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GDB

Section: GNU Development Tools (1)
Updated: 2017-10-25
Index

NAME

gdb - The GNU Debugger

SYNOPSIS

gdb [-help] [-nh] [-nx] [-q][-batch] [-cd=dir] [-f][-b bps]
[-tty=dev] [-s symfile][-e prog] [-se prog][-c core] [-p procID]
[-x cmds] [-d dir][prog|prog procID|prog core]

DESCRIPTION

The purpose of a debugger such as GDB is to allow you to see what isgoing on ``inside'' another program while it executes --- or what anotherprogram was doing at the moment it crashed.

GDB can do four main kinds of things (plus other things in support ofthese) 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 theeffects of one bug and go on to learn about another.

You can use GDB to debug programs written in C, C@t{++}, Fortran andModula-2.

GDB is invoked with the shell command "gdb". Once started, it readscommands from the terminal until you tell it to exit with the GDBcommand "quit". You can get online help from GDB itselfby using the command "help".

You can run "gdb" with no arguments or options; but the mostusual way to start GDB is with one argument or two, specifying anexecutable program as the argument:

        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 wantto debug a running process:

        gdb program 1234        gdb -p 1234

would attach GDB to process 1234 (unless you also have a filenamed 1234; GDB does check for a core file first).With option -p you can omit the program filename.

Here are some of the most frequently needed GDB commands:

break [file:]functiop: Set a breakpoint at function (in file).
run [arglist]: Start your program (with arglist, if specified).
bt: Backtrace: display the program stack.
print expr: Display the value of an expression.
c: Continue running your program (after stopping, e.g. at a breakpoint).
next: Execute next program line (after stopping); step over anyfunction 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.
step: Execute next program line (after stopping); step into anyfunction calls in the line.
help [name]: Show information about GDB command name, or general informationabout using GDB.
quit: 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 onlineas the "gdb" entry in the "info" program.

OPTIONS

Any arguments other than options specify an executablefile and core file (or process ID); that is, the first argumentencountered with noassociated 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 haveboth long and short forms; both are shown here. The long forms are alsorecognized if you truncate them, so long as enough of the option ispresent to be unambiguous. (If you prefer, you can flag optionarguments with + rather than -, though we illustrate themore usual convention.)

All the options and command line arguments you give are processedin sequential order. The order makes a difference when the -xoption is used.

-help

-h

List all options, with brief explanations.

-symbols=file

-s file

Read symbol table from file file.

-write

Enable writing into executable and core files.

-exec=file

-e file

Use file file as the executable file to execute whenappropriate, and for examining pure data in conjunction with a coredump.

-se=file

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

-core=file

-c file

Use file file as a core dump to examine.

-command=file

-x file

Execute GDB commands from file file.

-ex command

Execute given GDB command.

-directory=directory

-d directory

Add directory to the path to search for source files.

-nh

Do not execute commands from ~/.gdbinit.

-nx

-n

Do not execute commands from any .gdbinit initialization files.

-quiet

-q

``Quiet''. Do not print the introductory and copyright messages. Thesemessages are also suppressed in batch mode.

-batch

Run in batch mode. Exit with status 0 after processing all the commandfiles specified with -x (and .gdbinit, if not inhibited).Exit with nonzero status if an error occurs in executing the GDBcommands in the command files.

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

        Program exited normally.

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

-cd=directory

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

-fullname

-f

Emacs sets this option when it runs GDB as a subprocess. It tellsGDB to output the full file name and line number in a standard,recognizable fashion each time a stack frame is displayed (whichincludes each time the program stops). This recognizable format lookslike two \032 characters, followed by the file name, line numberand character position separated by colons, and a newline. TheEmacs-to-GDB interface program uses the two \032characters 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 serialinterface used by GDB for remote debugging.

-tty=device

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

COPYRIGHT

Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.3 orany later version published by the Free Software Foundation; with theInvariant Sections being ``Free Software'' and ``Free Software NeedsFree 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 modifythis GNU Manual. Buying copies from GNU Press supports the FSF indeveloping GNU and promoting software freedom.''

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