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 */ };
<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.