sub truncate_front
{
        my $fh = shift;
        my $pos = shift;
        my $length = (stat ($fh))[7];

        my $buf;
        my $buf_size = ($pos < 1024 ? $pos : 1024);
        my $status;

        if ($pos <= 0)
        {
                return ($length);
        }
        elsif ($pos >= $length)
        {
                truncate ($fh, 0) or die ('truncate: ' . $!);
                return (0);
        }
        
        $length = 0;
        
        while (1)
        {       
                my $read_pos = $pos + $length;
                seek ($fh, $read_pos, 0) or die ('seek: ' . $!);
                $status = read ($fh, $buf, $buf_size);
                
                if (!defined ($status))
                {       
                        die ('read: ' . $!);
                }
                elsif ($status == 0)
                {       
                        last;
                }
                
                seek ($fh, $length, 0) or die ('seek: ' . $!);
                print $fh $buf;
                
                $length += $status;
        }
        
        if (defined ($status))
        {       
                truncate ($fh, $length) or die ('truncate: ' . $!);
        }
        
        return ($length);
}