22.2. 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 infinte 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:
handle SIGALRM nopass
handle SIGALRM pass
handle SIGLARM pass
As a further example, to hook at the begining and end of the echo
command, and to add extra text to the beginning and end of the message,
you could define:
(gdb) echo Hello World
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.
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.