How to close a non-blocking socket?

Question

I believe that if we call close system call on a non-blocking socket it returns immediately, then how to handle the response? whether it is closed or not? in other words wha

Answer 1

if we call close system call on a non-blocking socket it returns immediately

The socket is always closed: the connection may still be writing to the peer. But your question embodies a fallacy: if you call close() on any socket it will return immediately. Closing and writing to a socket is asynchronous. You can control that with SO_LINGER as per the other answer, although I suspect that only applies to blocking mode. Probably you should put the socket back into blocking mode before closing with a positive SO_LINGER if that's what you need to do.