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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 communication
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 pro-
cesses 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.
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), getpeer-
name(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 con-
nection 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 sock-
addr_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 get-
sockopt(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 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 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 Messages
Ancillary data is sent and received using sendmsg(2) and recvmsg(2). For historical rea-
sons 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 por-
tion contains an integer array of the file descriptors. The passed file descrip-
tors behave as though they have been created with dup(2).
SCM_CREDENTIALS
Send or receive Unix credentials. This can be used for authentication. The cre-
dentials 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 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 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 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 gen-
erating a file system socket object. See the appropriate manual pages for more informa-
tion.
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 system honor the per-
missions 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 sys-
tems 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.
EXAMPLE
See bind(2).
SEE ALSO
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), creden-
tials(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)
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