UNIX(7) Linux Programmer's Manual UNIX(7)NAMEunix - sockets for local interprocess communication
SYNOPSIS
#include <sys/socket.h>
#include <sys/un.h>
unix_socket = socket(AF_UNIX, type, 0);
error = socketpair(AF_UNIX, type, 0, int *sv);
DESCRIPTION
The AF_UNIX (also known as AF_LOCAL) socket family is used to communi‐
cate between processes on the same machine efficiently. Traditionally,
UNIX domain sockets can be either unnamed, or bound to a filesystem
pathname (marked as being of type socket). Linux also supports an
abstract namespace which is independent of the filesystem.
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 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 domain sockets support passing file descriptors or process creden‐
tials 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
filesystem pathname using bind(2). When the address of the socket
is returned by getsockname(2), getpeername(2), and accept(2), its
length is
offsetof(struct sockaddr_un, sun_path) + 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 socket‐
pair(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'). The socket's address in
this namespace is given by the additional bytes in sun_path that are
covered by the specified length of the address structure. (Null
bytes in the name have no special significance.) The name has no
connection with filesystem pathnames. When the address of an
abstract socket is returned by getsockname(2), getpeername(2), and
accept(2), the returned addrlen is greater than sizeof(sa_family_t)
(i.e., greater than 2), and the name of the socket is contained in
the first (addrlen - sizeof(sa_family_t)) bytes of sun_path. The
abstract socket namespace is a nonportable Linux extension.
Socket options
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
in an 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.
Autobind feature
If a bind(2) call specifies addrlen as sizeof(sa_family_t), or the
SO_PASSCRED socket option was specified for a socket that was not
explicitly bound to an address, then the socket is autobound to an
abstract address. The address consists of a null byte followed by 5
bytes in the character set [0-9a-f]. Thus, there is a limit of 2^20
autobind addresses. (From Linux 2.1.15, when the autobind feature was
added, 8 bytes were used, and the limit was thus 2^32 autobind
addresses. The change to 5 bytes came in Linux 2.3.15.)
Sockets API
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 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 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 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 authen‐
tication. The credentials are passed as a struct ucred ancil‐
lary 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 (before including any header files) 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 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.
Ioctls
The following ioctl(2) calls return information in value. The correct
syntax is:
int value;
error = ioctl(unix_socket, ioctl_type, &value);
ioctl_type can be:
SIOCINQ
Returns the amount of queued unread data in the receive buffer.
The socket must not be in LISTEN state, otherwise an error (EIN‐
VAL) is returned. SIOCINQ is defined in <linux/sockios.h>.
Alternatively, you can use the synonymous FIONREAD, defined in
<sys/ioctl.h>.
ERRORS
EADDRINUSE
The specified local address is already in use or the filesystem
socket object already exists.
ECONNREFUSED
The remote address specified by connect(2) was not a listening
socket. This error can also occur if the target 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 that the value
AF_UNIX was not specified in the sun_type field of passed
addresses, or the socket was 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.
ENOENT The pathname in the remote address specified to connect(2) did
not exist.
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 SIG‐
PIPE 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
versus SOCK_STREAM)
ESOCKTNOSUPPORT
Unknown socket type.
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.
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 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) 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 domain
sockets. Portable programs should not rely on this feature for secu‐
rity.
Binding to a socket with a filename creates a socket in the filesystem
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
filesystem 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 nonancillary 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).
For an example of the use of SCM_RIGHTS see cmsg(3).
SEE ALSOrecvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabili‐
ties(7), credentials(7), socket(7)COLOPHON
This page is part of release 3.65 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 2012-05-10 UNIX(7)
