gdb - The GNU Debugger at Linux.org

Back to main site | Back to man page index

GDB(1) GNU Development Tools GDB(1)

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 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:

· 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@t{++}, 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:

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
gdb -p 1234

would attach GDB to process 1234 (unless you also have a file named 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.

type the text of the program in the vicinity of where it is presently stopped.

step
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.

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 online as the "gdb" entry in the "info" program.

OPTIONS
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 short 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. (If you prefer, you can flag option arguments with + rather than -, though we
illustrate the more usual convention.)

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.

-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 when appropriate, and for examining pure data in
conjunction with a core dump.

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

-core=file
-c file
Use file file as a core dump to examine.

-command=file
-x file
Execute GDB commands from file file.

-ex command
Execute given GDB command.

-batch
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.

-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 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.

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

SEE ALSO
The full documentation for GDB is maintained as a Texinfo manual. If the "info" and "gdb" programs and GDB's
Texinfo documentation are properly installed at your site, the command

info gdb

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."