View source for Unix domain socket

{{Short description|Communications endpoint for exchanging data between processes}}
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 <code>domain</code> argument value <code>AF_UNIX</code> that is passed to the function that creates a socket [[system resource]]. The same communication domain is also selected by <code>AF_LOCAL</code>. <ref name="man-unix-sockets"></ref>

Valid <code>type</code> argument values for a UDS are:<ref name="man-unix-sockets">{{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}}</ref>
* <code>SOCK_STREAM</code> (compare to [[Transmission Control Protocol|TCP]]) – for a stream-oriented socket
* <code>SOCK_DGRAM</code> (compare to [[User Datagram Protocol|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)
* <code>SOCK_SEQPACKET</code> (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 (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 descriptor]]s across a UDS connection using the <code>sendmsg()</code> and <code>recvmsg()</code> 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.<ref name="neohapsis">{{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}}</ref><ref name="linux-cmsg-man-page">{{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}}</ref> This can be used to implement a rudimentary form of [[capability-based security]].<ref name="wheeler-secure-linux-howto">{{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}}</ref>

==See also==
* {{Annotated link|Network socket}}
* {{Annotated link|Berkeley sockets}}
* {{Annotated link|Pipeline (Unix)}}
* {{Annotated link|Netlink}}

==References==
{{Reflist}}

==External links==
* {{man|sh|socket|SUS||create a socket}}
* {{man|sh|socketpair|SUS||create a pair of connected sockets}}
* {{man|sh|sendmsg|SUS||send a message on a socket}}
* {{man|sh|recvmsg|SUS||receive a message from a socket}}
* {{man|3|cmsg|Linux||socket ancillary data, including sending/receiving [[file descriptor]]s}}
* [https://untroubled.org/ucspi-unix/ ucspi-unix], UNIX-domain socket client-server command-line tools
* [https://lists.freebsd.org/pipermail/freebsd-performance/2005-February/001143.html Unix sockets vs Internet sockets]
* [https://beej.us/guide/bgipc/html/multi/index.html Unix Sockets - Beej's Guide to Unix IPC]

{{Inter-process communication}}
{{Use dmy dates|date=January 2019}}

[[Category:Network socket]]
[[Category:Unix]]

[[fr:Berkeley sockets#Socket unix]]