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unix — sockets for local interprocess communication
#include <sys/socket.h> #include <sys/un.h>
unix_socket =
socket( |
AF_UNIX, |
| type, | |
0); |
error =
socketpair( |
AF_UNIX, |
| type, | |
| 0, | |
int *sv); |
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.
A UNIX domain socket address is represented in the following structure:
struct sockaddr_un { sa_family_t sun_family;char sun_path[UNIX_PATH_MAX];};
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.
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_PASSCREDEnables 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.
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.)
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 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_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. Thus structure is defined in
<sys/socket.h> as follows:
struct ucred { pid_t pid;uid_t uid;gid_t gid;};
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.
The following ioctl(2) calls return
information in value. The correct syntax
is:
intvalue;error= ioctl(unix_socket,ioctl_type, &value);
ioctl_type can
be:
SIOCINQReturns 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>
The specified local address is already in use or the filesystem socket object already exists.
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.
Remote socket was unexpectedly closed.
User memory address was not valid.
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.
connect(2) called on an already connected socket or a target address was specified on a connected socket.
The pathname in the remote address specified to connect(2) did not exist.
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 AF_UNIX.
Remote socket does not match the local socket type
(SOCK_DGRAM versus
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 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.
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), credentials(7), socket(7)
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/.
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This man page is Copyright (C) 1999 Andi Kleen <akmuc.de>. %%%LICENSE_START(VERBATIM_ONE_PARA) 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. %%%LICENSE_END 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 2008-05-27, mtk, Provide a clear description of the three types of address that can appear in the sockaddr_un structure: pathname, unnamed, and abstract. |