Computer network programming

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In computing, network programming, essentially identical to socket programming or client-server programming, involves writing computer programs that communicate with other programs across a computer network. The program or process initiating the communication is called a client process, and the program waiting for the communication to be initiated is the server process. The client and server processes together form a distributed system. The communication between the client and server process may either be connection-oriented (such as an established TCP virtual circuit or session), or connectionless (based on UDP datagrams).

A program that can act both as a client and a server is based on peer-to-peer communication.

Sockets are usually implemented by an API library such as Berkeley sockets, first introduced in 1983. Most implementations are based on Berkeley sockets, for example Winsock introduced 1991. Other socket API implementations exist, such as the STREAMS-based Transport Layer Interface (TLI).

These are examples of functions or methods typically provided by the API library:

socket() creates a new socket of a certain socket type, identified by an integer number, and allocates system resources to it.
bind() is typically used on the server side, and associates a socket with a socket address structure, i.e. a specified local port number and IP address.
listen() is used on the server side, and causes a bound TCP socket to enter listening state.
connect() is used on the client side, and assigns a free local port number to a socket. In case of a TCP socket, it causes an attempt to establish a new TCP connection.
accept() is used on the server side. It accepts a received incoming attempt to create a new TCP connection from the remote client, and creates a new socket associated with the socket address pair of this connection.
send() and recv(), or write() and read(), or recvfrom() and sendto(), are used for sending and receiving data to/from a remote socket.
close() causes the system to release resources allocated to a socket. In case of TCP, the connection is terminated.