如何在 winsock 中使“发送”非阻塞

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 (eg, 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.

Answer 2

API ioctlsocket 可以做到。你可以像下面这样使用它。但是你为什么不在winsock 中使用I/O 模型呢？

ioctlsocket(hsock,FIOBIO,(unsigned long *)&ul);

Answer 3

My memory is fuzzy here since it's probably been 15 years since I've used UDP non-blocking.

However, there are some things of which you should be aware.

1. Send only smallish packets if you're going over a public network. The PATH MTU can trip you up if either the client or the server is not written to take care of incomplete packets.

2. Make sure you check that you have sent the number of bytes you think you have to send. It can get weird when you're expecting to see 300 bytes sent and the receiving end only gets 248. Both client side and server side have to be aware of this issue.

3. See here for some good advice from the Linux folks.

4. See here for the Unix Socket FAQ for UDP

5. This is a good, general network programming FAQ and example page.

Answer 4

如何测量 Send 花费的时间，然后只是睡眠时间丢失了 20 毫秒？