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UNIX(7) Linux Programmer’s Manual UNIX(7)
NAME
unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local interprocess commu-
nication
SYNOPSIS
#include <sys/socket.h>
#include <sys/un.h>
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 communicate between
processes on the same machine efficiently. Unix sockets can be either anonymous
(created by socketpair(2)) or associated with a file 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 data-
gram oriented socket that preserves message boundaries. Unix sockets are always
reliable and don’t reorder datagrams.
Unix sockets support passing file descriptors or process credentials to other pro-
cesses using ancillary data.
ADDRESS FORMAT
A unix address is defined as a filename in the filesystem or as a unique string in
the abstract namespace. Sockets created by socketpair(2) are anonymous. For non-
anonymous sockets the target address can be set using connect(2). The local
address can be set using bind(2). When a socket is connected and it doesn’t
already have a local address a unique address in the abstract namespace will be
generated automatically.
#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. sun_path contains the zero-terminated pathname
of the socket in the file system. If sun_path starts with a zero byte it refers to
the abstract namespace maintained by the Unix protocol module. The socket’s
address in this namespace is given by the rest of the bytes in sun_path. Note that
names in the abstract namespace are not zero-terminated.
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.
ANCILLARY MESSAGES
Ancillary data is sent and received using sendmsg(2) and recvmsg(2). For histori-
cal 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 process. 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 authentication. The
credentials are passed as a struct ucred ancillary 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 specify values that do not match his own.
The sender must specify its own process ID (unless it has the capability
CAP_SYS_ADMIN), its user ID, effective user ID or set user ID (unless it has
CAP_SETUID), and its group id, effective group ID or set group ID (unless it has
CAP_SETGID). To receive a struct ucred message the SO_PASSCRED option must be
enabled on the socket.
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 filesystem honour 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) permission on the directory the socket is created in.
Connecting to the socket object requires read/write permission. This behavior dif-
fers from many BSD-derived systems which ignore permissions for Unix sockets.
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/recv
at least one byte of non-ancillary data in the same send/recv_msg call.
Unix domain stream sockets do not support the notion of out-of-band data.
ERRORS
ENOMEM Out of memory.
ECONNREFUSED
connect(2) called with a socket object that isn’t listening. This can happen
when the remote socket does not exist or the filename is not a socket.
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.
EOPNOTSUPP
Stream operation called on non-stream oriented socket or tried to use the
out-of-band data option.
EPROTONOSUPPORT
Passed protocol is not PF_UNIX.
ESOCKTNOSUPPORT
Unknown socket type.
EPROTOTYPE
Remote socket does not match the local socket type (SOCK_DGRAM vs.
SOCK_STREAM)
EADDRINUSE
Selected local address is already taken or filesystem socket object already
exists.
EISCONN
connect(2) called on an already connected socket or a target address was
specified on a connected socket.
ENOTCONN
Socket operation needs a target address, but the socket is not connected.
ECONNRESET
Remote socket was unexpectedly closed.
EPIPE Remote socket was closed on a stream socket. If enabled, a SIGPIPE is sent
as well. This can be avoided by passing the MSG_NOSIGNAL flag to sendmsg(2)
or recvmsg(2).
EFAULT User memory address was not valid.
EPERM The sender passed invalid credentials in the struct ucred.
Other errors can be generated by the generic socket layer or by the filesystem
while generating a filesystem socket object. See the appropriate manual pages for
more information.
SEE ALSO
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7),
socket(7)
Linux Man Page 2002-12-02 UNIX(7)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
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