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.\"     @(#)unix.4	8.1 (Berkeley) 6/9/93
.\"
.Dd $Mdocdate: July 6 2011 $
.Dt UNIX 4
.Os
.Sh NAME
.Nm unix
.Nd UNIX-domain protocol family
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/un.h>
.Sh DESCRIPTION
The
.Ux Ns -domain
protocol family is a collection of protocols
that provides local (on-machine) interprocess
communication through the normal
.Xr socket 2
mechanisms.
The
.Ux Ns -domain
family supports the
.Dv SOCK_STREAM ,
.Dv SOCK_SEQPACKET ,
and
.Dv SOCK_DGRAM
socket types and uses
filesystem pathnames for addressing.
.Sh ADDRESSING
.Ux 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
.Ux 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
.Ux 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
.Ux 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
.Ux Ns -domain
protocol family is comprised of simple
transport protocols that support the
.Dv SOCK_STREAM ,
.Dv SOCK_SEQPACKET ,
and
.Dv SOCK_DGRAM
abstractions.
.Dv SOCK_STREAM
and
.Dv SOCK_SEQPACKET
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.
.Sh SEE ALSO
.Xr socket 2 ,
.Xr netintro 4