Dear monks, brother xiper has posted a question (329204) on using Tk's waitVariable with (MMF) tied variables. I will post the code here again.

use strict;
use Tk;
use Win32::MMF::Shareable;

print "Process starting...\n";

my $ns = tie my $wait, 'Win32::MMF::Shareable', 'wait' or die;
$wait = 0;

my $pid = fork;
if (!$pid)
{
  $SIG{HUP} = \&stop;
  while( 1 ) { print "tick()\n"; sleep 1 }
}

my $mw = MainWindow->new;
$mw->Button( -text => '$wait',                   -command => sub { pri
+nt "\$wait is $wait\n" } )->pack;
$mw->Button( -text => 'waitVariable( \\$wait )', -command => \&start )
+->pack;
$mw->Button( -text => 'local $wait++',           -command => \&stop  )
+->pack;
$mw->Button( -text => 'remote $wait++',          -command => sub{ kill
+( 'HUP', $pid ) } )->pack;

MainLoop;

kill( 'TERM', $pid );

sub start
{
  print "waiting for \$wait (was $wait)\n";
  $mw->waitVariable( \$wait );
  print "finished waiting for \$wait (is now $wait)\n";
}

sub stop
{
  print "\$wait++ by pid $$\n";
  $wait++;
}

[download]

Basically the code does a waitVariable on $wait, which should be updated by the child process upon receiving a 'HUP' signal. When I ran the code, the value of $wait was actually incremented by the child process, however the Tk's waitVariable didn't catch the updated variable, eventhough the value of the variable did change.

After hours of going through the Tk source, I couldn't figure out a sensible explanation to why this is happenning. I suspect it's because Tk's wait function goes too low level that it 'bypassed' the perl tie() handler and watches a local copy of the variable instead?

Can someone please give an explanation to this strangeness, and a possible cure? Thanks!

In reply to Tk and waitVariable on tied variables by Roger

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.