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NAME
unix - 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 communicate 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 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 domain 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), 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 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'). 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 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 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 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 (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 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.
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 (EINVAL) 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 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 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 appropriate 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 filesystem 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 domain sockets. Portable
programs should not rely on this feature for security.
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 ALSO
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), credentials(7), socket(7)
COLOPHON
This page is part of release 3.54 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)