Originální popis anglicky:
unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local interprocess
communication
Návod, kniha: Linux Programmer's Manual
#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.
#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.
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_PASSCRED
- Enables 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.
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_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.
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 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 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.
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 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) 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/recv at least one byte of non-ancillary data in the same send/recv_msg
call.
Unix domain stream sockets do not support the notion of out-of-band data.
- 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 filesystem
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 information.
recvmsg(2),
sendmsg(2),
socket(2),
socketpair(2),
cmsg(3),
capabilities(7),
socket(7)
