UNIXDomainSocketDateien

UNIX domain sockets are an inter-process communication mechanism that enables processes on the same host to communicate using the operating system’s IPC facilities. They use the AF_UNIX (or AF_LOCAL) address family and support stream, datagram, and, on some systems, sequenced-packet semantics. Unlike Internet domain sockets, they do not use IP addressing or routing; data stays on the local machine and typically incurs lower overhead.

Addressing can be filesystem-based or, on Linux, in an abstract namespace. In pathname-based sockets, the socket is bound to a path in the filesystem, creating a socket file that clients connect to by specifying the same path. Permissions on the socket file control access. In the abstract namespace, the socket name is stored in the kernel and does not appear in the filesystem; Linux supports this with a leading null byte in the sun_path.

The common use pattern is: server creates a socket, binds to an address, listens, and accepts connections; a client creates a socket and connects to the same address. For stream sockets (SOCK_STREAM) the connection is established and then data is exchanged with send/recv or read/write; for datagram sockets (SOCK_DGRAM) each send/recv operation is message-oriented. UNIX domain sockets also support passing file descriptors to a peer using ancillary data (SCM_RIGHTS) and can carry credentials (SCM_CREDENTIALS) for authentication.

Security considerations include restricting access with filesystem permissions and carefully managing the lifecycle of path-based sockets (e.g., removing the socket file on shutdown). They are widely used for local IPC in databases, desktop environments, and system services due to their speed and simplicity. Portability varies: abstract namespace exists on Linux, while BSD/macOS typically use only path-based sockets.