How to locate the source file of a subroutine call when debugging a complex CPAN distribution?

Question

I am trying to debug a Tk program for a while now. The problem seems to be with a deiconify() call from a top level window, however I am not able to locate the source file where the deiconify() sub is defined. Here is a made-up example just to illustrate what I mean:

test.pl:

use strict;
use warnings;
use Tk;

my $mw = MainWindow->new( -title => "Main Window" );
$mw->Label(-text => "Debugging", -font => "Times 20")->pack( );
$mw->Button(
    -text    => 'Quit',
    -command => sub { exit },
)->pack;
$mw->Button(
    -text    => 'Show window',
    -command => \&show_window,
)->pack;

my $tl = $mw->Toplevel( -title => "Toplevel 1" );
$tl->Button(
    -text    => 'Quit',
    -command => sub { exit },
)->pack;
$tl->withdraw();
MainLoop;

sub show_window {
    #$DB::single = 1;
    $tl->deiconify();  # <--- Where is this sub defined??
    $tl->raise();
}

I first tried to grep the distribution for the sub name¹:

find . -name '*.pm' -exec grep -Hn deiconify {} \;
find . -name '*.xs' -exec grep -Hn deiconify {} \;
find . -name '*.al' -exec grep -Hn deiconify {} \;
find . -name '*.h' -exec grep -Hn deiconify {} \;

Then I tried to run the script under the debugger

perl -d test.pl

and set a breakpoint just before the call to $tl->deiconify() (see above). When I pressed s at the breakpoint

  DB<1> s
Tk::Submethods::CODE(0x56245540c658)(/home/hakon/perlbrew/perls/perl-5.24.1/lib/site_perl/5.24.1/x86_64-linux/Tk/Submethods.pm:37):
37:      *{$package.'::'.$sub} = sub { shift->$fn($sub,@_) };

it shows the line where deiconify() was defined as anonymous subroutine ( Line 37 of Tk::Submethods ), but when I press s again to step into the anonymous subroutine, it just step over it and returns to line 32 in test.pl.

I suspect the method must be defined in Tk::wm somehow (since it is related to the Window manager) maybe by the auto load mechanism or by dynaloader?

Footnotes

_{1. The commands was run from the toplevel directory of the Tk distribution. To set it up, run this first:}

 cpan -g Tk
 tar zxvf Tk-804.033.tar.gz
 cd Tk-804.033

That line is being shown because it's the code for the `deiconify` method: it is converted into a call to the method named by `$fn` that passes `$sub` (presumably the string `"deiconify"`) as the first parameter. It's presumably XS code if the debugger isn't showing it. What is in `$fn` at that point? — Borodin, Apr 05 '17 at 21:15

score 2 · Accepted Answer · answered Apr 06 '17 at 06:55

Grepping the source a little more liberally (specifically including *.c files) finds the string This procedure is invoked to process the "wm deiconify" Tcl command in the file pTk/mTk/win/tkWinWm.c. It's in a comment introducing the C function WmDeiconifyCmd, which certainly looks like it's the actual implementation of that functionality.

I haven't bothered to look into exactly how the Tk module XS code exposes that C function to the Perl level, but if that's what you're really interested in, you now know the endpoints and just have to fill in the middle :-)

Once the function is exposed to the Perl level, the line in Tk::Submethods that you quote above is clearly where it gets injected into the appropriate symbol table so it can be called via $tl->deiconify().

How to locate the source file of a subroutine call when debugging a complex CPAN distribution?

1 Answers1