gdb - The GNU Debugger
gdb [-help] [-nx] [-q] [-batch] [-cd=dir] [-f] [-b bps] [-tty=dev]
[-s symfile] [-e prog] [-se prog] [-c core] [-x cmds] [-d dir]
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
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 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
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
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:
Set a breakpoint at function (in file).
Start your program (with arglist, if specified).
bt Backtrace: display the program stack.
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.
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.
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
-h List all options, with brief explanations.
Read symbol table from file file.
-write Enable writing into executable and core files.
Use file file as the executable file to execute when appropriate,
and for examining pure data in conjunction with a core
Read symbol table from file file and use it as the executable
Use file file as a core dump to examine.
Execute GDB commands from file file.
Add directory to the path to search for source files.
-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.
-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.
Run GDB using directory as its working directory, instead of
the current directory.
-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.
Run using device for your program's standard input and output.
`gdb' entry in info; Using GDB: A Guide to the GNU Source-Level Debug-
ger, Richard M. Stallman and Roland H. Pesch, July 1991.
Copyright (c) 1991 Free Software Foundation, Inc.
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
GNU Tools 4nov1991 gdb(1)
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