unix -- UNIX-domain protocol family
#include <sys/types.h>
#include <sys/un.h>
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
and SOCK_DGRAM socket types and uses file system pathnames for 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 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.
The UNIX-domain protocol family is comprised of simple transport protocols
that support the SOCK_STREAM and SOCK_DGRAM abstractions.
SOCK_STREAM 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.
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 credentials
of its client, and vice versa by the client to verify the credentials
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)
caller) 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.
socket(2), intro(4)
"An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.
"An Advanced 4.3 BSD Interprocess Communication Tutorial", PS1, 8.
FreeBSD 5.2.1 July 15, 2001 FreeBSD 5.2.1 [ Back ] | 