Perl Hacks

Hack #59. Customize the Debugger

Write your own debugger commands.

Adding a command to the debugger (or modifying an existing one) by editing the debugger is a difficult job; to do this, you have to patch the debugger source in perl5db.pl and replace it. Sometimes you don't have the necessary privileges to do this, and given the complexity of the debugger, it's a difficult job—especially because you can't debug the debugger.

Yet modifying your tools the way you want them is important. Fortunately, Devel::Command module makes this much simpler. With Devel::Command, you write simple modules to define your commands, and the debugger finds them and loads them for you automatically.

The Hack

Writing a command is simple. There are only a few things to remember:

Input and output

The debugger reads input from DB::IN and writes to DB::OUT. If you want your command to work just like a native debugger command, you need to use these filehandles for input and output. Generally, you'll only need to print to DB::OUT.

Debugger context versus program context

To evaluate an expression in the context of the program that's being debugged (for example, you want to pass the value of a variable in the program to your command), call the subroutine &eval on it. To evaluate something in the debugger's context, use plain old eval.

A "hello, world" command looks like:

package Devel::Command::HelloWorld;
use base 'Devel::Command';

sub command
{
    print DB::OUT "Hello world!\\n";
    1;
}

1;

Devel::Command defaults to ...