CODEWITHSUNDEEP
c++windowswinsock
Zhi Wang
Zhi Wang

Reputation: 1168

How to make 'send' non-blocking in winsock

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?

Upvotes: 6

Views: 28726

Answers (4)

user2507519
user2507519

Reputation: 62

The API ioctlsocket can do it.You can use it as below.But why don't you use I/O models in winsock?

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

Upvotes: 3

user1961169
user1961169

Reputation: 799

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.

Upvotes: 15

Nicholaz
Nicholaz

Reputation: 1429

How about measuring the time Send takes and then just sleeping the time missing up to the 20ms?

Upvotes: 0

JimR
JimR

Reputation: 16123

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.

Upvotes: 0

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