Greeting brethren/sisteren,
I have a problem that I'm trying to figure out, but I seem to be stumpped. First, the relevant code: 
#!/usr/local/perl5.006/bin/perl -w

use strict;
my $var = shift;

print "var: $var\n";
my $temp = sprintf "%lx", $var;
print "temp: $temp\n";
my $packed = pack "H*", $temp;

print "packed: $packed\n";

$temp = unpack "H*", $packed;
print "temp: $temp\n";
my $unpacked = hex $temp;

print "unpacked: $unpacked\n";

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...and now the output: 
>pack.pl 12
var: 12
temp: c
packed: À
temp: c0
unpacked: 192
>

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I don't understand why when I unpack the variable $packed, it doesn't come back with what I packed it with. What I also find strange is that the behavior alternates depending on what range the input is in. Here's what I've found (x represents the input value): 
16^0 <= x < 16^1 (# of hex digits is odd)  bad
16^1 <= x < 16^2 (# of hex digits is even) good
16^2 <= x < 16^3 (# of hex digits is odd)  bad
16^3 <= x < 16^4 (# of hex digits is even) good
...

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Definately a pattern. The way I figure it, pack is packing this in to the appropriate number of bytes, and when it doesn't have enough to fill an entire byte, it 0 pads to the right. Don't know how to exploit that, however (other than a hack with logarithms to find out what range it's in and act accordingly)

any help would be appreciated,
thor

In reply to Pack/unpack irregularity by thor

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