|GDB(1)||GNU Development Tools||GDB(1)|
GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:
- Start your program, specifying anything that might affect its behavior.
- Make your program stop on specified conditions.
- Examine what has happened, when your program has stopped.
- Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.
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 command "quit". You can get online help from GDB itself by using the command "help".
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:
- break [file:]function
- Set a breakpoint at function (in file).
- run [arglist]
- Start your program (with arglist, if specified).
- Backtrace: display the program stack.
- print expr
- 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.
- edit [file:]function
- look at the program line where it is presently stopped.
- list [file:]function
- 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.
- help [name]
- Show information about GDB command name, or general information about using GDB.
- Exit from GDB.
For full details on GDB, see Using GDB: A Guide to the GNU Source-Level Debugger, by Richard M. Stallman and Roland H. Pesch. The same text is available online as the "gdb" entry in the "info" program.
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.
- -s file
- Read symbol table from file file.
- Enable writing into executable and core files.
- -e file
- Use file 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 file and use it as the executable file.
- -c file
- Use file file as a core dump to examine.
- -x file
- Execute GDB commands from file file.
- -ex command
- Execute given GDB command.
- -d directory
- Add directory to the path to search for source files.
- Do not execute commands from ~/.gdbinit.
- Do not execute commands from any .gdbinit 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 GDB using directory as its working directory, instead of the current directory.
- 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.
- -b bps
- Set the line speed (baud rate or bits per second) of any serial interface used by GDB for remote debugging.
- Run using device for your program's standard input and output.
should give you access to the complete manual.
Using GDB: A Guide to the GNU Source-Level Debugger, Richard M. Stallman and Roland H. Pesch, July 1991.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software" and "Free Software Needs Free Documentation", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom."