gdb(1) GNU Tools gdb(1)

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:

o Start your program, specifying anything that might
affect its behavior.

o Make your program stop on specified conditions.

o Examine what has happened, when your program has
stopped.

o Change things in your program, so you can experi-
ment 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 For-
tran 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, speci-
fying 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 argu-
ment, 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 differ-
ence 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’ ini-
tialization 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 copy-
right messages. These messages are also suppressed
in batch mode.

-batch Run in batch mode. Exit with status 0 after pro-
cessing 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 fil-
ter, for example to download and run a program on
another computer; in order to make this more use-
ful, the message

Program exited normally.

(which is ordinarily issued whenever a program run-
ning 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 sub-
process. 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 recog-
nizable format looks like two ‘ 32’ characters,
followed by the file name, line number and charac-
ter 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 ver-
sions 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 con-
ditions 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.

GNU Tools 4nov1991 4