
Dump of assembler code for function main:
5 {
0x08048330 <main+0>: push %ebp
0x08048331 <main+1>: mov %esp,%ebp
0x08048333 <main+3>: sub $0x8,%esp
0x08048336 <main+6>: and $0xfffffff0,%esp
0x08048339 <main+9>: sub $0x10,%esp
6 printf ("Hello.\n");
0x0804833c <main+12>: movl $0x8048440,(%esp)
0x08048343 <main+19>: call 0x8048284 <puts@plt>
7 return 0;
8 }
0x08048348 <main+24>: mov $0x0,%eax
0x0804834d <main+29>: leave
0x0804834e <main+30>: ret
End of assembler dump.
|
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
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
set disassemble-next-line
show disassemble-next-line
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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 section Using GDB with Different Languages).
print expr
print /f expr
print
print /f
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 section 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 section Examining the Symbol Table.
10.1 Expressions 10.2 Ambiguous Expressions 10.3 Program Variables Program variables 10.4 Artificial Arrays Artificial arrays 10.5 Output Formats Output formats 10.6 Examining Memory Examining memory 10.7 Automatic Display Automatic display 10.8 Print Settings Print settings 10.9 Value History Value history 10.10 Convenience Variables Convenience variables 10.11 Registers 10.12 Floating Point Hardware Floating point hardware 10.13 Vector Unit 10.14 Operating System Auxiliary Information Auxiliary data provided by operating system 10.15 Memory Region Attributes Memory region attributes 10.16 Copy Between Memory and a File Copy between memory and a file 10.17 How to Produce a Core File from Your Program Cause a program dump its core 10.18 Character Sets Debugging programs that use a different character set than GDB does 10.19 Caching Data of Remote Targets Data caching for remote targets 10.20 Search Memory Searching memory for a sequence of bytes
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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
4.1 Compiling for Debugging.
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 section 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:
@
::
{type} addr
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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
multiple-symbols setting.
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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 section 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,
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 |
This use of `::' is very rarely in conflict with the very similar use of the same notation in C++. GDB also supports use of the C++ scope resolution operator in GDB expressions.
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. For example, GCC, the GNU C/C++ compiler, usually supports the `-gstabs+' option. `-gstabs+' produces debug info in a format that is superior to formats such as COFF. You may be able to use DWARF 2 (`-gdwarf-2'), which is also an effective form for debug info. See section `Options for Debugging Your Program or GCC' in Using the GNU Compiler Collection (GCC). See section 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 section incomplete type, for more about this.
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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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 section 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 section 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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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
d
u
o
t
a
(gdb) p/a 0x54320 $3 = 0x54320 <_initialize_vx+396> |
The command info symbol 0x54320 yields similar results.
See section info symbol.
c
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
s
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.
r
For example, to print the program counter in hex (see section 10.11 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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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
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.
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.
b
h
w
g
Each time you specify a unit size with x, that size becomes the
default unit the next time you use x. (For the `s' and
`i' formats, the unit size is ignored and is normally not written.)
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 section 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.
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 section 20. Debugging Remote Programs), you may wish to verify the program's image in the
remote machine's memory against the executable file you downloaded to
the target. The compare-sections command is provided for such
situations.
compare-sections [section-name]
"qCRC"
remote request.
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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
display does not repeat if you press RET again after using it.
display/fmt expr
display/fmt addr
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 section Registers).
undisplay dnums...
delete display dnums...
undisplay does not repeat if you press RET after using it.
(Otherwise you would just get the error `No display number ...'.)
disable display dnums...
enable display dnums...
display
info display
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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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
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
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
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
set print symbol-filename off
show print symbol-filename
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
show print max-symbolic-offset
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.
Other settings control how different kinds of objects are printed:
set print array
set print array on
set print array off
show print array
set print array-indexes
set print array-indexes on
set print array-indexes off
show print array-indexes
set print elements number-of-elements
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 zero means that the printing is unlimited.
show print elements
set print frame-arguments value
all
scalars
.... 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
.... 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
set print repeats
"<repeats n times>", where n is the number of
identical repetitions, instead of displaying the identical elements
themselves. Setting the threshold to zero will cause all elements to
be individually printed. The default threshold is 10.
show print repeats
set print null-stop
show print null-stop
set print pretty on
$1 = {
next = 0x0,
flags = {
sweet = 1,
sour = 1
},
meat = 0x54 "Pork"
}
|
set print pretty off
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}
|
This is the default format.
show print pretty
set print sevenbit-strings on
\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
show print sevenbit-strings
set print union on
set print union off
"{...}"
instead.
show print union
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
$1 = {it = Tree, form = {...}}
|
set print union affects programs written in C-like languages
and in Pascal.
These settings are of interest when debugging C++ programs:
set print demangle
set print demangle on
show print demangle
set print asm-demangle
set print asm-demangle on
show print asm-demangle
set demangle-style style
auto
gnu
g++) encoding algorithm.
This is the default.
hp
aCC) encoding algorithm.
lucid
lcc) encoding algorithm.
arm
cfront-generated executables. GDB would
require further enhancement to permit that.
show demangle-style
set print object
set print object on
set print object off
show print object
set print static-members
set print static-members on
set print static-members off
show print static-members
set print pascal_static-members
set print pascal_static-members on
set print pascal_static-members off
show print pascal_static-members
set print vtbl
set print vtbl on
vtbl commands do not work on programs compiled with the HP
ANSI C++ compiler (aCC).)
set print vtbl off
show print vtbl
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Values printed by the print command are saved in the GDB
value history. This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded
(for example with the file or symbol-file commands).
When the symbol table changes, the value history is discarded,
since the values may contain pointers back to the types defined in the
symbol table.
The values printed are given history numbers by which you can
refer to them. These are successive integers starting with one.
print shows you the history number assigned to a value by
printing `$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 *$.next |
You can print successive links in the chain by repeating this command--which you can do by just typing RET.
Note that the history records values, not expressions. If the value of
x is 4 and you type these commands:
print x set x=5 |
then the value recorded in the value history by the print command
remains 4 even though the value of x has changed.
show values
show
values does not change the history.
show values n
show values +
show values + produces no display.
Pressing RET to repeat show values n has exactly the
same effect as `show values +'.
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GDB provides convenience variables that you can use within GDB to hold on to a value and refer to it later. These variables exist entirely within GDB; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is why you can use them freely.
Convenience variables are prefixed with `$'. Any name preceded by `$' can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see section Registers). (Value history references, in contrast, are numbers preceded by `$'. See section 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
show conv.
init-if-undefined $variable = expression
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.
$_
$_ is automatically set by the x command to
the last address examined (see section 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 $__.
$__
$__ 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
$_exitcode is automatically set to the exit code when
the program being debugged terminates.
$_siginfo
$_siginfo is bound to extra signal information
inspection (see extra signal information).
On HP-UX systems, if you refer to a function or variable name that begins with a dollar sign, GDB searches for a user or system name first, before it searches for a convenience variable.
GDB also supplies some convenience functions. These have a syntax similar to convenience variables. A convenience function can be used in an expression just like an ordinary function; however, a convenience function is implemented internally to GDB.
help function
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You can refer to machine register contents, in expressions, as variables
with names starting with `$'. The names of registers are different
for each machine; use info registers to see the names used on
your machine.
info registers
info all-registers
info registers regname ...
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 pointer(10) 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 section 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').
However, GDB must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if GDB is unable to locate the saved registers, the selected stack frame makes no difference.
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Depending on the configuration, GDB may be able to give you more information about the status of the floating point hardware.
info float
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Depending on the configuration, GDB may be able to give you more information about the status of the vector unit.
info vector
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GDB provides interfaces to useful OS facilities that can help you debug your program.
When GDB runs on a Posix system (such as GNU or Unix
machines), it interfaces with the inferior via the ptrace
system call. The operating system creates a special sata structure,
called struct user, for this interface. You can use the
command info udot to display the contents of this data
structure.
info udot
struct user maintained by the OS
kernel for the program being debugged. GDB displays the
contents of struct user as a list of hex numbers, similar to
the examine command.
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
On some targets, GDB can access operating-system-specific information and display it to user, without interpretation. For remote targets, this functionality depends on the remote stub's support of the `qXfer:osdata:read' packet, see qXfer osdata read.
info os processes
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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...
mem auto
delete mem nums...
disable mem nums...
enable mem nums...
info mem
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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
wo
rw
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8
16
32
64
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cache
nocache
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set mem inaccessible-by-default [on|off]
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
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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
The format parameter may be any one of:
binary
ihex
srec
tekhex
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
restore filename [binary] bias start end
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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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 section 18.1 Commands to Specify 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]
Note that this command is implemented only for some systems (as of this writing, GNU/Linux, FreeBSD, Solaris, Unixware, and S390).
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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 section 20. Debugging Remote Programs) 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 host-charset 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 target-charset TABTAB, GDB will list the host character sets it supports.
set charset charset
show charset
show host-charset
show target-charset
set target-wide-charset charset
wchar_t type. To
display the list of supported wide character sets, type
set target-wide-charset TABTAB.
show target-wide-charset
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 '+' $10 = 78 '+' (gdb) |
The IBM1047 character set uses the number 78 to encode the `+' character.
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GDB can cache data exchanged between the debugger and a remote target (see section 20. Debugging Remote Programs). Such caching generally improves performance, because it reduces the overhead of the remote protocol by bundling memory reads and writes into large chunks. Unfortunately, GDB does not currently know anything about volatile registers, and thus data caching will produce incorrect results when volatile registers are in use.
set remotecache on
set remotecache off
ON, use data
caching. By default, this option is OFF.
show remotecache
info dcache
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Memory can be searched for a particular sequence of bytes with the
find command.
find [/sn] start_addr, +len, val1 [, val2, ...]
find [/sn] start_addr, end_addr, val1 [, val2, ...]
s and n are optional parameters. They may be specified in either order, apart or together.
b
h
w
g
All values are interpreted in the current language. This means, for example, that if the current source language is C/C++ then searching for the string "hello" includes the trailing '\0'.
If the value size is not specified, it is taken from the value's type in the current language. This is useful when one wants to specify the search pattern as a mixture of types. Note that this means, for example, that in the case of C-like languages a search for an untyped 0x42 will search for `(int) 0x42' which is typically four bytes.
You can use strings as search values. Quote them with double-quotes
(").
The string value is copied into the search pattern byte by byte,
regardless of the endianness of the target and the size specification.
The address of each match found is printed as well as a count of the number of matches found.
The address of the last value found is stored in convenience variable `$_'. 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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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 section 10.3 Program Variables, for more information about debugging optimized code.
11.1 Inline Functions How GDB presents inlining
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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:
finish command. This is a limitation of compiler-generated
debugging information; after finish, you can step to the next line
and print a variable where your program stored the return value.
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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 section 4.1 Compiling for Debugging.
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 9.1 Printing Source Lines.
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
macro expand-once expression
macro exp1 expression
info macro macro
macro define macro replacement-list
macro define macro(arglist) replacement-list
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
macro
define command, described above; it cannot remove definitions present
in the program being debugged.
macro list
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-2' 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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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 section 19. Specifying a Debugging Target. 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 D.6 Tracepoint Packets.
This chapter describes the tracepoint commands and features.
13.1 Commands to Set Tracepoints 13.2 Using the Collected Data 13.3 Convenience Variables for Tracepoints
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Before running such a trace experiment, an arbitrary number of tracepoints can be set. A tracepoint is actually a special type of breakpoint (see section 5.1.1 Setting Breakpoints), 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. Conditional expressions and ignore counts on tracepoints have no effect, and tracepoints cannot run GDB commands when they are hit. Tracepoints may not be thread-specific either.
This section describes commands to set tracepoints and associated conditions and actions.
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trace location
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 section 9.2 Specifying a 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 doesn'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.
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.
The convenience variable $tpnum records the tracepoint number
of the most recently set tracepoint.
delete tracepoint [num]
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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These commands are deprecated; they are equivalent to plain disable and enable.
disable tracepoint [num]
enable tracepoint command.
enable tracepoint [num]
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passcount [n [num]]
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
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actions [num]
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 and
while-stepping.
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 while stepping. 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 $fp, $sp > end end |
collect expr1, expr2, ...
$regs
$args
$locals
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 command info scope (see section info scope) is
particularly useful for figuring out what data to collect.
while-stepping n
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 > |
You may abbreviate while-stepping as ws or
stepping.
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info tracepoints [num]
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:
passcount n command
while-stepping n command
actions command. The actions
are prefixed with an `A' so as to distinguish them from commands.
(gdb) info trace
Num Type Disp Enb Address What
1 tracepoint keep y 0x0804ab57 in foo() at main.cxx:7
pass count 1200
step count 20
A while-stepping 20
A collect globfoo, $regs
A end
A collect globfoo2
A end
(gdb)
|
This command can be abbreviated info tp.
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tstart
tstop
Note: a trace experiment and data collection may stop automatically if any tracepoint's passcount is reached (see section 13.1.3 Tracepoint Passcounts), or if the trace buffer becomes full.
tstatus
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 |
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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.
13.2.1 tfind nHow to select a trace snapshot 13.2.2 tdumpHow to display all data for a snapshot 13.2.3 save-tracepoints filenameHow to save tracepoints for a future run
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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
tfind 0 (since 0 is the number of the first snapshot).
tfind none
tfind end
tfind
tfind -
tfind tracepoint num
tfind pc addr
tfind outside addr1, addr2
tfind range addr1, addr2
tfind line [file:]n
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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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) |
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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 section 23.1.3 Command Files).
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(int) $trace_frame
(int) $tracepoint
(int) $trace_line
(char []) $trace_file
(char []) $trace_func
$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.
(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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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.
14.1 How Overlays Work A general explanation of overlays. 14.2 Overlay Commands Managing overlays in GDB. 14.3 Automatic Overlay Debugging GDB can find out which overlays are mapped by asking the inferior. 14.4 Overlay Sample Program A sample program using overlays.
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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.
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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