The listen() call performs two tasks for a specified stream socket:

1. Completes the necessary binding if bind() has not been called for the socket.
2. Creates a connection request queue of a specified length to queue incoming connection requests.

The listen() call indicates a readiness to accept client connection requests. It transforms an active socket into a passive socket. A passive socket can never be used as an active socket to initiate connection requests.

Calling listen() is the third of four steps that a server performs to accept a connection. It is called after allocating a stream socket with socket(), and after binding a name to the socket with bind(). It must be called before calling accept() to accept a connection request from a client.