The UNIX-domain protocol family is comprised of simple transport protocols that support the SOCK_STREAM and
SOCK_DGRAM abstractions. SOCK_STREAM sockets also support the communication of UNIX file descriptors through the use of the msg_control field in the msg argument to sendmsg(2) and recvmsg(2).
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 <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