From Linux Man Pages
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 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 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 kernel 2.6.4) SOCK_SEQPACKET, for a connection-oriented socket that pre-
serves 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 address is defined as a filename in the filesystem or as a unique string in the abstract namespace. Sock-
ets created by socketpair(2) are anonymous. For non-anonymous sockets the target address can be set using con-
nect(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 null byte ( '), then 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 automat-
ically. 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 reasons the ancillary mes-
sage 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 infor-
mation 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 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_SET-
GID). 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 pro-
grams. (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. Connect-
ing 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() or recvmsg() 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.
RELATED
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), socket(7)
CATEGORY