Hi Monks

I'm a little confused regarding seeking with the x command with unpack.

This came about in this node in my thread yesterday.

In a format such as PNG, where the first field to extract is not at the start of the packed format, you seek with x before extracting. However, when the string to be unpacked is 'too short' in that it doesn't have enough bytes to seek with 'x', Perl will die rather than safely stop parsing.

I have reduced this down to the following test-case. Is this expected behaviour with x or am I attempting to approach the problem incorrectly?

Many thanks.

#!/usr/bin/env perl
use strict;
use warnings;
use 5.016;

my $str = "\x00\x00\x01\x00\x00\x02";
my @vals;

# This works - exact multiple of 'x2C'
say "length:", length($str);
@vals = unpack "(x2C)*", $str;
say join "\n", @vals;

$str .= "\x00";

# This works - not a multiple of 'SC'
say "length:", length($str);
@vals = unpack "(SC)*", $str;
say join "\n", @vals;

# This fails - not a multiple of 'x2C'
say "length:", length($str);
@vals = unpack "(x2C)*", $str;
# ^-- dies here with: 'x' outside of string in unpack
say join "\n", @vals;

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In reply to Seeking with 'x' in unpack and out of bounds reads by mxb

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