Truncate typically gets rid of the end of a file. But if you want to delete the first x bytes of a file, there's no trivial way to do it. That's what this snippet does.
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);
}

[download]

In reply to Truncate the front of a file by flocto

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.