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:

• 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. This can be used to implement a rudimentary form of capability-based security.