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 address-
     ing.

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-pathnames 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 integers repre-
     senting 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.

     The effective credentials (i.e., the user ID and group list) the 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 (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 influ-
     ence the credentials presented to its peer except by calling the appropriate system call (e.g., connect(2) or listen(2)) under different
     effective credentials.

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