A Unix domain socket (UDS), also called a local socket or inter-process communication (IPC) socket, is a communication endpoint used for data exchange between processes running on the same Unix or Unix-like operating system.

The term Unix domain socket refers to the domain argument value AF_UNIX passed to the system call that creates the socket. The same communication domain can also be selected with AF_LOCAL.^[1]

Valid type argument values for a UDS are:^[1]

• SOCK_STREAM (compare to TCP) – a stream-oriented socket
• SOCK_DGRAM (compare to UDP) – a datagram-oriented socket that preserves message boundaries; on most Unix implementations, Unix domain datagram sockets are reliable and do not reorder datagrams
• SOCK_SEQPACKET (compare to SCTP) – a connection-oriented sequenced-packet socket that preserves message boundaries and delivers messages in the order 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 instead of using an underlying network protocol, communication takes place entirely within the operating system kernel. A UDS may use the file system as its address namespace. Some operating systems, such as Linux, provide additional namespaces. Processes refer to a UDS through a file system inode, allowing two processes to communicate by opening the same socket.

In addition to sending data, processes can pass file descriptors over a UDS connection by using the sendmsg() and recvmsg() system calls. This allows one process to grant another process access to a file descriptor that it would not otherwise be able to use.^[2][3] This can be used to implement a rudimentary form of capability-based security.^[4]

• Network socket – Software-based endpoint of network communications
• Berkeley sockets – Inter-process communication API
• Pipeline (Unix) – Mechanism for inter-process communication using message passing
• Netlink – Linux kernel interface for inter-process communication between processes

• socket – System Interfaces Reference, The Single UNIX Specification, Version 5 from The Open Group
• socketpair – System Interfaces Reference, The Single UNIX Specification, Version 5 from The Open Group
• sendmsg – System Interfaces Reference, The Single UNIX Specification, Version 5 from The Open Group
• recvmsg – System Interfaces Reference, The Single UNIX Specification, Version 5 from The Open Group
• cmsg(3) – Linux Programmer's Manual – Library Functions
• ucspi-unix, UNIX-domain socket client-server command-line tools
• Unix sockets vs Internet sockets
• Unix Sockets - Beej's Guide to Unix IPC