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 pro‐
vides local (on-machine) interprocess communication through the normal
socket(2) mechanisms. The UNIX-domain family supports the SOCK_STREAM
and SOCK_DGRAM socket types and uses file system pathnames for address‐
ing.
ADDRESSING
UNIX-domain addresses are variable-length file system 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 file system. 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 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
file system 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 proto‐
cols that support the SOCK_STREAM and SOCK_DGRAM abstractions.
SOCK_STREAM sockets also support the communication of UNIX file descrip‐
tors 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 await‐
ing delivery, or that are purposely not received, are automatically
closed by the system when the destination socket is closed.
The effective credentials (i.e., the user ID and group list) of a peer on
a SOCK_STREAM socket may be obtained using the LOCAL_PEERCRED socket
option. This may be used by a server to obtain and verify the creden‐
tials of its client, and vice versa by the client to verify the creden‐
tials of the server. These will arrive in the form of a filled in struct
xucred (defined in <sys/ucred.h>). The credentials presented to the
server (the listen(2) caller) are those of the client when it called
connect(2); the credentials presented to the client (the connect(2) call‐
er) are those of the server when it called listen(2). This mechanism is
reliable; there is no way for either party to influence the credentials
presented to its peer except by calling the appropriate system call
(e.g., connect(2) or listen(2)) under different effective credentials.
UNIX domain sockets support a number of socket options which can be set
with setsockopt(2) and tested with getsockopt(2):
LOCAL_CREDS This option may be enabled on a SOCK_DGRAM 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 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
LOCAL_CONNWAIT Used with SOCK_STREAM sockets, this option causes the
connect(2) function to block until accept(2) has been
called on the listening socket.
SEE ALSOsocket(2), intro(4)
"An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.
"An Advanced 4.3 BSD Interprocess Communication Tutorial", PS1, 8.
BSD July 15, 2001 BSD
