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@(#)unix.4 8.1 (Berkeley) 6/9/93
.Dd $Mdocdate: March 8 2021 $ .Dt UNIX 4 .Os .Sh NAME .Nm unix .Nd UNIX-domain protocol family .Sh SYNOPSIS n sys/types.h n sys/un.h .Sh DESCRIPTION The x Ns -domain protocol family is a collection of protocols that provides local (on-machine) interprocess communication through the normal .Xr socket 2 mechanisms. The x 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 x Ns -domain addresses are variable-length filesystem pathnames of at most 104 characters. The include file n sys/un.h defines this address: d -literal -offset indent struct sockaddr_un { u_char sun_len; u_char sun_family; char sun_path[104]; }; .Ed
p Binding a name to a x 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.
p The x 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 x 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 x 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 x file descriptors through the use of the .Ar msg_control field in the .Ar msg argument to .Xr sendmsg 2 and .Xr recvmsg 2 .
p 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 n 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.
p
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
.Rs
4.4BSD PSD:20
.%A S. Sechrest
.%T An Introductory 4.4BSD Interprocess Communication Tutorial
.%B 4.4BSD Programmer's Supplementary Documents (PSD)
.Re
.Rs
4.4BSD PSD:21
.%A S. J. Leffler
.%A R. S. Fabry
.%A W. N. Joy
.%A P. Lapsley
.%A S. Miller
.%A C. Torek
.%T An Advanced 4.4BSD Interprocess Communication Tutorial
.%B 4.4BSD Programmer's Supplementary Documents (PSD)
.Re