questions about multi threading for sockets/tcp-connections.

Question

I have a server that connects to multiple clients using TCP/IP connections, using C in Unix. Since it won't have more than 20 connections at a time, I figured I would use a thread per connection/socket. But the problem is writing to the sockets as I'll be sending user prompted msgs to clients. Once each socket is handled by a thread, how do I interact with the created thread to write to the sockets? Should each thread just read from the sockets and I'll write to sockets in the main program? Not sure if that's a good way to go about it.

Answer 1

Sounds like you'd probably be better with a single thread and multiplexing the sockets (using select, poll etc). This will avoid the race conditions and locking requirements which will otherwise make the program more difficult to write.

Unless you are doing significant processor-intensive work, or waiting for IO on behalf of these clients, you'll get no benefit from using threads anyway, but the race conditions will still be there.

So I'd say, get a working implementation using a single thread, THEN if in performance testing you discover that it is lacking, refactor it to use multithreading if that seems like the best option to beat the performance problems (of course you'll be profiling it etc).

Answer 2

Having the main thread write to the sockets is fine, you only need to worry about having multiple threads writing to a socket at the same time.

However, I'd test the performance of using a single thread and select/poll before bothering with the muti threaded approach.

Answer 3

My rule of thumb is that any given socket should only be operated on by a single thread(*). So if you spawn a separate I/O thread for each socket, and your main thread wants something written to an I/O thread's socket, then the main thread should send that data to the I/O thread, whereupon the I/O thread can write it to the socket.

Of course, this means you need to have a good communications method between the main thread and the I/O thread; which you could do by spawning a socket-pair for each I/O thread and having the I/O threads select()/poll() on their end of the socket-pair (to handle data coming from the main thread) as well as on their network socket.

But once you've done that, you're dealing with complexity of using select()/poll() AND multithreading, which is a lot of complexity overhead. So unless you absolutely need multithreading for some reason, I agree with the previous posters -- it's better to just handle all the sockets in a single thread, via select() or poll().

(*) It's possible to have multiple threads reading/writing to the same socket at the same time, but it's error-prone. In particular, startup and shutdown sequences can be tricky to get 100% right. That's why I try to avoid 'sharing' a given socket amongst multiple threads.