Using pack/unpack on a PNG file

[sierpinski]

Hello monks,

I've been reading about PNG files and pack/unpack for a little while, and thought I understood it enough to perform a simple operation, but apparently I was mistaken. I created a PNG file with another script, and now I'm trying to get to know pack/unpack so I can read/write the associated binary data. The script doesn't error, but only makes a single byte file with "very short lines (no magic)" as a result. Here is the code:

#!/usr/bin/perl

use strict;
use warnings;

my $ifh = open("<:raw", "./file-bar.png");

my $data = unpack('a*', $ifh);

my $outputfile = "newfile.png";

open (my $ofh, '>:raw', $outputfile) 
    or die "Could not open '$outputfile' $!";

print $ofh pack('a*', $data);
close $ofh;
print("Image created.\n");

Here are the file types of the initial and created files:

$ file file-bar.png 
file-bar.png: PNG image data, 400 x 300, 4-bit colormap, interlaced
$ file newfile.png 
newfile.png: very short file (no magic)

I've tried to piece together bits of code from various sources, so that's why it's not of the same style, but feel there's no point in cleaning up code that doesn't work yet. Any help is greatly appreciated.

[AnomalousMonk]

my $ifh = open("<:raw", "./file-bar.png");

Wishful, also quite surprising that Perl seems happy with it even with warnings and strictures enabled. In any event, the "successful" result (1) of open-ing the file is assigned to  $ifh and this is packed and written (apparently also "successfully") to another file. But it's still junk.

c:\@Work\Perl\monks\sierpinski>perl -wMstrict -MData::Dump -le
"my $ifh = open('<:raw', 'junk.txt');
 dd $ifh;
"
1

Can anyone explain to me how this works — with strictures enabled?

[choroba]

Documented behaviour:

(...) if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, so use strict "refs" should not be in effect.)

system 'echo abc > 1.txt';
open '<:raw', '1.txt' or die $!;
print while readline '<:raw';
unlink '1.txt';

Update: I wouldn't recomend it, though, as it's confusing to a reader :-) The file is still opened with default layers:

print PerlIO::get_layers("<:raw");
# unixperlio

[AnomalousMonk]

This is considered a symbolic reference, so use strict "refs" should not be in effect.

Yes, but if strictures are enabled the example still runs (and strictures were also enabled above), so I'm still confused:

c:\@Work\Perl\monks\sierpinski>perl -wMstrict -le
"system 'echo abc > 1.txt';
 open '<:raw', '1.txt' or die $!;
 print while readline '<:raw';
"
abc

Update: I wouldn't recomend it, though, as it's confusing to a reader ... Darn right!

[RichardK]

unpack takes a string as it's second parameter not a file handle. See the basic debugging checklist for lots of helpful advice on tracking down problems.

[Anonymous Monk]

Also, this is wishful thinking :) my $ifh = open("<:raw", "./file-bar.png");

[sierpinski]

Somehow in searching for info, I didn't come across Pack/Unpack Tutorial (aka How the System Stores Data) until now, and it's shining a huge light on the whole thing, so it seems I should be able to figure this out after all.

[AnomalousMonk]

See also perlpacktut.

Update: In general, pack/unpack are almost as much fun as regexes!

[AnomalousMonk]

Also consider:

c:\@Work\Perl\monks\sierpinski>perl -wMstrict -le
"my $s = 'some string -- could be any length';
 ;;
 my $up = unpack 'a*', $s;
 $s eq $up or die 'A: not identity operation';
 ;;
 my $p = pack 'a*', $up;
 $s eq $p or die 'B: not identity operation';
 ;;
 print 'a-ok';
"
a-ok