NAME
gdb - The GNU Debugger

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

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++, and Modula-2.
Fortran support will be added when a GNU Fortran compiler is ready.

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

would attach GDB to process 1234 (unless you also have a file named
‘1234’; GDB does check for a core file first).

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).

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 any
function calls in the line.

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.

-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.

-directory=directory

-d directory
Add directory to the path to search for source files.

-nx

-n Do not execute commands from any ‘.gdbinit’ initialization
files. Normally, the commands in these files are executed
after all the command options and arguments have been
processed.

-quiet

-q "Quiet". Do not print the introductory and copyright
messages. These messages are also suppressed in batch mode.

-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 ‘ 32’ 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 ‘ 32’ 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
‘gdb’ entry in info; Using GDB: A Guide to the GNU Source-Level
Debugger, Richard M. Stallman and Roland H. Pesch, July 1991.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission
notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided
that the entire resulting derived work is distributed under the
terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for
modified versions, except that this permission notice may be
included in translations approved by the Free Software Foundation
instead of in the original English.