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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] [prog[core|procID]]


        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 Fortran.

        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:

Command Description
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.
Continue running your program (after stopping, e.g. at a break point).
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.
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.


        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.

Option Description
List all options, with brief explanations.
-s file
Read symbol table from file file.
-write Enable writing into executable and core files.
-e file
Use file 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 file and use it as the executable file.
-c file
Use file file as a core dump to examine.
-x file
Execute GDB commands from file file.
-d directory
Add directory to the path to search for source files.
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''. 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; 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.
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.