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.\"     @(#)unix.4	8.1 (Berkeley) 6/9/93
.\"
.Dd January 7, 1998
.Dt UNIX 4
.Os
.Sh NAME
.Nm unix
.Nd UNIX-domain protocol family
.Sh SYNOPSIS
.Fd #include \*[Lt]sys/types.h\*[Gt]
.Fd #include \*[Lt]sys/un.h\*[Gt]
.Sh DESCRIPTION
The
.Tn UNIX Ns -domain
protocol family is a collection of protocols
that provides local (on-machine) interprocess
communication through the normal
.Xr socket 2
mechanisms.
The
.Tn UNIX Ns -domain
family supports the
.Dv SOCK_STREAM
and
.Dv SOCK_DGRAM
socket types and uses
filesystem pathnames for addressing.
.Sh ADDRESSING
.Tn UNIX Ns -domain
addresses are variable-length filesystem pathnames of
at most 104 characters.
The include file
.Aq Pa sys/un.h
defines this address:
.Bd -literal -offset indent
struct sockaddr_un {
	u_char	sun_len;
	u_char	sun_family;
	char	sun_path[104];
};
.Ed
.Pp
Binding a name to a
.Tn UNIX Ns -domain
socket with
.Xr bind 2
causes a socket file to be created in the filesystem.
This file is
.Em not
removed when the socket is closed\(em\c
.Xr unlink 2
must be used to remove the file.
.Pp
The length of
.Tn UNIX Ns -domain
address, required by
.Xr bind 2
and
.Xr connect 2 ,
can be calculated by the macro
.Fn SUN_LEN
defined in
.Aq Pa sys/un.h .
The
.Ar sun_path
field must be terminated by a NUL character to be used with
.Fn SUN_LEN ,
but the terminating NUL is
.Em not
part of the address.
.Pp
The
.Tn UNIX Ns -domain
protocol family does not support broadcast addressing or any form
of
.Dq wildcard
matching on incoming messages.
All addresses are absolute- or relative-pathnames
of other
.Tn UNIX Ns -domain
sockets.
Normal filesystem access-control mechanisms are also
applied when referencing pathnames; e.g., the destination
of a
.Xr connect 2
or
.Xr sendto 2
must be writable.
.Sh PROTOCOLS
The
.Tn UNIX Ns -domain
protocol family comprises simple
transport protocols that support the
.Dv SOCK_STREAM
and
.Dv SOCK_DGRAM
abstractions.
.Dv SOCK_STREAM
sockets also support the communication of
.Ux
file descriptors through the use of the
.Ar msg_control
field in the
.Ar msg
argument to
.Xr sendmsg 2
and
.Xr recvmsg 2 .
.Pp
Any valid descriptor may be sent in a message.
The file descriptor(s) to be passed are described using a
.Ar struct cmsghdr
that is defined in the include file
.Aq Pa sys/socket.h .
The type of the message is
.Dv 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.
.Pp
The received descriptor is a
.Em duplicate
of the sender's descriptor, as if it were created with a call to
.Xr dup 2 .
Per-process descriptor flags, set with
.Xr fcntl 2 ,
are
.Em 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.
.Pp
There is one
.Tn socket-level
.Xr setsockopt 2 / Ns Xr getsockopt 2
option available in the
.Nm
domain.
The
.Dv LOCAL_CREDS
option may be enabled on a
.Dv SOCK_DGRAM
or a
.Dv SOCK_STREAM
socket.  This option provides a mechanism for the receiver to
receive the credentials of the process as a
.Xr 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
.Pa \*[Lt]sys/socket.h\*[Gt]
as follows:
.Bd -literal
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 */
};
.Ed
.Pp
The
.Fn SOCKCREDSIZE
macro computes the size of the sockcred structure for a specified number
of groups.
The cmsghdr fields have the following values:
.Bd -literal
cmsg_len = sizeof(struct cmsghdr) + SOCKCREDSIZE(ngroups)
cmsg_level = SOL_SOCKET
cmsg_type = SCM_CREDS
.Ed
.Sh SEE ALSO
.Xr socket 2 ,
.Xr intro 4
.Rs
.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 7
.Re
.Rs
.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 8
.Re