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UNIX(4)			 BSD 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
     and SOCK_DGRAM socket types and uses file system pathnames	for address-

     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 and SOCK_DGRAM abstractions.
     SOCK_STREAM sockets also support the communication	of UNIX	file descrip-
     tors 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 in-
     clude 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 op-
     tion.  This may be	used by	a server to obtain and verify the credentials
     of	its client, and	vice versa by the client to verify the credentials 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 credentials 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 a SOCK_DGRAM	or a
		     SOCK_STREAM socket.  This option provides a mechanism 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 follows:

		     struct sockcred {
		       id_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 =	sizeof(struct cmsghdr) + 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.

BSD				 July 15, 2001				   BSD


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