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UNIX(4) 		     Kernel Interfaces Manual			 UNIX(4)

NAME
     unix -- UNIX-domain protocol family

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

DESCRIPTION
     The  Unix-domain protocol family is a collection of protocols that provides
     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  pathnames
     for addressing.

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  path-
     names; e.g., the destination of a connect(2) or sendto(2) must be writable.

PROTOCOLS
     The  Unix-domain protocol family is comprised of simple transport protocols
     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  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 de-
     fined 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 de-
     livery, 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 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 (de-
     fined in <sys/ucred.h>).  The credentials presented to the server (the lis-
     ten(2) caller) are those of the client when it called connect(2); the  cre-
     dentials  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 ex-
     cept 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 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 {
		       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  con-
		     nect(2)  function	to block until accept(2) has been called
		     on the listening socket.

SEE ALSO
     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 9.0			 October 5, 2009			 UNIX(4)
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