[Chicago-talk] Reading & writing variable length packed

[Jay Strauss]

On Mon, Mar 16, 2009 at 1:52 PM, Jay Strauss <me at heyjay.com> wrote:
> 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
>

Yep, I get different results.  I never used "read" before, and it
doesn't work like I was thinking or like you were thinking either.
The offset isn't the offset into the file handle, its the offset into
the target scalar.  Took me a long time to figure that error out :)

Jay