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RedHat 9 (Linux i386) - man page for unix (redhat section 7)

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

       unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local interprocess communication.

       #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 pro-
       cesses 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.

       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  get-
       sockopt(2) by specifying SOL_SOCKET as the socket family.

       SO_PASSCRED enables the receiving of the credentials of the sending process ancillary mes-
       sage. 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.

       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

	      Send or receive a set of open file descriptors from another process.  The data por-
	      tion contains a integer array of the file descriptors.  The passed file descriptors
	      behave as like they have been created with dup(2).

	      Send or receive unix credentials.  This can be used for authentication.	The  cre-
	      dentials 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 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.

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

       In  the	Linux implementation, sockets which are visible in the filesystem honour the per-
       missions 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 sys-
       tems 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 you need to send/read at least one byte.

       ENOMEM Out of memory.

	      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.

	      Stream  operation  called on non-stream oriented socket or tried to use the out-of-
	      band data option.

	      Passed protocol is not PF_UNIX.

	      Unknown socket type.

	      Remote socket does not match the local socket type (SOCK_DGRAM vs.  SOCK_STREAM)

	      Selected local address is already taken or filesystem socket object already exists.

	      connect(2) called on an already connected socket or a target address was	specified
	      on a connected socket.

	      Socket operation needs a target address, but the socket is not connected.

	      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  gen-
       erating a filesystem socket object. See the appropriate manual pages for more information.

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

       This man page was written by Andi Kleen.

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

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