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.\"     @(#)unix.4	8.1 (Berkeley) 6/9/93
.\" $FreeBSD: src/share/man/man4/unix.4,v 1.3.2.4 2002/04/22 08:59:53 dd Exp $
.\"
.Dd November 1, 2015
.Dt UNIX 4
.Os
.Sh NAME
.Nm unix
.Nd UNIX-domain protocol family
.Sh SYNOPSIS
.In sys/types.h
.In 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_DGRAM
and
.Dv SOCK_SEQPACKET
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
.In sys/un.h
defines this address:
.Bd -literal -offset indent
struct sockaddr_un {
u_char		sun_len;
sa_family_t	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
.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_DGRAM
and
.Dv SOCK_SEQPACKET
abstractions.
.Dv SOCK_STREAM ,
.Dv SOCK_DGRAM
and
.Dv SOCK_SEQPACKET
sockets also support the communication of
.Ux
file descriptors through the use of the
.Va msg_control
field in the
.Fa 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
.Vt "struct cmsghdr"
that is defined in the include file
.In 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 via
.Li dup(fd)
or
.Li fcntl(fd, F_DUPFD_CLOEXEC, 0)
depending on whether
.Dv MSG_CMSG_CLOEXEC
is passed in the
.Xr recvmsg 2
call.
Descriptors that are awaiting delivery, or that are
purposely not received, are automatically closed by the system
when the destination socket is closed.
.Pp
The effective credentials (i.e., the user ID and group list) of a
peer on a
.Dv SOCK_STREAM
socket may be obtained using the
.Dv 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
.Vt "struct xucred"
(defined in
.In sys/ucred.h ) .
The credentials presented to the server (the
.Xr listen 2
caller) are those of the client when it called
.Xr connect 2 ;
the credentials presented to the client (the
.Xr connect 2
caller) are those of the server when it called
.Xr 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.,
.Xr connect 2
or
.Xr listen 2 )
under different effective credentials.
.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