Re^2: Problem Transmitting Data via TCP/IP

[scorpio17]

in reply to Re: Problem Transmitting Data via TCP/IP
in thread Problem Transmitting Data via TCP/IP

Thank you! I knew it had to be something simple like that.

But what do you mean "wrongly coded"? How should htonl() be defined?

When I change my code to this:

  #  $socket->send( htonl($request_length) ) or die "3a Couldn't write
+ to socket: $!";
    $socket->send( pack('l>',$request_length) ) or die "3a Couldn't wr
+ite to socket: $!";
    $socket->send( $request ) or die "3b Couldn't write to socket: $!"
+;

I get the expected output in the logging console, and the server seems to write the response to the socket, but my client fails to read the response from the socket.

I get this message now:

5 Couldn't read data from server:

So something is still not quiet right... but I'm getting closer!

[BrowserUk]

But what do you mean "wrongly coded"? How should htonl() be defined?

Your htonl:

sub htonl {
  my $input = shift;
  my $output = unpack( 'N*', pack( 'L*', $input ) );
  return $output;
}

1. First, takes the number and "packs it" (converts it to binary) as a signed long:

   Which means that the return from that first pack is 4 bytes and encoded in whatever byte-order (endianess) your current platform uses:

   $packed = pack 'l', 17;;
   
   print length $packed;;
   4
   
   print ord( substr $packed, $_, 1 ) for 0 .. 3;;
   17
   0
   0
   0
   

   On my intel system, that means little-endian (the low byte comes first).

2. But then, you unpack (convert binary to ascii), those 4 bytes, treating them a big-endian long ('N'), resulting in:

   $unpacked = unpack 'N', $packed;;
   
   print length( $unpacked );;
   9
   
   print ord( substr $unpacked, $_, 1 ) for 0 .. 9;;
   50
   56
   53
   50
   49
   50
   54
   55
   50
   0
   

   Resulting in a 9-byte ascii encode string containing the number: 285212672; which is meaningless.

   htonl() could be correctly coded as sub htonl{ pack 'l>', $_[0] }; if you see the need for wrapping a built-in function in a silly named wrapper :)

Similarly, you will need to fix your ntohl(); something like this: sub ntohl{ unpack 'l>', $_[0] } would suffice.

That may fix the second part of your problem.

[scorpio17]

Thanks again. I've never taken the time to learn how pack and unpack work as well as I should, and it looks like that mistake has finally caught up with me.

For what it's worth - the original C++ code (which I didn't write) used the functions htonl() and ntohl(). I was not familiar with those, but could tell that it had something to do with encoding/decoding data, so I googled for a perl implementation, and found one in the Net::Inet module on CPAN. It's written to take an array of inputs, but I knew that in my case I'd only be sending in a single argument.

As it turns out, my code did have another error: recv() returns undef if there's an error, so I had to change this:

# this is broken !!!
$socket->recv($data, $reponse_length) or die "Couldn't read data from 
+server: $!";

into this:

unless (defined $socket->recv($data, $reponse_length)) {
  die "Couldn't read data from server: $!";
}

But everything seems to be working now -thank you!!