PERL Beginners - pack/unpack question

Author

pack/unpack question

Beau E. Cox

2006-01-22, 7:55 am

Hi -

I really thought I understood pack/unpack,
but this has me stumped. I can't find anything to
explain the operation of the following script
in the documentation:

use strict;
use warnings;

my $buffer = pack( "NN", 22, 0 );
printf "buffer length = %s\n", length $buffer;
print "buffer ",
join( " ", map { sprintf "%02x", ord( $_ ) } split "", $buffer ),
"\n";
my( $len1, $len2 ) = unpack "NN", $buffer;
printf "len1 = %s len2 = %s\n",
$len1 || 'undef', $len2 || 'undef';

which gives this result:

$ perl tpack.pl
buffer length = 8
buffer 00 00 00 16 00 00 00 00
len1 = 22 len2 = undef

Why is len2 undefined instead of 0? Any ideas?
I am using perl 5.8.7 on Gentoo.

--
Aloha => Beau;

Xavier Noria

2006-01-22, 7:55 am

Beau E. Cox wrote:

> I really thought I understood pack/unpack,
> but this has me stumped. I can't find anything to
> explain the operation of the following script
> in the documentation:
>
> use strict;
> use warnings;
>
> my $buffer = pack( "NN", 22, 0 );
> printf "buffer length = %s\n", length $buffer;
> print "buffer ",
> join( " ", map { sprintf "%02x", ord( $_ ) } split "", $buffer ),
> "\n";
> my( $len1, $len2 ) = unpack "NN", $buffer;
> printf "len1 = %s len2 = %s\n",
> $len1 || 'undef', $len2 || 'undef';
>
> which gives this result:
>
> $ perl tpack.pl
> buffer length = 8
> buffer 00 00 00 16 00 00 00 00
> len1 = 22 len2 = undef
>
> Why is len2 undefined instead of 0? Any ideas?

That test does not show len2 is undefined, you are just seeing that the
expression

$len2 || 'undef'

evaluates to the string 'undef'.

That's because $len2 is the false value 0, and so || evaluates the right
operand, which is the string 'undef'. Since the string 'undef' is true
in boolean context that's what the expression returns. In particular,
you are not even getting the special scalar value undef, you are getting
a string that happens to contain the characters 'u', 'n', 'd', 'e', and 'f'.

Your understanding of pack/unpack is OK:

% perl -wle '$x = pack "NN", 22, 0; print for unpack "NN", $x'
22
0

The lesson learned here is that tests of that kind should be as minimal
as possible.

-- fxn

Beau E. Cox

2006-01-22, 7:55 am

Thanks - my stupid error.

On Sunday 22 January 2006 12:58 am, Xavier Noria wrote:
> Beau E. Cox wrote:
>
> That test does not show len2 is undefined, you are just seeing that the
> expression
>
> $len2 || 'undef'
>
> evaluates to the string 'undef'.
>
> That's because $len2 is the false value 0, and so || evaluates the right
> operand, which is the string 'undef'. Since the string 'undef' is true
> in boolean context that's what the expression returns. In particular,
> you are not even getting the special scalar value undef, you are getting
> a string that happens to contain the characters 'u', 'n', 'd', 'e', and
> 'f'.
>
> Your understanding of pack/unpack is OK:
>
> % perl -wle '$x = pack "NN", 22, 0; print for unpack "NN", $x'
> 22
> 0
>
> The lesson learned here is that tests of that kind should be as minimal
> as possible.
>
> -- fxn

--
Aloha => Beau;

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