unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL — Sockets for local interprocess communication
#include <sys/socket.h> #include <sys/un.h>
unix_socket = socket( |
PF_UNIX, | |
| type, | ||
0); |
error
= socketpair( |
PF_UNIX, | |
| type, | ||
| 0, | ||
| int * | sv); |
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 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.
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.
struct sockaddr_un { sa_family_t sun_family;char sun_path[UNIX_PATH_MAX];};
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.
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_PASSCREDEnables 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.
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 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_RIGHTSSend 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_CREDENTIALSSend 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;uid_t uid;gid_t gid;};
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.
Selected local address is already taken or filesystem socket object already exists.
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.
Remote socket was unexpectedly closed.
User memory address was not valid.
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.
connect(2) called on an already connected socket or a target address was specified on a connected socket.
Out of memory.
Socket operation needs a target address, but the socket is not connected.
Stream operation called on non-stream oriented socket or tried to use the out-of-band data option.
The sender passed invalid credentials in the struct ucred.
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).
Passed protocol is not PF_UNIX.
Remote socket does not match the local socket type
(SOCK_DGRAM vs.
SOCK_STREAM)
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.
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.)
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 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(2) or recvmsg(2) call.
Unix domain stream sockets do not support the notion of out-of-band data.
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), credentials(7), socket(7)
This page is part of release 2.79 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/.
|
This man page is Copyright (C) 1999 Andi Kleen <akmuc.de>. Permission is granted to distribute possibly modified copies of this page provided the header is included verbatim, and in case of nontrivial modification author and date of the modification is added to the header. Modified, 2003-12-02, Michael Kerrisk, <mtk.manpagesgmail.com> Modified, 2003-09-23, Adam Langley Modified, 2004-05-27, Michael Kerrisk, <mtk.manpagesgmail.com> Added SOCK_SEQPACKET |