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NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

       #include <sys/socket.h>
       #include <sys/un.h>

       unix_socket = socket(PF_UNIX, type, 0);
       error = socketpair(PF_UNIX, type, 0, int *sv);

DESCRIPTION    [Toc]    [Back]

       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    [Toc]    [Back]

       A  unix	address  is  defined  as  a filename in the filesystem or as a
       unique string in the abstract namespace.  Sockets  created  by  socket-
       pair(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.

SOCKET OPTIONS    [Toc]    [Back]

       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.


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

       SCM_CREDENTIALS    [Toc]    [Back]
	      Send or receive unix credentials.  This can be used for  authentication.
   The  credentials are passed as a struct ucred ancillary

	      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    [Toc]    [Back]

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

NOTES    [Toc]    [Back]

       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 you need to send/read at  least
       one byte.

ERRORS    [Toc]    [Back]

       ENOMEM Out of memory.

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

       EPROTONOSUPPORT    [Toc]    [Back]
	      Passed protocol is not PF_UNIX.

       ESOCKTNOSUPPORT    [Toc]    [Back]
	      Unknown socket type.

       EPROTOTYPE    [Toc]    [Back]
	      Remote socket does not match the local socket  type  (SOCK_DGRAM
	      vs.  SOCK_STREAM)

       EADDRINUSE    [Toc]    [Back]
	      Selected	local  address	is  already taken or filesystem socket
	      object already exists.

       EISCONN    [Toc]    [Back]
	      connect(2) called on an already connected  socket  or  a	target
	      address was specified on a connected socket.

       ENOTCONN    [Toc]    [Back]
	      Socket  operation  needs a target address, but the socket is not

       ECONNRESET    [Toc]    [Back]
	      Remote socket was unexpectedly closed.

       EPIPE  Remote socket was closed on a stream socket. If enabled, a  SIG-
	      PIPE  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.

SEE ALSO    [Toc]    [Back]

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

CREDITS    [Toc]    [Back]

       This man page was written by Andi Kleen.

Linux Man Page			  1999-05-07			       UNIX(7)
[ Back ]
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