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Re^2: Problem Transmitting Data via TCP/IP

by scorpio17 (Monsignor)
on Aug 09, 2012 at 19:12 UTC ( #986590=note: print w/ replies, xml ) Need Help??


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!


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Re^3: Problem Transmitting Data via TCP/IP
by BrowserUk (Pope) on Aug 09, 2012 at 19:33 UTC
    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.


    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
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    The start of some sanity?

      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!!

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