Reconstruct binary data saved with unpack("H*", $b)

andreas1234567 has asked for the wisdom of the Perl Monks concerning the following question:

Honorable Monks,

An application reads binary data from an external system, and saves it using $h = unpack("H*", $b). If I try to reconstruct the binary data with $b = pack("H*", $h), the binary data will sometimes have a trailing null byte. Is it possible to make sure the binary data as it were (i.e. never add the null byte)?

A little code sample to illustrate the problem indicates that is works when input length is even only:

$ cat pack.t 
use strict;
use warnings;
use Test::More qw /no_plan/;

my @arr = (0 .. 9, 'a' .. 'f');
my $hexdata = q{};

while (@arr) {
  $hexdata .= shift @arr;
  my $binary_data = pack("H*", $hexdata);
  cmp_ok(unpack('H*', $binary_data), 'eq', $hexdata);
}

__END__

$ perl pack.t 
not ok 1
#   Failed test at pack.t line 11.
#          got: '00'
#     expected: '0'
ok 2
not ok 3
#   Failed test at pack.t line 11.
#          got: '0120'
#     expected: '012'
ok 4
not ok 5
#   Failed test at pack.t line 11.
#          got: '012340'
#     expected: '01234'
ok 6
not ok 7
#   Failed test at pack.t line 11.
#          got: '01234560'
#     expected: '0123456'
ok 8
not ok 9
#   Failed test at pack.t line 11.
#          got: '0123456780'
#     expected: '012345678'
ok 10
not ok 11
#   Failed test at pack.t line 11.
#          got: '0123456789a0'
#     expected: '0123456789a'
ok 12
not ok 13
#   Failed test at pack.t line 11.
#          got: '0123456789abc0'
#     expected: '0123456789abc'
ok 14
not ok 15
#   Failed test at pack.t line 11.
#          got: '0123456789abcde0'
#     expected: '0123456789abcde'
ok 16
1..16
# Looks like you failed 8 tests of 16.
[download]

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Comment on Reconstruct binary data saved with unpack("H*", $b) Select or Download Code

Replies are listed 'Best First'.
Re: Reconstruct binary data saved with unpack("H*", $b) by Anonymous Monk on Jul 27, 2011 at 12:41 UTC
I think the solution is to use u template (uuencoding) The problem with your sample data is it doesn't come in pairs -- this is where the null is coming from, its padding -- H* demands pairs	[reply]
Re: Reconstruct binary data saved with unpack("H*", $b) by ikegami (Patriarch) on Jul 27, 2011 at 19:27 UTC
Unless your ok with just using `chop` on the result — at which point it will only work if the input length is odd — it is impossible to recreate your original data since multiple inputs encode identically. `$ perl -e'print pack "H", "abc";' \| od -t x1 0000000 ab c0 0000002 $ perl -e'print pack "H", "abc0";' \| od -t x1 0000000 ab c0 0000002` [download] If you don't care about preserving leading zeroes, you can use `my $bin = pack("H", ( length($hex) % 2 ? '0' : '' ).$hex); ... ( my $hex = unpack('H', $bin) ) =~ s/^0+(?!\z)//;` [download] If you don't care about preserving leading zeroes, you can use `my $bin = pack("H", "".reverse $hex); ... ( my $hex = reverse unpack('H', $bin) ) =~ s/^0+(?!\z)//;` [download] If you do care about preserving leading zeroes, you can use `my $bin = pack("NH", length($hex), $hex) ... my ($length, $hex) = unpack('NH', $bin); substr($hex, $length, length($hex), '');` [download]	[reply] [d/l] [select]


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