UNIX(7) Linux Programmers Manual UNIX(7)
NAME
unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local inter
process communication
SYNOPSIS
#include
#include
unix_socket = socket(PF_UNIX, type, 0);
error = socketpair(PF_UNIX, type, 0, int *sv);
DESCRIPTION
The PF_UNIX (also known as PF_LOCAL) socket family is used to communi
cate between processes on the same machine efficiently. Traditionally,
Unix sockets can be either unnamed, or bound to a file system pathname
(marked as being of type socket). Linux also supports an abstract
namespace which is independent of the file system.
Valid types are: SOCK_STREAM, for a stream-oriented socket and
SOCK_DGRAM, for a datagram-oriented socket that preserves message
boundaries (as on most Unix implementations, Unix domain datagram sock
ets are always reliable and dont reorder datagrams); and (since Linux
2.6.4) SOCK_SEQPACKET, for a connection-oriented socket that preserves
message boundaries and delivers messages in the order that they were
sent.
Unix sockets support passing file descriptors or process credentials to
other processes using ancillary data.
Address Format
A Unix domain socket address is represented in the following structure:
#define UNIX_PATH_MAX 108
struct sockaddr_un {
sa_family_t sun_family; /* AF_UNIX */
char sun_path[UNIX_PATH_MAX]; /* pathname */
};
sun_family always contains AF_UNIX.
Three types of address are distinguished in this structure:
* pathname: a Unix domain socket can be bound to a null-terminated
file system pathname using bind(2). When the address of the socket
is returned by getsockname(2), getpeername(2), and accept(2), its
length is sizeof(sa_family_t) + strlen(sun_path) + 1, and sun_path
contains the null-terminated pathname.
* unnamed: A stream socket that has not been bound to a pathname using
bind(2) has no name. Likewise, the two sockets created by socket
pair(2) are unnamed. When the address of an unnamed socket is
returned by getsockname(2), getpeername(2), and accept(2), its
length is sizeof(sa_family_t), and sun_path should not be inspected.
* abstract: an abstract socket address is distinguished by the fact
that sun_path[0] is a null byte (\0). All of the remaining bytes
in sun_path define the "name" of the socket. (Null bytes in the
name have no special significance.) The name has no connection with
file system pathnames. The sockets address in this namespace is
given by the rest of the bytes in sun_path. When the address of an
abstract socket is returned by getsockname(2), getpeername(2), and
accept(2), its length is sizeof(struct sockaddr_un), and sun_path
contains the abstract name. The abstract socket namespace is a non-
portable Linux extension.
Socket Options
For historical reasons these socket options are specified with a
SOL_SOCKET type even though they are PF_UNIX specific. They can be set
with setsockopt(2) and read with getsockopt(2) by specifying SOL_SOCKET
as the socket family.
SO_PASSCRED
Enables the receiving of the credentials of the sending process
ancillary message. When this option is set and the socket is
not yet connected a unique name in the abstract namespace will
be generated automatically. Expects an integer boolean flag.
(Un)supported Features
The following paragraphs describe domain-specific details and unsup
ported features of the sockets API for Unix domain sockets on Linux.
Unix domain sockets do not support the transmission of out-of-band data
(the MSG_OOB flag for send(2) and recv(2)).
The send(2) MSG_MORE flag is not supported by Unix domain sockets.
The SO_SNDBUF socket option does have an effect for Unix domain sock
ets, but the SO_RCVBUF option does not. For datagram sockets, the
SO_SNDBUF value imposes an upper limit on the size of outgoing data
grams. This limit is calculated as the doubled (see socket(7)) option
value less 32 bytes used for overhead.
Ancillary Messages
Ancillary data is sent and received using sendmsg(2) and recvmsg(2).
For historical reasons the ancillary message types listed below are
specified with a SOL_SOCKET type even though they are PF_UNIX specific.
To send them set the cmsg_level field of the struct cmsghdr to
SOL_SOCKET and the cmsg_type field to the type. For more information
see cmsg(3).
SCM_RIGHTS
Send or receive a set of open file descriptors from another pro
cess. The data portion contains an integer array of the file
descriptors. The passed file descriptors behave as though they
have been created with dup(2).
SCM_CREDENTIALS
Send or receive Unix credentials. This can be used for authen
tication. The credentials are passed as a struct ucred ancil
lary message.
struct ucred {
pid_t pid; /* process ID of the sending process */
uid_t uid; /* user ID of the sending process */
gid_t gid; /* group ID of the sending process */
};
The credentials which the sender specifies are checked by the
kernel. A process with effective user ID 0 is allowed to spec
ify values that do not match its own. The sender must specify
its own process ID (unless it has the capability CAP_SYS_ADMIN),
its user ID, effective user ID, or saved set-user-ID (unless it
has CAP_SETUID), and its group ID, effective group ID, or saved
set-group-ID (unless it has CAP_SETGID). To receive a struct
ucred message the SO_PASSCRED option must be enabled on the
socket.
ERRORS
EADDRINUSE
Selected local address is already taken or file system socket
object already exists.
ECONNREFUSED
connect(2) called with a socket object that isnt listening.
This can happen when the remote socket does not exist or the
filename is not a socket.
ECONNRESET
Remote socket was unexpectedly closed.
EFAULT User memory address was not valid.
EINVAL Invalid argument passed. A common cause is the missing setting
of AF_UNIX in the sun_type field of passed addresses or the
socket being in an invalid state for the applied operation.
EISCONN
connect(2) called on an already connected socket or a target
address was specified on a connected socket.
ENOMEM Out of memory.
ENOTCONN
Socket operation needs a target address, but the socket is not
connected.
EOPNOTSUPP
Stream operation called on non-stream oriented socket or tried
to use the out-of-band data option.
EPERM The sender passed invalid credentials in the struct ucred.
EPIPE Remote socket was closed on a stream socket. If enabled, a SIG
PIPE is sent as well. This can be avoided by passing the
MSG_NOSIGNAL flag to sendmsg(2) or recvmsg(2).
EPROTONOSUPPORT
Passed protocol is not PF_UNIX.
EPROTOTYPE
Remote socket does not match the local socket type (SOCK_DGRAM
vs. SOCK_STREAM)
ESOCKTNOSUPPORT
Unknown socket type.
Other errors can be generated by the generic socket layer or by the
file system while generating a file system socket object. See the
appropriate manual pages for more information.
VERSIONS
SCM_CREDENTIALS and the abstract namespace were introduced with Linux
2.2 and should not be used in portable programs. (Some BSD-derived
systems also support credential passing, but the implementation details
differ.)
NOTES
In the Linux implementation, sockets which are visible in the file sys
tem honor the permissions of the directory they are in. Their owner,
group and their permissions can be changed. Creation of a new socket
will fail if the process does not have write and search (execute) per
mission on the directory the socket is created in. Connecting to the
socket object requires read/write permission. This behavior differs
from many BSD-derived systems which ignore permissions for Unix sock
ets. Portable programs should not rely on this feature for security.
Binding to a socket with a filename creates a socket in the file system
that must be deleted by the caller when it is no longer needed (using
unlink(2)). The usual Unix close-behind semantics apply; the socket
can be unlinked at any time and will be finally removed from the file
system when the last reference to it is closed.
To pass file descriptors or credentials over a SOCK_STREAM, you need to
send or receive at least one byte of non-ancillary data in the same
sendmsg(2) or recvmsg(2) call.
Unix domain stream sockets do not support the notion of out-of-band
data.
EXAMPLE
See bind(2).
SEE ALSO
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabili
ties(7), credentials(7), socket(7)
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-06-17 UNIX(7)
|
