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_DGRAM and
SOCK_SEQPACKET 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;
sa_family_t 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 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 is
comprised of simple transport protocols that support the
SOCK_STREAM, SOCK_DGRAM and
SOCK_SEQPACKET abstractions.
SOCK_STREAM, SOCK_DGRAM 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 via
dup(fd) or fcntl(fd,
F_DUPFD_CLOEXEC, 0) depending on whether
MSG_CMSG_CLOEXEC is passed in the
recvmsg(2) call. 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.
SEE ALSO
An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7.
An Advanced 4.3 BSD Interprocess Communication Tutorial, PS1, 8.