C++ Sockets Send() Thread-Safety

Question

I am coding sockets server for 1000 clients maxmimum, the server is about my game, i'm using non-blocking sockets and about 10 threads that receive data simultaneously from different sockets (first thread receives from 0-100,second from 101-200 and so on..)

but if thread 1 wants to send data to all 1000 clients and thread 2 also wants to send data to all 1000 clients at the same time, is that safe? are there any chances of the data being messed in the other (client) side?

if yes, i guess the only problem that can happen is that sometimes client would receive 2 or 10 packets as 1 packet, is that correct? if yes, is there any solution to that :(

Answer 1

Since you are sending data using different sockets, there must not be any problem. Rather when these different threads access same data you have to ensure data integrity.

Answer 2

send() is not atomic in most implementations, so sending to 1000 different sockets from multiple threads could lead to mixed-up messages arriving on the client side, and all kinds of weirdness. (I know nothing, see Nicolai's and Robert's comments below the rest of my comment still stands though (in terms of being a solution to your problem))

What I would do is use threads for sending like you use them for receiving. One thread to manage sending to one (or more) sockets that ensures that you don't write to one socket from multiple threads at the same time.

Also look here for some additional discussion and more interesting links. If you're on windows, the winsock programmers faq is an invaluable resource, for your issue see here.

Answer 3

Are you using UDP or TCP sockets?

If UDP, each write should be encapsulated in a separate packet and should be carried to the other side intact. The order may be swapped (as it may for any UDP packet) but they should be whole.

If TCP, there's no concept of packets on the transport layer and any 10 writes on one side may be bundled up on the other side in one read. TCP writes may also only accept part of your buffer so even if the send() function is atomic, your write isn't necessarily. In this case you'd need to synchronize it.

Answer 4

The usual pattern of dealing with many sockets is to have a dedicated thread polling for I/O events with select(2), poll(2), or better kqueue(2) or epoll(4) (depending on the platform) acting as socket event dispatcher. The sockets are usually handled in non-blocking mode. Then one might have pool of threads reacting to the events and either do reads and writes directly or via lower level buffers/queues.

All sorts of techniques are applicable here - from queues to event subscription whiteboards. It gets tricky with multiplexing accepts/reads/writes/EOFs on the I/O level and with event arbitration on the application level. Several libraries like libevent and boost::asio help structure the lower level (the ACE library is also in this space, but I'd hate recommending it to anybody). You would have to come up with application-level protocols and state machines yourself (again boost::statechart might be of help).

Some good links to get better understanding of what you are up against (this is probably the millionth time they are mentioned here on SO):

• The C10K problem
• High-Performance Server Architecture

Apologies for not offering a concrete solution, but this is a very wide design question and most decisions depend heavily on the context (lots of fun though). Hope this helps a bit.