UNIX(7) 						     Linux Programmer's Manual							   UNIX(7)

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 socket type.  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.
       Unix sockets are always reliable and don't reorder datagrams.

       Unix sockets support passing file descriptors or process credentials to other processes as ancillary data to datagrams.

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  automati-
       cally.

	      #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.

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 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 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 a integer array of the file descrip-
	      tors.  The passed file descriptors behave as like 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 mes-
	      sage.

	      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 his own.  The sender must specify its own process id (unless it has 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.

VERSIONS

       SCM_CREDENTIALS and the abstract namespace were introduced with Linux 2.2 and should not be used in portable programs.

NOTES

       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 dif-
       fers from many BSD derived systems which ignore permissions for Unix sockets. Portable programs should not rely on this feature	for  secu-
       rity.

       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 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.

       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  appro-
       priate manual pages for more information.

SEE ALSO

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

CREDITS

       This man page was written by Andi Kleen.

Linux Man Page							    1999-05-07								   UNIX(7)