20.2 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

(No value for GDBP) echo Hello World
<<<---Hello World--->>>
(No value for GDBP)

You can define a hook for any single-word command in No value for GDBN, but not for command aliases; you should define a hook for the basic command name, e.g. backtrace rather than bt. If an error occurs during the execution of your hook, execution of No value for GDBN commands stops and No value for GDBN 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.