c++ - How to make 'send' non-blocking in winsock

Question

I am making a program which sends UDP packets to a server at a fixed interval, something like this:

while (!stop) {
    Sleep(fixedInterval);

    send(sock, pkt, payloadSize, flags);
 }

However the periodicity cannot be guaranteed because send is a blocking call (e.g., when fixedInterval is 20ms, and a call to send is > 20ms ). Do you know how I can turn the send into a non-blocking operation?

Answer 1

You need to use a non-blocking socket. The send/receive functions are the same functions for blocking or non-blocking operations, but you must set the socket itself to non-blocking.

u_long mode = 1;  // 1 to enable non-blocking socket
ioctlsocket(sock, FIONBIO, &mode);

Also, be aware that working with non-blocking sockets is quite different. You'll need to make sure you handle WSAEWOULDBLOCK errors as success! :)

So, using non-blocking sockets may help, but still will not guarantee an exact period. You would be better to drive this from a timer, rather than this simple loop, so that any latency from calling send, even in non-blocking mode, will not affect the timing.