sparky8342 has asked for the wisdom of the Perl Monks concerning the following question:

Can anyone explain the following behaviour?

#!/usr/bin/perl

sub extract {
   my($i,$y) = @_;
   print "Unpack $i: " . (join ',',unpack('w*',$y)) . "\n";
}

$x=4294967296;

foreach(1..9) {
   $z=pack('w*',$x);
   &extract($_,$z);
}

[download]
Output:

Unpack 1: 2082839,9,0,0,0
Unpack 2: 34126102543,0,0,0
Unpack 3: 34126037007,0,0,0
Unpack 4: 34126037007,0,0,0
Unpack 5: 34126037007,0,0,0
Unpack 6: 34126037007,0,0,0
Unpack 7: 34126037007,0,0,0
Unpack 8: 34126037007,0,0,0
Unpack 9: 34126037007,0,0,0
This only happens once $x >= 4294967296
Thanks.

Replies are listed 'Best First'.
Re: Unpack weirdness
by jmcnamara (Monsignor) on May 30, 2002 at 14:49 UTC

    It looks like a bug: 
        $ perl5.6.0 -le 'print unpack "w", pack "w", 2**32'
    2082834

    $ perl5.7.2 -le 'print unpack "w", pack "w", 2**32'
    4294967296

    [download]

    --
    John.

[reply]
[d/l]
      I do get the same results for 5.6.0 and 5.7.2, but if I go further back in time, I get different results. Note that 2 ** 32 is bigger than what fits in a 32 bit integer, so it could matter whether your version of Perl stores the value in an 64 integer, or a double.

      What follows are result of 

          print unpack "w*", pack "w*", 2 ** 32;

      [download]

      of all versions of Perl since 5.003_07 (the first version that produces any output).

      Abigail

      Read more... (6 kB)
[reply]
[d/l]
        Just out of curiousity, could you go into the set up that produced these results? I'm imagining something in which you just type a few simple commands on a script and blammo. 
        ()-()
 \"/
  `
[reply]

Back to Seekers of Perl Wisdom