The
PF_UNIX
(also called
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 an socket object in the filesystem namespace (and
subject to the usual filesystem permission checks).
Since Linux 2.2 an abstract name space independent from the file system
is supported too.
Valid types are
SOCK_STREAM
for a stream oriented socket type and
SOCK_DGRAM
for a datagram oriented socket type that preserves message boundaries. Unix
sockets are always reliable.
Unix sockets support passing file descriptors or process credentials to other
processes using ancillary data.
ADDRESS FORMAT
A unix address is defined as a unique string either in the filesystem or
in the abstract namespace. Sockets create by
socketpair(2)
don't have an address. For other 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 have a local address already a
unique address in the abstract namespace will be generated automatically.
sun_family
always contains AF_UNIX (or AF_LOCAL which is a synonym)
sun_path
contains the null terminated pathname of the filesystem socket object.
If
sun_path
starts with a 0 byte it refers to the abstract namespace maintained by the Unix
protocol module. After that a non-zero terminated byte sequence of
the passed length number of bytes - 1 follows.
SOCKET OPTIONS
For historical reasons these socket options are specified with a
SOL_SOCKET
type. They are PF_UNIX specific though. 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 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. They are PF_UNIX specific though. 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 file descriptor.
The data portion contains a integer array of the
file descriptors.
SCM_CREDENTIALS
Send or receive the credentials of the sending process. 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 */
};
During sending only root processes are allowed specify credentials they don't
own. On receiving the current credentials of the sending process are passed,
unless the user specified different credentials (and had the rights to do that).
To receive the message the
SO_PASSCRED
option must be enabled.
VERSIONS
SCM_CREDENTIALS
and the abstract namespace were introduced with Linux 2.2.
NOTES
In Linux PF_UNIX sockets visible in the filesystem honor the permissions of the
the directory they are part of. It is also possible to change their owner, groups
and permissions. To create a new socket (bind) write and executable
permission to the directory containing the socket is needed, for connecting
read/write permissions to the socket object in the filesystem.
This behavior differs from many BSD derived systems which
ignore permissions for Unix sockets. Portable programs should not rely on
this feature.
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 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.
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.
