struct sockaddr_un { u_char sun_len; u_char sun_family; char sun_path[104]; };
Binding a name to a UNIX -domain socket with bind(2) causes a socket file to be created in the filesystem. This file is not removed when the socket is closed---unlink(2) must be used to remove the file.
The UNIX -domain protocol family does not support broadcast addressing or any form of ``wildcard'' matching on incoming messages. All addresses are absolute- or relative-pathnames of other UNIX -domain sockets. Normal filesystem access-control mechanisms are also applied when referencing pathnames; e.g., the destination of a connect(2) or sendto(2) must be writable.
Any valid descriptor may be sent in a message. The file descriptor(s) to be passed are described using a struct cmsghdr that is defined in the include file Aq Pa sys/socket.h . The type of the message is SCM_RIGHTS and the data portion of the messages is an array of integers representing the file descriptors to be passed. The number of descriptors being passed is defined by the length field of the message; the length field is the sum of the size of the header plus the size of the array of file descriptors.
The received descriptor is a duplicate of the sender's descriptor, as if it were created with a call to dup(2). Per-process descriptor flags, set with fcntl(2), are not passed to a receiver. Descriptors that are awaiting delivery, or that are purposely not received, are automatically closed by the system when the destination socket is closed.