When writing multithreaded programs that work with sockets, a common pattern is to have two threads per socket—one thread writes the data to the socket, another one reads the data. Obviously, the socket library used for this should be threadsafe to start with and support a read from a socket in one thread and a write to the socket in the other thread without corrupting the library's socket state. The raw system calls read() and write() support this. Your experience with the more sophisticated libraries may vary.