UNIX(4) - Device Drivers Manual #
UNIX(4) - Device Drivers Manual
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.
SEE ALSO #
S. Sechrest, “An Introductory 4.4BSD Interprocess Communication Tutorial”, 4.4BSD Programmer’s Supplementary Documents (PSD).
S. J. Leffler, R. S. Fabry, and W. N. Joy, and P. Lapsley, and S. Miller, and C. Torek, “An Advanced 4.4BSD Interprocess Communication Tutorial”, 4.4BSD Programmer’s Supplementary Documents (PSD).
OpenBSD 7.5 - August 21, 2021