scorpio17 has asked for the wisdom of the Perl Monks concerning the following question:
Dear Monks,
I need help writing a TCP/IP based client. I have a working client, written in C++, but I want to convert it to perl, because the ultimate goal here is to build a CGI-based web version of the client. The client is supposed to send a request to the server, then read back a response. Both client and server are running on linux (Ubuntu 10.04LTS).
The orignal client works as follows:
1) Open socket connection given host name and port.
2) Send the request length as a network ordered long integer (32-bits).
Here's how it does this:
std::string Request("abcdefghijklmnopq"); // just an example, for test +ing uint32_t msgLen = Request.length(); // msgLen = 17 for this test + case msgLen = htonl(msgLen); // convert to network ordered long int Socket().Write( reinterpret_cast< char * >(&msgLen), sizeof(msgLen));
Note: Socket() is just an accessor to the socket object (singleton).
sizeof(msgLen) = 4, since this is a 32-bit unsigned int (4 bytes).
3) Now that the server knows how much data we're going to send, go ahead and send the request:
Socket().Write( Request.begin(), Request.length() );
4) Read the length of the response:
Socket().Read(&msgLen, sizeof(msgLen)); msgLen = ntohl(msgLen); // convert back from network ordered long int Socket().Read(&buffer, msgLen); // read response into buffer
On the server side, when a request comes in, it gets processesed as follows:
char msg2[256]; memset(msg2,0,256); Socket().Read(msg2,4u); // first read 4 bytes with buffer size log.Printf("1. msg2: %04X %04X %04X %04X\n", msg2[0], msg2[1], msg2[2] +, msg2[3]); log.Printf("2. msg2: %d %d %d %d\n", msg2[0], msg2[1], msg2[2], msg2[3 +]); unsigned long nlen = *(unsigned long *)msg2; // convert char string ( +bytes) to int log.Printf("before ntohl: nlen = %d (hex: %04X)", nlen, nlen); nlen = ntohl(nlen); // convert back from network ordered long int log.Printf("after ntohl: nlen = %d (hex: %04X)", nlen, nlen); Socket().Read(msg2, nlen); // read the request log.Printf("Request: %s", msg2);
I've added the "log" statements to print output to a debugging console.
When I run the (working) C++ client, here's what I see:
New client connection accepted 1. msg2: 0000 0000 0000 0011 2. msg2: 0 0 0 17 before ntohl: nlen = 285212672 (hex: 11000000) after ntohl: nlen = 17 (hex: 0011) Request: abcdefghijklmnopq Response: hello world Closing connection.
When I run my perl version of the client, I see this:
New client connection accepted msg2: 0032 0038 0035 0032 msg2: 2 8 5 2 msg2: 50 56 53 50 before ntohl: nlen = 842348594 (hex: 32353832) after ntohl: nlen = 842544434 (hex: 32383532)
Note that the 4 bytes I'm reading into msg2 are NOT the expected '0 0 0 17', but are instead the ASCII codes for the digits '2 8 5 2', and the expected value for nlen (before ntohl) is 285212672.
So, it looks like I'm sending the data as "characters" and not as "bytes". I'm guessing maybe I should be using pack/unpack to force the data into "binary mode"???
Here is my perl code for the client:
#!/usr/bin/perl use strict; use IO::Socket; use IO::Select; $| = 1; my $host = 'localhost'; my $port = '3000'; my $data; my $socket = IO::Socket::INET->new( PeerAddr => $host, PeerPort => $port, Proto => 'tcp', Type => SOCK_STREAM, Blocking => 1, Timeout => 10, ) or die "Couldn't connect to $host:$port : $!"; my $sel = IO::Select->new($socket) or die "IO::Select error $!"; my $request = "abcdefghijklmpnoq"; my $request_length = length($request); print "Request:\n$request\n"; print "length(request) = $request_length\n"; # 17 my $x = htonl($request_length); print "x = $x\n"; # 285212672 printf("x = %x\n", $x); # 11000000 if ($sel->can_write(10)) { $socket->autoflush(1); # write request length # THIS IS WHERE I THINK SOMETHING IS GOING WRONG - THE SERVER GETS T +HE WRONG LENGTH HERE!!! $socket->send( htonl($request_length) ) or die "3a Couldn't write to + socket: $!"; # write request $socket->send( $request ) or die "3b Couldn't write to socket: $!"; if ($sel->can_read(10)) { # read response length $socket->recv($data, 4) or die "5 Couldn't read data from server: +$!"; my $reponse_length = ntohl($data); if ($sel->can_read(10)) { # read response $socket->recv($data, $reponse_length) or die "6 Couldn't read da +ta from server: $!"; print "Response:\n$data\n"; } else { print "can't read 2: $@ : $!"; } } else { print "can't read 1: $@ : $!"; } } else { print "can't write: $@ : $!"; } $socket->close(); sub htonl { my $input = shift; my $output = unpack('N*',pack('L*',$input)); return $output; } sub ntohl { my $input = shift; my $output = unpack('L*', pack('N*', $input)); return $output; }
After the two writes, it seems to get stuck, eventually timing out and dying with "Can't read 1:". But obviously, if the initial length doesn't get communicated correctly, everything downstream will malfunction.
Any help will be greatly appreciated - thanks!
update: added readmore tags
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Re: Problem Transmitting Data via TCP/IP
by BrowserUk (Patriarch) on Aug 09, 2012 at 18:44 UTC | |
by scorpio17 (Canon) on Aug 09, 2012 at 19:12 UTC | |
by BrowserUk (Patriarch) on Aug 09, 2012 at 19:33 UTC | |
by scorpio17 (Canon) on Aug 09, 2012 at 20:12 UTC |