UNIX(4) BSD Kernel Interfaces 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 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 path-
names 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 abstrac-
tions. 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 repre-
senting 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 {
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)
BSD
May 29, 2011 BSD