Non-blocking socket on Windows doesn't return after send() call

Question

I'm hoping someone can explain a situation -- any situation -- in which a non-blocking Windows socket would not immediately return after using send() on it. On other platforms, my code works as intended, so it appears to be a Windows-specific issue.

The way I can tell it isn't returning is quite simple: I cout a message immediately before and after the send() function, and upon running, the console only shows the first message. I can't seem to reproduce it in a simplified example, so I apologize for the lack of code. I will try to provide more specifics if necessary.

Edit:

The specific code where the socket is created and set as non-blocking is the following. I'm assuming that there is indeed no way a non-blocking socket would behave this way, and that something else is causing it to stop before the second message is shown. Thank you for your help anyway...I will try my best to get to the bottom of it, but right now I'm quite mystified.

SOCKET new_sock = accept(sock, (struct sockaddr*)&addr, &addr_size);

#ifdef _WIN32
    unsigned long ul;
    ioctlsocket(new_sock, FIONBIO, &ul);
#else
    fcntl(new_sock,F_SETFL,O_NONBLOCK);
#endif

setsockopt(new_sock, SOL_SOCKET, SO_LINGER, (unsigned char *)&lg, sizeof(lg));
setsockopt(new_sock, SOL_SOCKET, SO_OOBINLINE, (unsigned char *)&opt, sizeof(int));

Answer 1

If your "cout" message display does not contain an std::endl, the call might not be blocking without you noticing !

Answer 2

Send will block on filling the ip stack buffers on a connected socket. If you have a connected socket and you start throwing data at it faster then the ip stack can roll it out the wire with the attendant ack nak paddywacks then the send will block.

I have not seen that properly set non blocking sends will block though. They will error with wouldblock. So I would look to how you are setting non blocking io. Are you sure you are setting non blocking on the resultant socket not the base socket... ie the socket returned from the accept.

Answer 3

if this is the actual code, you havn't initialized ul . Do:

unsigned long ul = 1;
ioctlsocket(new_sock, FIONBIO, &ul);