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 length of UNIX-domain address, required by
bind(2) and
connect(2), can be calculated
by the macro
SUN_LEN() defined in
<sys/un.h>. The
sun_path field must be terminated by a NUL character to
be used with
SUN_LEN(), but the terminating NUL is
not part of the address. The
NetBSD
kernel ignores any user-set value in the
sun_len member
of the structure.
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 comprises 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.
A UNIX-domain socket supports two socket-level options for use with
setsockopt(2) and
getsockopt(2):
The
LOCAL_CREDS
option may be enabled on a
SOCK_DGRAM
,
SOCK_SEQPACKET
, or
a
SOCK_STREAM
socket. This option provides a mechanism
for the receiver to receive the credentials of the process as a
recvmsg(2) control message. The
msg_control field in the msghdr structure points to a buffer that contains a
cmsghdr structure followed by a variable length sockcred structure, defined in
<sys/socket.h> as follows:
struct sockcred {
pid_t sc_pid; /* process id */
uid_t sc_uid; /* real user id */
uid_t sc_euid; /* effective user id */
gid_t sc_gid; /* real group id */
gid_t sc_egid; /* effective group id */
int sc_ngroups; /* number of supplemental groups */
gid_t sc_groups[1]; /* variable length */
};
The
LOCAL_PEEREID
option may be used with
getsockopt(2) to get the PID
and effective user and group IDs of a
SOCK_STREAM
or
SOCK_SEQPACKET
peer when it did
connect(2) or
bind(2). The returned structure is
struct unpcbid {
pid_t unp_pid; /* process id */
uid_t unp_euid; /* effective user id */
gid_t unp_egid; /* effective group id */
};
as defined in
<sys/un.h>.
The
SOCKCREDSIZE() macro computes the size of the sockcred
structure for a specified number of groups. The cmsghdr fields have the
following values:
cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups))
cmsg_level = SOL_SOCKET
cmsg_type = SCM_CREDS
EXAMPLES
The following code fragment shows how to bind a socket to pathname:
const char *pathname = "/path/to/socket";
struct sockaddr_un addr;
int ret;
memset(&addr, 0, sizeof(addr));
addr.sun_family = AF_LOCAL;
if (strlen(pathname) ≥ sizeof(addr.sun_path))
goto too_long;
strncpy(addr.sun_path, pathname, sizeof(addr.sun_path));
ret = bind(s, (const struct sockaddr *)&addr, SUN_LEN(&addr));
if (ret != 0)
goto bind_failed;
...
COMPATIBILITY
The
sun_len field exists only in system derived from
4.4BSD. On systems which don't have the
SUN_LEN() macro, the
following definition is recommended:
#ifndef SUN_LEN
#define SUN_LEN(su) sizeof(struct(sockaddr_un))
#endif
SEE ALSO
socket(2),
CMSG_DATA(3),
intro(4)
Stuart Sechrest, An
Introductory 4.4BSD Interprocess Communication Tutorial. (see
/usr/share/doc/psd/20.ipctut)
Samuel J. Leffler,
Robert S. Fabry, William N.
Joy, Phil Lapsley, Steve
Miller, and Chris Torek,
Advanced 4.4BSD IPC Tutorial. (see
/usr/share/doc/psd/21.ipc)
HISTORY
The
sc_pid field was introduced in
NetBSD
8.0.