23.1.2 User-defined Command Hooks

You may define hooks, which are a special kind of user-defined command. Whenever you run the command ‘foo’, if the user-defined command ‘hook-foo’ exists, it is executed (with no arguments) before that command.

A hook may also be defined which is run after the command you executed. Whenever you run the command ‘foo’, if the user-defined command ‘hookpost-foo’ exists, it is executed (with no arguments) after that command. Post-execution hooks may exist simultaneously with pre-execution hooks, for the same command.

It is valid for a hook to call the command which it hooks. If this occurs, the hook is not re-executed, thereby avoiding infinite recursion.

In addition, a pseudo-command, ‘stop’ exists. Defining (‘hook-stop’) makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed.

For example, to ignore SIGALRM signals while single-stepping, but treat them normally during normal execution, you could define:

define hook-stop
handle SIGALRM nopass
end
define hook-run
handle SIGALRM pass
end
define hook-continue
handle SIGALRM pass
end

As a further example, to hook at the beginning and end of the echo command, and to add extra text to the beginning and end of the message, you could define:

define hook-echo
echo <<<---
end
define hookpost-echo
echo --->>>\n
end
(gdb) echo Hello World
<<<---Hello World--->>>
(gdb)

You can define a hook for any single-word command in GDB, but not for command aliases; you should define a hook for the basic command name, e.g. backtrace rather than bt. You can hook a multi-word command by adding hook- or hookpost- to the last word of the command, e.g. ‘define target hook-remote’ to add a hook to ‘target remote’.

If an error occurs during the execution of your hook, execution of GDB commands stops and GDB issues a prompt (before the command that you actually typed had a chance to run).

If you try to define a hook which does not match any known command, you get a warning from the define command.