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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 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.

     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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