Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Berkeley sockets application programming interfacecomprises a library for developing applications in the C programming language that perform inter-process communication, most commonly for communications across a computer network. Berkeley sockets originated with the 4.2BSD Unix operating system as an API. Only in 1989, however, could UC Berkeley release versions of its operating system and networking library free from the licensing constraints of AT&T's copyright-protected Unix. The Berkeley socket API forms the de facto standard abstraction for network sockets. Most other programming languages use an interface similar to the C API. The API is also used for Unix domain sockets, which are an interface to interprocess communication channels within a single computer. The STREAMS-based Transport Layer Interface API offers an alternative to the socket API. However, recent systems that provide the TLI API also provide the Berkeley socket API. The Berkeley socket interface, an application programming interface, allows communications between hosts or between processes on one computer, using the concept of an Internet socket.