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OpenDarwin 7.2.1 - man page for unix (opendarwin section 4)

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UNIX(4) 			   BSD 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 and SOCK_DGRAM socket types and uses filesystem
     pathnames for addressing.

ADDRESSING
     UNIX-domain 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-domain socket with bind(2) causes a socket file to be created in
     the filesystem.  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-path-
     names of other UNIX-domain 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.

PROTOCOLS
     The UNIX-domain 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 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 inte-
     gers 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.

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.

BSD					   June 9, 1993 				      BSD
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