gdb [OPTIONS] [prog|prog procID|prog core]
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, C++, Fortran and Modula-2.
GDB is invoked with the shell command
gdb. Once started, it reads
commands from the terminal until you tell it to exit with the GDB
exit. You can get online help from GDB itself
by using the command
You can run
gdb with no arguments or options; but the most
usual way to start GDB is with one argument or two, specifying an
executable program as the argument:
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 or use option
-p, if you want to debug a running process:
gdb program 1234 gdb -p 1234
would attach GDB to process
1234. With option -p you
can omit the program filename.
Here are some of the most frequently needed GDB commands:
Set a breakpoint at function or line (in file).
Start your program (with arglist, if specified).
Backtrace: display the program stack.
Display the value of an expression.
Continue running your program (after stopping, e.g. at a breakpoint).
Execute next program line (after stopping); step over any function calls in the line.
look at the program line where it is presently stopped.
type the text of the program in the vicinity of where it is presently stopped.
Execute next program line (after stopping); step into any function calls in the line.
Show information about GDB command name, or general information about using GDB.
Exit from GDB.
Any arguments other than options specify an executable file and core file (or process ID); that is, the first argument encountered with no associated option flag is equivalent to a --se option, and the second, if any, is equivalent to a -c option if it’s the name of a file. Many options have both long and abbreviated forms; both are shown here. The long forms are also recognized if you truncate them, so long as enough of the option is present to be unambiguous.
The abbreviated forms are shown here with ‘-’ and long forms are shown with ‘--’ to reflect how they are shown in --help. However, GDB recognizes all of the following conventions for most options:
All the options and command line arguments you give are processed in sequential order. The order makes a difference when the -x option is used.
List all options, with brief explanations.
Read symbol table from file.
Enable writing into executable and core files.
Use file as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.
Read symbol table from file and use it as the executable file.
Use file as a core dump to examine.
Execute GDB commands from file.
Execute given GDB command.
Execute GDB command before loading the inferior.
Add directory to the path to search for source files.
Do not execute commands from ~/.config/gdb/gdbinit, ~/.gdbinit, ~/.config/gdb/gdbearlyinit, or ~/.gdbearlyinit
Do not execute commands from any .gdbinit or .gdbearlyinit initialization files.
“Quiet”. Do not print the introductory and copyright messages. These messages are also suppressed in batch mode.
Run in batch mode. Exit with status
0 after processing all the command
files specified with -x (and .gdbinit, if not inhibited).
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.
Run in batch mode, just like --batch, but totally silent. All GDB output is supressed (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
stdout, will also be made silent.
--args prog [arglist]
Change interpretation of command line so that arguments following this option are passed as arguments to the inferior. As an example, take the following command:
gdb ./a.out -q
It would start GDB with -q, not printing the introductory message. On the other hand, using:
gdb --args ./a.out -q
starts GDB with the introductory message, and passes the option to the inferior.
Attach GDB to an already running program, with the PID pid.
Open the terminal user interface.
Read all symbols from the given symfile on the first access.
Do not read symbol files.
Run in DBX compatibility mode.
GDB’s exit code will be the same as the child’s exit code.
Print details about GDB configuration and then exit.
Print version information and then exit.
Run GDB using directory as its working directory, instead of the current directory.
Run GDB using directory as its data directory. The data directory is where GDB searches for its auxiliary files.
Emacs sets this option when it runs GDB as a subprocess. It tells GDB to output the full file name and line number in a standard, recognizable fashion each time a stack frame is displayed (which includes each time the program stops). This recognizable format looks like two ‘\032’ characters, followed by the file name, line number and character position separated by colons, and a newline. The Emacs-to-GDB interface program uses the two ‘\032’ characters as a signal to display the source code for the frame.
Set the line speed (baud rate or bits per second) of any serial interface used by GDB for remote debugging.
Set timeout, in seconds, for remote debugging.
Run using device for your program’s standard input and output.