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

       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  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 3.53 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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