Problem using Connect(), send(), recv, with UDP sockets

4
votes

For my Uni assignment I have to create a fast action paced networked game and so have chosen to use UDP as opposed to TCP. I am aware of a lot of the differences in programming both UDP and TCP and have read through most of the relevant parts of MSDN's documentation on winsock. On MSDN it states that creating a UDP socket via the connect() function should bind the socket to the address and port specified and as a result be able use the send() and recv() functions with the created socket.

For my application I create a client and use connect() using the loopback address which sends a number of packets via the send() function. The client, after calling select(), then receives the packets it sent out. However the result I get from the recv() function is SOCKET_ERROR and the error description using WSAGetLastError() is "An existing connection was forcibly closed by the remote host".

If i use the bind() function and use sendto() to send data over the loopback address, I recv() packets without any errors... Does anyone know why the connect() function is not doing what it is supposed to do, and has anyone been able to use UDP sockets with the connect() function?

4
c++cwinsock2
UDP is a connectionless protocol. Where on MSDN did it say to use connect()? I think you must have been reading some TCP documentation.... – Tony Delroy
connect() does do something with UDP: it sets the address that the UDP socket will send packets to (and receive packets from). So that later on you can just call send() instead of sendto(). Not terribly useful, really, but not completely useless either. – Jeremy Friesner
@Tony: no, connect is fully valid on connectionless sockets. It just fixes the socket to only allow sending to/receiving from a single remote address (so you can use send and recv rather than sendto and recvfrom). – R.. GitHub STOP HELPING ICE

4 Answers

16
votes

You will need to call bind() if you want your program to receive UDP packets. connect() only sets the address that the socket will send packets to if you call send(); it does not associate the socket with a local UDP port to receive on; for that you must call bind().

4
votes

"UNIX Network Programming" points out that a connect call made on a UDP client side socket figures out and stores all the state about the destination socket address in advance (masking, selecting interface, etc.), saving the cost of doing so on every ::sendto call. This book claims that ::send vs ::sendto can be up to 3x faster because of this reduced overhead - data can go straight to the NIC driver bypassing most IP stack processing. High performance game programmer's may want to consider this.

1
votes

you should check Beej's Guide to Network Programming Using Internet Sockets, there are nice examples that address your question.

-3
votes

Keep in mind that the UDP protocol is a "connectionless" protocol meaning that you never ever connect to the host, you just send out data. So you can see that connect as an action is meaningless for UDP.

For UDP you should use sendto() and recvfrom() in these function you specify the address and the buffers and that's about it, everything else that is comfortably handled for you in TCP is gone you have to handle things on your own.

In the MSDN documentation its mentioned that you can in fact somehow use the normal send/recv functions with UDP but why would you when you have separate functions already? Like other commented already connect() for UDP does something else it's not essentially a "connect" operation but a sort of a filter to set up send()/recv() for UDP usage.