unix

Sommaire

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 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.

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 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.

Socket Options

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.

(Un)supported Features

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 Messages

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 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.

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.
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.

VERSIONS

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.)

NOTES

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.

EXAMPLE

See bind(2).

SEE ALSO

recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), credentials(7), socket(7)