16 Sockets

This chapter describes the GNU facilities for interprocess communication using sockets.

A socket is a generalized interprocess communication channel. Like a pipe, a socket is represented as a file descriptor. Unlike pipes sockets support communication between unrelated processes, and even between processes running on different machines that communicate over a network. Sockets are the primary means of communicating with other machines; telnet, rlogin, ftp, talk and the other familiar network programs use sockets.

Not all operating systems support sockets. In the GNU library, the header file sys/socket.h exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets these functions always fail.

Incomplete: We do not currently document the facilities for broadcast messages or for configuring Internet interfaces. The reentrant functions and some newer functions that are related to IPv6 aren't documented either so far.

• Socket Concepts: Basic concepts you need to know about.
• Communication Styles: Stream communication, datagrams and other styles.
• Socket Addresses: How socket names (``addresses'') work.
• Interface Naming: Identifying specific network interfaces.
• Local Namespace: Details about the local namespace.
• Internet Namespace: Details about the Internet namespace.
• Misc Namespaces: Other namespaces not documented fully here.
• Open/Close Sockets: Creating sockets and destroying them.
• Connections: Operations on sockets with connection state.
• Datagrams: Operations on datagram sockets.
• Inetd: Inetd is a daemon that starts servers on request. The most convenient way to write a server is to make it work with Inetd.
• Socket Options: Miscellaneous low-level socket options.
• Networks Database: Accessing the database of network names.