What does it mean that accept() creates a new socket?

Question

My question is based on the following understanding

• Socket is defined by ip:port. Both server and client will have their own socket
• Socket connection is defined by five touples server_ip, server_port, client_ip, client_port, protocol
• A socket descriptor is an integer value that identifies a socket

I tried to implement my own server where socket() and accept() returns different socket descriptor value (Is it always the case?). My question is why is it said that accept() creates a new socket if no new port is opened in server and ip:port is same for both the socket descriptors returned by socket() and accept(). If new socket is created by accept() how is it different than the socket created by socket()?

Answer 1

1. I tried to implement my own server where socket() and accept() returns different socket descriptor value (Is it always the case?).

Yes.

2. My question is why is it said that accept() creates a new socket if no new port is opened in server and ip:port is same for both the socket descriptors returned by socket() and accept(). If new socket is created by accept() how is it different than the socket created by socket()?

Because the initial socket is used to wait for communication while the second is used to communicate. A call to socket (+bind+listen) prepare a communication end-point, aka socket (or server socket) to receive incoming calls. A call to accept on a well prepared communication point, waits for an incoming call, and when this happens that creates a communication channel (2 end-points + protocol) represented by the connected socket returned by the call.

C API may confuse you because both are called socket, but are really not for the same use. In some other languages/API differentiation is made. For example in Java you have ServerSocket that is used to wait for incoming calls, and Socket that are used to communicate.