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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,
     SOCK_SEQPACKET, and SOCK_DGRAM socket types and uses file system path-
     names for 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
     pathnames;	e.g., the destination of a connect(2) or sendto(2) must	be

     The UNIX-domain sockets support the communication of UNIX file descrip-
     tors and process credentials through the use of the msg_control field in
     the msg argument to sendmsg(2) and	recvmsg(2).  The items to be passed
     are described using a struct cmsghdr that is defined in the include file

     To	send file descriptors, 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 via dup(fd) or fcntl(fd, F_DUPFD_CLOEXEC, 0) depending on
     whether MSG_CMSG_CLOEXEC is passed	in the recvmsg(2) call.	 Descriptors
     that are awaiting delivery, or that are purposely not received, are auto-
     matically closed by the system when the destination socket	is closed.

     Credentials of the	sending	process	can be transmitted explicitly using a
     control message of	type SCM_CREDS with a data portion of type struct
     cmsgcred, defined in <sys/socket.h> as follows:

     struct cmsgcred {
       pid_t cmcred_pid;	     /*	PID of sending process */
       uid_t cmcred_uid;	     /*	real UID of sending process */
       uid_t cmcred_euid;	     /*	effective UID of sending process */
       gid_t cmcred_gid;	     /*	real GID of sending process */
       short cmcred_ngroups;	     /*	number of groups */
       gid_t cmcred_groups[CMGROUP_MAX];     /*	groups */

     The sender	should pass a zeroed buffer which will be filled in by the

     The group list is truncated to at most CMGROUP_MAX	GIDs.

     The process ID cmcred_pid should not be looked up (such as	via the
     KERN_PROC_PID sysctl) for making security decisions.  The sending process
     could have	exited and its process ID already been reused for a new

     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 mecha-
		     nism for the receiver to receive the credentials of the
		     process calling write(2), send(2),	sendto(2) or
		     sendmsg(2)	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 current implementation	truncates the group list to at
		     most CMGROUP_MAX groups.

		     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

		     On	SOCK_STREAM and	SOCK_SEQPACKET sockets credentials are
		     passed only on the	first read from	a socket, then the
		     system clears the option on the socket.

		     This option and the above explicit	struct cmsgcred	both
		     use the same value	SCM_CREDS but incompatible control
		     messages.	If this	option is enabled and the sender at-
		     tached a SCM_CREDS	control	message	with a struct
		     cmsgcred, it will be discarded and	a struct sockcred will
		     be	included.

		     Many setuid programs will write(2)	data at	least par-
		     tially controlled by the invoker, such as error messages.
		     Therefore,	a message accompanied by a particular sc_euid
		     value should not be trusted as being from that user.

     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.

     LOCAL_PEERCRED  Requested via getsockopt(2) on a SOCK_STREAM socket re-
		     turns credentials of the remote side.  These will arrive
		     in	the form of a filled in	xucred structure, defined in
		     <sys/ucred.h> as follows:

		     struct xucred {
		       u_int cr_version;	     /*	structure layout version */
		       uid_t cr_uid;		     /*	effective user id */
		       short cr_ngroups;	     /*	number of groups */
		       gid_t cr_groups[XU_NGROUPS];  /*	groups */
		     The cr_version fields should be checked against
		     XUCRED_VERSION define.

		     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

		     To	reliably obtain	peer credentials on a SOCK_DGRAM
		     socket refer to the LOCAL_CREDS socket option.

     connect(2), dup(2), fcntl(2), getsockopt(2), listen(2), recvmsg(2),
     sendto(2),	setsockopt(2), socket(2), CMSG_DATA(3),	intro(4)

     "An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.

     "An Advanced 4.3 BSD Interprocess Communication Tutorial",	PS1, 8.

BSD				August 19, 2018				   BSD


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