gdb - The GNU debugger


gdb [-help] [-nx] [-q] [-batch] [-cd=dir]
	[-f] [-b bps] [-tty=dev] [-s symfile]
	[-e prog] [-se prog] [-c core] [-d dir]
	[-x file] [-epoch] [-mapped] [-nw | -w]
	[-readnow] [-version]
	[ prog [ core | procID]]


You can use gdb(1) to view what is going on inside another program while it executes, or to view what another program was doing at the moment it crashed.

The gdb utility can do the following four main things (plus other things in support of these) to help you catch bugs while a program is executing:

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.

The gdb debugger is invoked with the shell command gdb(1). 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 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 specify a process identifier (ID) as a second argument if you want to debug a running process. The following code attaches gdb to process 1234 (unless you also have a file named 1234; gdb checks for a core file first):

gdb program 1234


The following is a list of commonly used gdb commands:

break [file:]function
Set a breakpoint at function (in file).
run [arglist]
Start your program (with arglist, if specified).
Backtrace: display the program stack.
print expr
Display the value of an expression.
Continue running your program (after stopping, for example, at a breakpoint).
Execute next program line (after stopping); step over any function calls in the line.
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.
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 PDF file, gdb_info.pdf.


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 is 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 of 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 as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.
Read symbol table from file and use it as the executable file.
-core=file, -c file
Use file as a core dump to examine.
-x file
Execute gdb commands from 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.
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. To make this more useful, the following message (which is ordinarily issued when a program running under gdb control terminates) is not issued when running in batch mode:
Program exited normally.
Run gdb using directory, instead of the current directory, as its working 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 form 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.
Run using device for your program's standard input and output.
-mapped, -m
Use mapped symbol files. This feature is not available on Interix.
Output information in the format used by the Epoch emacs-gdb interface.
-readnow, -r
Fully read symbol files on first access.
-version, -v
Display version information, and then exit.
Run gdb as a command-line utility (default).
-windows, -w
Run gdb as an X Windows application.


Under some circumstances, when a function is disassembled, the disassembly may continue until the beginning of the next function. If the next function is aligned to some boundary, a few instructions of "noise" may appear at the end of the dissassembly; these can be ignored.

When disassembling or otherwise dealing with the range of instruction locations that includes the end of a function, gdb treats the end as the last byte before the beginning of the next function. On Intel processors, there may be a .align that causes a few bytes of empty space (containing random data) to be included in the range that nominally includes the end. This can be safely ignored.


Appendix B in the Professional SDK User's Guide contains an introduction to gdb.

The file /pubs/gdb_info.pdf file contains the text of the info(1) files about gdb.

gdb(1) entry in info(1); note that info(1) is not distributed with Interix products.

Using GDB: A Guide to the GNU Source-Level Debugger, 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 English.