UNIX(7) Linux Programmer's Manual UNIX(7)NAME
unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local inter‐
process 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 communi‐
cate 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 sock‐
ets are always reliable and don't reorder datagrams); and (since kernel
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 address is defined as a filename in the filesystem or as a
unique string in the abstract namespace. Sockets created by socket‐
pair(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 automati‐
cally.
#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-termi‐
nated pathname of the socket in the file system. If sun_path starts
with a null byte ('' '), then it refers to the abstract namespace main‐
tained 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.
(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
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 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.
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 filesys‐
tem 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) 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() 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 SIG‐
PIPE 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 appro‐
priate manual pages for more information.
SEE ALSOrecvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabili‐
ties(7), socket(7)Linux Man Page 2004-05-27 UNIX(7)
