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 provides local (on-machine) interprocess
communication through the normal
socket(2) mechanisms. The UNIX-domain family
supports the SOCK_STREAM,
SOCK_SEQPACKET, 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 socket 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 protocols that support the
SOCK_STREAM, SOCK_SEQPACKET,
and SOCK_DGRAM abstractions.
SOCK_STREAM and
SOCK_SEQPACKET sockets also support the
communication of UNIX file descriptors 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 include 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.