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

       unix, AF_UNIX, AF_LOCAL - Sockets for local interprocess communication

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

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

       The  AF_UNIX (also known as AF_LOCAL) socket family is used to communi‐
       cate between processes on the same machine efficiently.	Traditionally,
       Unix  sockets can be either unnamed, or bound to a file system pathname
       (marked as being 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 sock‐
       ets are always reliable and don't reorder datagrams); and (since	 Linux
       2.6.4)  SOCK_SEQPACKET, for a connection-oriented socket that preserves
       message boundaries and delivers messages in the order  that  they  were

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

   Address Format
       A Unix domain socket address is represented in the following structure:

	   #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.

       Three types of address are distinguished in this structure:

       *  pathname: a Unix domain socket can be	 bound	to  a  null-terminated
	  file	system pathname using bind(2).	When the address of the socket
	  is returned by getsockname(2), getpeername(2),  and  accept(2),  its
	  length  is  sizeof(sa_family_t) + strlen(sun_path) + 1, and sun_path
	  contains the null-terminated pathname.

       *  unnamed: A stream socket that has not been bound to a pathname using
	  bind(2)  has	no name.  Likewise, the two sockets created by socket‐
	  pair(2) are unnamed.	When the  address  of  an  unnamed  socket  is
	  returned  by	getsockname(2),	 getpeername(2),  and  accept(2),  its
	  length is sizeof(sa_family_t), and sun_path should not be inspected.

       *  abstract: an abstract socket address is distinguished	 by  the  fact
	  that	sun_path[0] is a null byte ('\0').  All of the remaining bytes
	  in sun_path define the "name" of the socket.	 (Null	bytes  in  the
	  name have no special significance.)  The name has no connection with
	  file system pathnames.  The socket's address in  this	 namespace  is
	  given	 by the rest of the bytes in sun_path.	When the address of an
	  abstract socket is returned by getsockname(2),  getpeername(2),  and
	  accept(2),  its  length  is sizeof(struct sockaddr_un), and sun_path
	  contains the abstract name.  The abstract socket namespace is a non-
	  portable Linux extension.

   Socket Options
       For  historical	reasons	 these	socket	options	 are  specified with a
       SOL_SOCKET type even though they are AF_UNIX specific.  They can be set
       with setsockopt(2) and read with getsockopt(2) by specifying SOL_SOCKET
       as the socket family.

	      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.

   Sockets API
       The  following  paragraphs  describe domain-specific details and unsup‐
       ported 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  use of MSG_TRUNC in the flags argument of recv(2) is not supported
       by Unix domain sockets.

       The SO_SNDBUF socket option does have an effect for Unix	 domain	 sock‐
       ets,  but  the  SO_RCVBUF  option  does not.  For datagram sockets, the
       SO_SNDBUF value imposes an upper limit on the size  of  outgoing	 data‐
       grams.	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 AF_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).

	      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).

	      Send or receive Unix credentials.	 This can be used for  authen‐
	      tication.	  The  credentials are passed as a struct ucred ancil‐
	      lary message.  Thus structure is defined	in  <sys/socket.h>  as

		  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 */

	      Since  glibc  2.8,  the  _GNU_SOURCE  feature test macro must be
	      defined in order to obtain the definition of this structure.

	      The credentials which the sender specifies are  checked  by  the
	      kernel.	A process with effective user ID 0 is allowed to spec‐
	      ify 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

	      Selected local address is already taken or  file	system	socket
	      object already exists.

	      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.

	      Remote socket was unexpectedly closed.

       EFAULT User memory address was not valid.

       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.

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

       ENOMEM Out of memory.

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

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

       EPERM  The sender passed invalid credentials in the struct ucred.

       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).

	      Passed protocol is not AF_UNIX.

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

	      Unknown socket type.

       Other  errors  can  be  generated by the generic socket layer or by the
       file system while generating a file  system  socket  object.   See  the
       appropriate manual pages for more information.

       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

       In the Linux implementation, sockets which are visible in the file sys‐
       tem  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) per‐
       mission 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	 sock‐
       ets.  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

       See bind(2).

       recvmsg(2), sendmsg(2), socket(2),  socketpair(2),  cmsg(3),  capabili‐
       ties(7), credentials(7), socket(7)

       This  page  is  part of release 3.22 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-12-01			       UNIX(7)

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