Hooks (Debugging with GDB)

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#### 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.

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