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

       unix - 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  domain  sockets  can  be either unnamed, or bound to a filesystem
       pathname (marked as being of type  socket).   Linux  also  supports  an
       abstract namespace which is independent of the filesystem.

       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 domain sockets support passing file descriptors or process creden‐
       tials 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
	  filesystem  pathname	using bind(2).	When the address of the socket
	  is returned by getsockname(2), getpeername(2),  and  accept(2),  its
	  length is

	      offsetof(struct sockaddr_un, sun_path) + 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').   The	 socket's  address  in
	  this namespace is given by the additional bytes in sun_path that are
	  covered by the specified length of  the  address  structure.	 (Null
	  bytes	 in  the  name have no special significance.)  The name has no
	  connection with  filesystem  pathnames.   When  the  address	of  an
	  abstract  socket  is returned by getsockname(2), getpeername(2), and
	  accept(2), the returned addrlen is greater than  sizeof(sa_family_t)
	  (i.e.,  greater  than 2), and the name of the socket is contained in
	  the first (addrlen - sizeof(sa_family_t)) bytes  of  sun_path.   The
	  abstract socket namespace is a nonportable 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
	      in an 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

   Autobind feature
       If a bind(2) call specifies  addrlen  as	 sizeof(sa_family_t),  or  the
       SO_PASSCRED  socket  option  was	 specified  for	 a socket that was not
       explicitly bound to an address, then the	 socket	 is  autobound	to  an
       abstract	 address.   The	 address consists of a null byte followed by 5
       bytes in the character set [0-9a-f].  Thus, there is a  limit  of  2^20
       autobind	 addresses.  (From Linux 2.1.15, when the autobind feature was
       added, 8 bytes  were  used,  and	 the  limit  was  thus	2^32  autobind
       addresses.  The change to 5 bytes came in Linux 2.3.15.)

   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 (before including any header files) 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

       The  following ioctl(2) calls return information in value.  The correct
       syntax is:

	      int value;
	      error = ioctl(unix_socket, ioctl_type, &value);

       ioctl_type can be:

	      Returns the amount of queued unread data in the receive  buffer.
	      The socket must not be in LISTEN state, otherwise an error (EIN‐
	      VAL) is returned.	  SIOCINQ  is  defined	in  <linux/sockios.h>.
	      Alternatively,  you  can use the synonymous FIONREAD, defined in

	      The specified local address is already in use or the  filesystem
	      socket object already exists.

	      The  remote  address specified by connect(2) was not a listening
	      socket.  This error can also occur if the target 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 that the value
	      AF_UNIX was not  specified  in  the  sun_type  field  of	passed
	      addresses, or the socket was in an invalid state for the applied

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

       ENOENT The  pathname  in the remote address specified to connect(2) did
	      not exist.

       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
	      versus SOCK_STREAM)

	      Unknown socket type.

       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.

       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  filesys‐
       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	domain
       sockets.	  Portable  programs should not rely on this feature for secu‐

       Binding to a socket with a filename creates a socket in the  filesystem
       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
       filesystem 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 nonancillary 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).

       For an example of the use of SCM_RIGHTS see cmsg(3).

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

       This page is part of release 3.65 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				  2012-05-10			       UNIX(7)

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