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up vote 2 down vote favorite
1

I have the following:

#! /usr/bin/perl
use Fcntl ':flock';
use strict;
...
    my (@list, $fh);
    open $fh, "<:encoding(utf8)", $file or die "$file: $!";
    flock $fh, LOCK_EX;
    use sigtrap 'handler' => sub {flock($fh, LOCK_UN);}, 'normal-signals'; # line 72

It runs but gives an error when I press ^C:

$ verify.pl
...
Can't use an undefined value as a symbol reference at ./verify.pl line 72.

The docs says it's OK

$ perldoc sigtrap
...
        use sigtrap 'handler' => \&my_handler, 'normal-signals';

I can't figure out what's wrong.

share|improve this question
up vote 1 down vote

I'm going to suggest that $fh is the undefined symbol reference. The use is run during the compilation phase (as if in a BEGIN {...} block), and, depending on where you think you're calling it, the $fh may never have been opened, or its scope may not be what you think.

Add some debugging to your handler to show, for example, the value of $fh and the refaddr of $fh. Also add to your open/flock code the same. I bet the refaddr isn't the same.

Since the sigtrap is registered globally, you may be best off with a global, e.g.:

my @handlers;
use sigtrap handler => sub { $_->() for grep defined, @handlers }, 'normal-signals';

{
  my ($fh, @list);
  open my $fh, ...
  flock $fh, LOCK_EX;
  my $handler = sub { flock $fh, LOCK_UN };

  @handlers = map { weaken $_ } grep defined, @handlers, $handler;
  # ... do stuff. When $handler goes out of scope, it'll go undef in @handlers
}
share|improve this answer
    
What is refaddr? perldoc.perl.org/search.html?q=refaddr – Chloe Jan 19 at 22:51
    
@Chloe Scalar::Util::refaddr – ThisSuitIsBlackNot Jan 19 at 23:15

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