UNIX(4) MachTen Programmer’s Manual UNIX(4)

NAME
unix - UNIX-domain protocol family

SYNOPSIS
#include <sys/types.h>
#include <sys/un.h>

DESCRIPTION
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
and SOCK_DGRAM socket types and uses filesystem pathnames for addressing.

ADDRESSING
UNIX-domain addresses are variable-length filesystem 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 filesystem. This file is not removed when the sock-
et is closed--unlink(2) must be used to remove the file.

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
filesystem access-control mechanisms are also applied when referencing
pathnames; e.g., the destination of a connect(2) or sendto(2) must be
writable.

PROTOCOLS
The UNIX-domain protocol family is comprised of simple transport proto-
cols that support the SOCK_STREAM and SOCK_DGRAM abstractions.
SOCK_STREAM sockets also support the communication of UNIX file descrip-
tors through the use of the msg_control field in the msg argument to
sendmsg(2) and recvmsg(2).

Any valid descriptor may be sent in a message. The file descriptor(s) to
be passed are described using a struct cmsghdr that is defined in the in-
clude file <sys/socket.h>. 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 with a call to dup(2). Per-process descriptor flags, set
with fcntl(2), are 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.