Perl Hacks

Hack #78. Introspect Your Subroutines

Trace any subroutine to its source.

You can name anonymous subroutines [Hack #57] and deparse them [Hack #56]. You can even peek at their closed-over lexical variables [Hack #76]. There are still more wonders in the world.

Someday you'll have to debug a running program and figure out exactly where package A picked up subroutine B. One option is to trace all import( ) calls, but that's even less fun than it sounds. Another option is to pull out the scariest and most powerful toolkit in the Perl hacker's toolbox: the B::* modules.

The Hack

Finding a misbehaving function means you need to know two of three things:

• The original package of the function

• The name of the file containing the function

• The line number in the file corresponding to the function

From there, your debugging should be somewhat easier. Perl stores all of this information for every CV^[11] it compiles. You just need a way to get to it.

The usual entry point is through the B module and its svref_2object( ) function, which takes a normal Perl data structure, grabs the underlying C representation, and wraps it in hairy-scary objects that allow you to peek (though not usually poke) at its guts.

It's surprisingly easy to report a subroutine's vital information:

use B;

sub introspect_sub
{
    my $sub      = shift;
    my $cv       = B::svref_2object( $sub );

    return join( ':',
        $cv->STASH->NAME( ), $cv->FILE( ), $cv->GV->LINE( ) . "\\n"
    );
}

introspect_sub( ) takes one argument, a reference to a subroutine. ...