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 processes on the same machine
efficiently. Unix sockets can be either anonymous (created
by socketpair(2)) or associated with a file of socket
type. 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 pre
serves message boundaries. Unix sockets are always reli
able and don't reorder datagrams.
Unix sockets support passing file descriptors or process
credentials to other processes as ancillary data to data
grams.
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 socketpair(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
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 zero byte 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 spe
cific. They can be set with setsockopt(2) and read with
getsockopt(2) by specifying SOL_SOCKET as the socket fam
ily.
SO_PASSCRED enables the receiving of the credentials of
the sending process ancillary message. When this option is
set and the socket is not connected yet an unique name in
the abstract namespace will be generated automatically.
Expects an integer boolean flag.
ANCILLARY MESSAGES
For historical reasons these ancillary message type 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 a inte
ger array of the file descriptors. The passed file
descriptors behave as like 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 his own. The sender
must specify its own process id (unless it has
CAP_SYS_ADMIN), its user id, effective user id or set user
id (unless it has CAP_SETUID), and its group id, effective
group id or 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 pro
grams.
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) permis
sion 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 sys
tem when the last reference to it is closed.
To pass file descriptors or credentials you need to
send/read at least one byte.
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 filesys
tem 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 informa
tion.
SEE ALSO
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3),
socket(7)
CREDITS
This man page was writen by Andi Kleen.
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