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UNIX(4)		       FreeBSD Kernel Interfaces Manual		       UNIX(4)

     unix -- UNIX-domain protocol family

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

     The UNIX-domain protocol family is	a collection of	protocols that pro-
     vides local (on-machine) interprocess communication through the normal
     socket(2) mechanisms.  The	UNIX-domain family supports the	SOCK_STREAM,
     SOCK_SEQPACKET, and SOCK_DGRAM socket types and uses file system path-
     names for addressing.

     UNIX-domain addresses are variable-length file system 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-domain socket with bind(2) causes	a socket file
     to	be created in the file system.	This file is not removed when the
     socket is closed -- unlink(2) must	be used	to remove the file.

     The length	of UNIX-domain address,	required by bind(2) and	connect(2),
     can be calculated by the macro SUN_LEN() defined in <sys/un.h>.  The
     sun_path field must be terminated by a NUL	character to be	used with
     SUN_LEN(),	but the	terminating NUL	is not part of the address.

     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
     file system access-control	mechanisms are also applied when referencing
     pathnames;	e.g., the destination of a connect(2) or sendto(2) must	be

     The UNIX-domain protocol family is	comprised of simple transport proto-
     cols that support the SOCK_STREAM,	SOCK_SEQPACKET,	and SOCK_DGRAM
     abstractions.  SOCK_STREAM	and SOCK_SEQPACKET 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	await-
     ing delivery, or that are purposely not received, are automatically
     closed by the system when the destination socket is closed.

     The effective credentials (i.e., the user ID and group list) of a peer on
     a SOCK_STREAM socket may be obtained using	the LOCAL_PEERCRED socket
     option.  This may be used by a server to obtain and verify	the creden-
     tials of its client, and vice versa by the	client to verify the creden-
     tials of the server.  These will arrive in	the form of a filled in	struct
     xucred (defined in	<sys/ucred.h>).	 The credentials presented to the
     server (the listen(2) caller) are those of	the client when	it called
     connect(2); the credentials presented to the client (the connect(2)
     caller) are those of the server when it called listen(2).	This mechanism
     is	reliable; there	is no way for either party to influence	the creden-
     tials presented to	its peer except	by calling the appropriate system call
     (e.g., connect(2) or listen(2)) under different effective credentials.

     UNIX domain sockets support a number of socket options which can be set
     with setsockopt(2)	and tested with	getsockopt(2):

     LOCAL_CREDS     This option may be	enabled	on SOCK_DGRAM, SOCK_SEQPACKET,
		     or	a SOCK_STREAM socket.  This option provides a mecha-
		     nism for the receiver to receive the credentials of the
		     process as	a recvmsg(2) control message.  The msg_control
		     field in the msghdr structure points to a buffer that
		     contains a	cmsghdr	structure followed by a	variable
		     length sockcred structure,	defined	in <sys/socket.h> as

		     struct sockcred {
		       uid_t sc_uid;	     /*	real user id */
		       uid_t sc_euid;	     /*	effective user id */
		       gid_t sc_gid;	     /*	real group id */
		       gid_t sc_egid;	     /*	effective group	id */
		       int   sc_ngroups;     /*	number of supplemental groups */
		       gid_t sc_groups[1];   /*	variable length	*/

		     The SOCKCREDSIZE()	macro computes the size	of the
		     sockcred structure	for a specified	number of groups.  The
		     cmsghdr fields have the following values:

		     cmsg_len =	CMSG_LEN(SOCKCREDSIZE(ngroups))
		     cmsg_level	= SOL_SOCKET
		     cmsg_type = SCM_CREDS

     LOCAL_CONNWAIT  Used with SOCK_STREAM sockets, this option	causes the
		     connect(2)	function to block until	accept(2) has been
		     called on the listening socket.

     socket(2),	intro(4)

     "An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.

     "An Advanced 4.3 BSD Interprocess Communication Tutorial",	PS1, 8.

FreeBSD	9.2			October	5, 2009			   FreeBSD 9.2


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