MSDN Documentation

Introduction to Sockets

Sockets provide a mechanism for interprocess communication (IPC) and network communication. They act as endpoints for sending and receiving data across a network. The socket API, often referred to as the Berkeley Sockets API, is a standard interface for network programming that has been adopted by many operating systems, including Windows through the Winsock API.

A socket can be thought of as an abstraction that allows applications to use network protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) without needing to understand the intricate details of the underlying network stack.

Socket Types

Sockets are typically characterized by their communication domain and socket type. The most common domain is the Internet domain (AF_INET for IPv4, AF_INET6 for IPv6). The primary socket types are:

• Stream Sockets (SOCK_STREAM): Provide a reliable, ordered, and bidirectional byte stream. TCP is the protocol typically used with stream sockets. Data is sent as a stream of bytes, and the protocol ensures delivery and order.
• Datagram Sockets (SOCK_DGRAM): Provide a fixed-size message, connectionless service. UDP is the protocol typically used with datagram sockets. Data is sent in discrete packets (datagrams), and delivery is not guaranteed.

Address Structures

To communicate, sockets need addresses that specify the network interface, port number, and IP address. Common structures include:

• sockaddr_in (for IPv4)
• sockaddr_in6 (for IPv6)

These structures typically contain:

struct sockaddr_in {
    sa_family_t    sin_family; /* Address family: AF_INET */
    in_port_t      sin_port;   /* Port number */
    struct in_addr sin_addr;   /* Internet address */
    char           sin_zero[8]; /* Not used */
};
                

Creating a Socket

The socket() function is used to create a new socket.

Connecting

For connection-oriented sockets (like SOCK_STREAM), connect() establishes a connection to a remote host.

Listening

Server applications use bind() to associate a socket with a specific local address and port, and then listen() to prepare the socket to accept incoming connections.

Accepting Connections

The accept() function is used by a server to retrieve a pending connection request. It creates a new socket for the communication with the client.

Sending and Receiving Data

Data is exchanged using send() (or write()) and recv() (or read()) for stream sockets, or sendto() and recvfrom() for datagram sockets.

Closing a Socket

The close() (or closesocket() in Winsock) function deallocates the socket descriptor and frees associated resources.

Full API Reference (Key Functions)

This section provides a brief overview of commonly used socket functions. For a complete list and detailed parameters, please refer to the platform-specific Winsock documentation.

Initialization and Cleanup (Winsock Specific)

Socket Creation and Configuration

Data Transfer