NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | EXAMPLE | SEE ALSO | COLOPHON
UNIX(7) Linux Programmer's Manual UNIX(7)
unix, AF_UNIX, AF_LOCAL - Sockets for local interprocess communication
#include <sys/socket.h>
#include <sys/un.h>
unix_socket = socket(AF_UNIX, type, 0);
error = socketpair(AF_UNIX, type, 0, int *sv);
The AF_UNIX (also known as AF_LOCAL) socket family is used to communicate
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 sockets are always reliable and don't
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.
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 socketpair(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
socket's 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.
For historical reasons these socket options are specified with a SOL_SOCKET
type even though they are AF_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.
The following paragraphs describe domain-specific details and unsupported
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 use of MSG_TRUNC in the flags argument of recv(2) is not supported by Unix
domain sockets.
The SO_SNDBUF socket option does have an effect for Unix domain sockets, but
the SO_RCVBUF option does not. For datagram sockets, the SO_SNDBUF value
imposes an upper limit on the size of outgoing datagrams. This limit is
calculated as the doubled (see socket(7)) option value less 32 bytes used for
overhead.
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 AF_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. Thus
structure is defined in <sys/socket.h> as follows:
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 */
};
Since glibc 2.8, the _GNU_SOURCE feature test macro must be defined in
order to obtain the definition of this structure.
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 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.
EADDRINUSE
Selected local address is already taken or file system socket object
already exists.
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.
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 SIGPIPE 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 AF_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.
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.)
In the Linux implementation, sockets which are visible in the file system
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) permission 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 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
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.
See bind(2).
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7),
credentials(7), socket(7)
This page is part of release 3.23 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 2008-12-01 UNIX(7)