Perl - binary unpack using pointer and index - perl

Question

I have a binary file that contain 3 files, a PNG, a PHP and a TGA file.
Here the file to give you the idea : container.bin
the file is build this way:
first 6 bytes are a pointer to the index, in this case 211794
Then you have all 3 files stacked one after the other
and at the ofset 211794, you have the index, that tell you where the file start and end
in this example you have:
[offset start] [offset end] [random data] [offset start] [name]
6 15149 asdf 6 Capture.PNG
15149 15168 4584 15149 index.php
15168 211794 12 15168 untilted.tga
meaning that capture.png start at offset 6, finish at offset 15149, then asdf is a random data, and the start offset is repeated again.
Now what I want to do is a perl to separate the file on this binary files.
The perl need to check the first 6 offset of the file (header), then jump to the index location, and use the list to extract the file out.

Answer 1

A mix of seek and read can be used to achieve the task:
#!/usr/bin/env perl
use strict;
use warnings;
use Fcntl 'SEEK_SET';
sub get_files_info {
my ( $fh, $offset ) = #_;
my %file;
while (<$fh>) {
chomp;
my $split_count = my ( $offset_start, $offset_end, $random_data, $offset_start_copy,
$file_name ) = split /\s/;
next if $split_count != 5;
if ( $offset_start != $offset_start_copy ) {
warn "Start of offset mismatch: $file_name\n";
next;
}
$file{$file_name} = {
'offset_start' => $offset_start,
'offset_end' => $offset_end,
'random_data' => $random_data,
};
}
return %file;
}
sub write_file {
my ( $fh, $file_name, $file_info ) = #_;
seek $fh, $file_info->{'offset_start'}, SEEK_SET;
read $fh, my $contents,
$file_info->{'offset_end'} - $file_info->{'offset_start'};
open my $fh_out, '>', $file_name or die 'Error opening file: $!';
binmode $fh_out;
print $fh_out $contents;
print "Wrote file: $file_name\n";
}
open my $fh, '<', 'container.bin' or die "Error opening file: $!";
binmode $fh;
read $fh, my $offset, 6;
seek $fh, $offset, SEEK_SET;
my %file = get_files_info $fh, $offset;
for my $file_name ( keys %file ) {
write_file $fh, $file_name, $file{$file_name};
}

Answer 2

The only real difficulty here is to make sure that both input and output files are read in binary mode. This can be achieved by using the :raw PerlIO layer when the files are opened.
This program seems to do what you want. It first locates and reads the index block into a string, and then opens that string for input and reads the start and end position and name of each of the constituent files. Thereafter processing each file is simple.
Be aware that unless the formatting of the index block is more strict than you say, you can rely only on the first, second, and last whitespace-separated fields on each line since random text could contain spaces. There is also no way to specify a file name containing spaces.
The output, using Data::Dump, is there to demonstrate correct functionality and is not necessary for the functioning of the program.
use v5.10;
use warnings;
use Fcntl ':seek';
use autodie qw/ open read seek close /;
open my $fh, '<:raw', 'container.bin';
read $fh, my $index_loc, 6;
seek $fh, $index_loc, SEEK_SET;
read $fh, my ($index), 1024;
my %contents;
open my $idx, '<', \$index;
while (<$idx>) {
my #fields = split;
next unless #fields;
$contents{$fields[-1]} = [ $fields[0], $fields[1] ];
}
use Data::Dump;
dd \%contents;
for my $file (keys %contents) {
my ($start, $end) = #{ $contents{$file} };
my $size = $end - $start;
seek $fh, $start, SEEK_SET;
my $nbytes = read $fh, my ($data), $size;
die "Premature EOF" unless $nbytes == $size;
open my $out, '>:raw', $file;
print { $out } $data;
close $out;
}
output
{
"Capture.PNG" => [6, 15149],
"index.php" => [15149, 15168],
"untilted.tga" => [15168, 211794],
}