Node:Hooks, Next:Command Files, Previous:Define, Up:Sequences

### 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:

         define hook-stop
         handle SIGALRM nopass
         end

         define hook-run
         handle SIGALRM pass
         end

         define hook-continue
         handle SIGLARM pass
         end

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:

         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`. 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.