gdb Man page

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.

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.

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 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.

-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.
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 \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.

COPRYRIGHT

Copyright (c) 1988-2016 Free Software Foundation, Inc.

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.”

gdb-7.11.1 2016-06-23 GDB(1)