UNIX(4) AerieBSD 1.0 Refernce Manual UNIX(4)


unix — UNIX-domain protocol family


#include <sys/types.h>
#include <sys/un.h>


The UNIX protocol family is a collection of protocols that provides local (on-machine) interprocess communication through the normal socket(2) mechanisms. The UNIX family supports the SOCK_STREAM and SOCK_DGRAM socket types and uses filesystem pathnames for addressing.


UNIX addresses are variable-length filesystem pathnames of at most 104 characters. The include file ‹sys/un.h› defines this address:

struct sockaddr_un {
	u_char	sun_len;
	u_char	sun_family;
	char	sun_path[104];

Binding a name to a UNIX socket with bind(2) causes a socket file to be created in the filesystem. This file is not removed when the socket is closed\(em\c unlink(2) must be used to remove the file.

The UNIX 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 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.


The UNIX 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 .Ux 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 structcmsghdr 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 is closed.


socket(2), netintro(4) .Rs .%T "An Introductory 4.3 BSD Interprocess Communication Tutorial" .%B PS1 .%N 7 .Re .Rs .%T "An Advanced 4.3 BSD Interprocess Communication Tutorial" .%B PS1 .%N 8 .Re

AerieBSD 1.0 Reference Manual August 26 2008 UNIX(4)