catcdbytes

#!/usr/bin/perl -wT

delete $ENV{PATH};
delete $ENV{BASH_ENV};
delete $ENV{ENV};

# cat's only a defined number of bytes from a cdrom.
# (ps. scsi cdrom = /dev/scd0)
# This is hack to prevent crash of some linux/PPC 2.2.(15pre4 etc) kernels 
# when normal 'cat' is used (/dev/cdrom read bug),
# It uses 'dd' cause I thought it would be more secure with reading (buffer
# sizes) than reading from perl.

$bigbufsize = 2048*1024; # 2MB
$sectorsize = 2048;

foreach (@ARGV) {
	if (/^(\d+)(?:\@(\d+))?$/) {
		if($length_bytes){
			&usage;
		}else{
			$length_bytes=$1; $start_bytes=$2;
		}
	}elsif (m|^(/dev/\w+)$|) {
		if($dev){&usage;
		}else{$dev=$1}
	}else{
		&usage;
	}
}
sub usage{
	die "usage:
	catcdbytes bytes[\@bytestart] [/dev/...]
	
	Reads a defined number of bytes [from a defined
	position] from [the first track of] a cdrom.
	This is useful to reconstruct partitions
	saved using my 'rawpartitionburner' script.
	\n";
}
&usage unless $length_bytes;
$start_bytes=0 unless $start_bytes;
$dev="/dev/cdrom" unless $dev;
# Alternative: /dev/scd0

use integer;

# first step: read bigbufsize pieces
{ 
	local $start= $start_bytes/$bigbufsize;
	local $startskip = $start_bytes - $start*$bigbufsize;
	local $end  = ($start_bytes+$length_bytes)/$bigbufsize;
	local $cnt = $end-$start;
	if ($cnt) {
		$secend = $end*$bigbufsize/$sectorsize; # for step 2
		print STDERR "Reading $cnt big chunks...\n";
		open IN, "/bin/dd bs=$bigbufsize skip=$start count=$cnt if=$dev|" or die $!;
		read IN,$buf,$bigbufsize; 
		print substr($buf,$startskip);
		while (read IN,$buf,$bigbufsize) { print $buf };
		close IN;
	}
}

# second step: read rest in sectors
{
	local($start,$startskip);
	
	if ($secend) {
		$start= $secend;
		$startskip=0;
		
	}else {
		# nothing has been read so far:
		$start= $start_bytes/$sectorsize;
		$startskip= $start_bytes - $start*$sectorsize;
	}	
	local $end= ($start_bytes+$length_bytes)/$sectorsize;
	local $cnt = $end-$start;
	if ($cnt) {
		$secend= $end;
		print STDERR "Reading $cnt sectors...\n";
		open IN, "/bin/dd bs=$sectorsize skip=$start count=$cnt if=$dev|" or die $!;
		read IN,$buf,$bigbufsize; # bufsize doesn't matter as long as it matches boundaries
		print substr($buf,$startskip);
		while (read IN,$buf,$bigbufsize) { print $buf };
		close IN;
	}else {
		undef $secend;
	}
}

# third step: read remaining bytes (from last sector)
{
	local($start,$startskip);
	
	if ($secend) {
		$start= $secend;
		$startskip=0;
	
	}else {
		# nothing has been read so far:
		$start= $start_bytes/$sectorsize;
		$startskip= $start_bytes - $start*$sectorsize;
	}	
	local $rest = $start_bytes+$length_bytes - $start*$sectorsize;
	local $len = $rest-$startskip;
	
	if ($rest) {
		print STDERR "Reading last sector ($len bytes)...\n";
		open IN, "/bin/dd bs=$sectorsize skip=$start count=1 if=$dev |" or die $!;
		read IN,$buf,$rest;
		print substr($buf,$startskip);
		close IN;
	}
}