Unix domain socket

{{short description|Communications endpoint for exchanging data between processes executing in the same operating system}}

A Unix domain socket (UDS), a.k.a. local socket, a.k.a. inter-process communication (IPC) socket, is a communication endpoint for exchanging data between processes executing in the same Unix or Unix-like operating system.

The name, Unix domain socket, refers to the domain argument value AF_UNIX that is passed to the function that creates a socket system resource. The same communication domain is also selected by AF_LOCAL.

Valid type argument values for a UDS are:{{cite web

| url = http://man7.org/linux/man-pages/man7/unix.7.html

| date = 30 April 2018

| title = Linux Programmer's Manual (unix - sockets for local interprocess communication)

| access-date = 22 February 2019

| df = dmy-all}}

• SOCK_STREAM (compare to TCP) – for a stream-oriented socket
• SOCK_DGRAM (compare to UDP) – for a datagram-oriented socket that preserves message boundaries (as on most UNIX implementations, UNIX domain datagram sockets are always reliable and don't reorder datagrams)
• SOCK_SEQPACKET (compare to SCTP) – for a sequenced-packet socket that is connection-oriented, preserves message boundaries, and delivers messages in the order that they were sent

The UDS facility is a standard component of a POSIX operating system.

The API for a UDS is similar to that of an Internet socket, but rather than using an underlying network protocol, all communication occurs entirely within the operating system kernel. A UDS may use the file system as its address name space. Some operating systems, like Linux, offer additional namespaces. Processes reference a UDS as a file system inode, so two processes can communicate by opening the same socket.

In addition to sending data, processes may send file descriptors across a UDS connection using the sendmsg() and recvmsg() system calls. This allows the sending processes to grant the receiving process access to a file descriptor for which the receiving process otherwise does not have access.{{cite web |url=http://archives.neohapsis.com/archives/postfix/2000-09/1476.html |date=30 September 2000 |title=Archive of the "Postfix Discussions" mailing list |access-date=29 September 2014 |archive-url=https://web.archive.org/web/20130518084034/http://archives.neohapsis.com/archives/postfix/2000-09/1476.html |archive-date=18 May 2013 |url-status=dead |df=dmy-all}}{{cite web |url=https://linux.die.net/man/3/cmsg |title=Linux man page - cmsg(3): access ancillary data |access-date=9 October 2018 |df=dmy-all}} This can be used to implement a rudimentary form of capability-based security.{{cite web |url=https://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/sockets.html |title="Secure Programming for Linux and Unix HOWTO", Section 3.4 "Sockets and Network Connections" |date=22 August 2004 |website=dwheeler.com |publisher=David A. Wheeler |access-date=29 September 2014}}