I must listen to incoming connections but at the same time I have to receive the messages from the already connected clients. However listen(1) makes the socket waiting so I can't receive them. What to do? (I have to get the messages as soon as the client sends them)

in short, you have 3 main options:

1. open a thread per client, that spawns after you accept(), and the run a loop in this context, that does read() => .... => write()

2. run a main loop that uses select() on clients after accept() for each, and handle dispatching yourself.

3. best option - use an async networking framework like tornado, gevent, twisted or a few more to handle this transparently.