papidave has asked for the wisdom of the Perl Monks concerning the following question:

Ok, this one has me entirely confused. Although I have a workaround (see below), the reason I even need a workaround escapes me.

The requirement is to be able to allow users to interrupt a certain operation (Ctrl-C) which may be called from an application that has $SIG{INT} set to 'IGNORE'. The initial code for this (Perl 5.8.8, Linux) was:

   my $intr = 0;
   my $save_handler = $SIG{INT};
   $SIG{INT} = sub { $intr++ };

   # do something here ...

   $SIG{INT} = $save_handler;

   # check value of $intr to see if I was interrupted...

[download]

What I found was that the code worked properly when $SIG{INT} had been blocked, but if called from code that did not alter $SIG{INT}, I got slammed with a "Use of uninitialized value in scalar assignment" warning.

Since I want my code to run clean, I did some investigation. First, the key INT exists, and has an undefined value. But I get the warning when I try to assign an undefined value back into the hash.

I can avoid the assignment by using a local $SIG{INT}, e.g.:

   my $intr = 0;
   {
      local $SIG{INT};
      $SIG{INT} = sub { $intr++ };

      # do something here ...

   } # end of scope for local handler

   # check value of $intr to see if I was interrupted...

[download]

... but this seems entirely bogus. Can someone suggest a good reason why %SIG would be initialized with values that I can't assign to it? I don't get this warning when I do a simple "$foo{INT} = undef;"

Replies are listed 'Best First'.
Re: can't assign signal handler an undef value
by shmem (Chancellor) on Dec 21, 2007 at 23:20 UTC
    Nothing special about the %SIG hash, but about assigning an undefined value. Try 
       my $intr = 0;
   my $save_handler = $SIG{INT};
   $SIG{INT} = sub { $intr++ };

   # do something here ...

   $SIG{INT} = $save_handler || ''; # assign handler back if true, els
+e ''

   # check value of $intr to see if I was interrupted...

    [download]

    update:

    Well, there is something special about the %SIG hash. It's not just a normal hash, because it is the %SIG hash. Assignments have to be defined (an empty string is false, but still defined), and the keys are fixed. E.g. you can't just make up signals: 

    qwurx [shmem] ~ > perl -wle '$SIG{BLORF} = undef'
No such signal: SIGBLORF at -e line 1.

    [download]

    You could also say 

       my $save_handler = $SIG{INT} || '';

    [download]

    to have a defined (but empty) value in $save_handler.

    --shmem 

    _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
[reply]
[d/l]
[select]
Re: can't assign signal handler an undef value
by sgifford (Prior) on Dec 22, 2007 at 05:46 UTC
    I see the same behavior on my system, and I agree it's very strange. Your workaround is cleaner than your original solution, though, in case something die's.

    --
    sgifford's Web page
[reply]
[d/l]

Back to Seekers of Perl Wisdom