[Chicago-talk] Reading & writing variable length packed

[Jay Strauss]

On Mon, Mar 16, 2009 at 9:30 AM, imran javaid <imranjj at gmail.com> wrote:
> This reminds me of a script I once wrote long ago to read a variable
> length file on a nine track tape.
> Here is a somewhat straight forward way to do it (did not test it and
> i am skipping some of the error checking on the return from read):
>
> open my $FILE, "<", $filename or die "Couldn't open file $filename: $!\n";
> my $buf;
> my $loc = 0;
> while(read($FILE, $buf, 2, $loc)) {
>  $loc += 2;
>  my $linenum = unpack("v", $buf);
>  my $ret = read($FILE, $buf, 1, $loc);
>  die if $ret != 1;
>  $loc++;
>  my $length = unpack("C", $buf);
>  if ($length == 255) {
>    $ret = read($FILE, $buf, 2, $loc);
>    die if $ret != 2;
>    $loc += 2;
>    $length = unpack("v", $buf);
>  }
>  $ret = read($FILE, $buf, $length, $loc);
>  die if $ret != $length;
>  print "LineNum: $linenum, Length: $length, Data: $buf\n";
> }

Hi Imran,

Thanks for the response.  I've done something sort of similar.  I'm
running into a problem now when I write the file.  I can't figure out
why my output file diffs from my orig.

I'll try yours and see if I get different results.

Jay