Question

I am planning to use unpack in the file. First i tested with a string. When i have a embedded space in a string the below script is showing it as null. When i tested with a file space is getting read properly. Not sure why it’s changing it to null when i do with string. Can i use unpack while reading a fixed length file? Do i need to consider trailing space or anything else?

#!/usr/local/bin/perl

use strict;
use warnings;
my $str="hek kaaa";
print "<$str>\n";
foreach(unpack("(A1)*", $str)) {
    print sprintf("%x", ord), " ";
}

Output:
<hek kaaa>
68 65 6b 0 6b 61 61 61

Thanks a lot for the response. Pasted my query below

When the perl program reads from a text file and unpacks using the format “A” is working properly. First line i have a embedded space in the first field between A and D. LABEL Variable gets printed as “A D3”. But when i had a variable $str=”A D3″ and unpacked the below way it’s having a null after A. How is it working differently when reading from a file and variable?

foreach(unpack("(A1)*", $str)) {
    print sprintf("%x", ord), " ";
}

it displays as the hex output as

41 0 44 33

cat test.txt
A D37845566974923342XYZ24023984
QRW49327408234028434ERD24448009

 my $file = 'test.txt';

open(my $data, '<', $file) or die "Could not open '$file'\n";
while (my $line = <$data>) {
    print $line;
    chomp $line;
    my ($label, $source, $dest, $type, $value) = unpack ("A4 A8 A8 A4 A8", $line);
    print "LABEL: $label SOURCE: $source DEST: $dest TYPE: $type VALUE: $value\n";
    print "length of a string:" . length($line) . "\n";
    foreach(unpack("(a1)*", $label)) {
       print sprintf("%x", ord), " ";
    }
    print "\n";

}

Answer 1

This little nugget in the pack/unpack documentation is easy to miss:

The “a”, “A”, and “Z” types gobble just one value, but
pack it as a string of length count, padding with nulls
or spaces as necessary. When unpacking, “A” strips
trailing whitespace and nulls, “Z” strips everything
after the first null, and “a” returns data verbatim.

As a workaround, you can use a a or Z in the template instead of A:

$ perl -e 'print ord unpack("A", " ")'
0

$ perl -e 'print ord unpack("a", " ")'
32

$ perl -e 'print ord unpack("Z", " ")'
32