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

NAME

       unix, AF_UNIX, AF_LOCAL - Sockets for local interprocess communication

SYNOPSIS

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

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

DESCRIPTION

       The  AF_UNIX  (also  known as AF_LOCAL) socket family is used to communicate between processes on the same machine efficiently.	Tradition-
       ally, 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 sockets 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 sent.

       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  con-
	  tains 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 ('').  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 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.

       SO_PASSCRED
	      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 unsupported 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 sockets, but the SO_RCVBUF option does not.  For datagram sockets, the
       SO_SNDBUF value imposes an upper limit on the size of outgoing datagrams.  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).

       SCM_RIGHTS
	      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).

       SCM_CREDENTIALS
	      Send or receive Unix credentials.  This can be used for authentication.  The credentials are passed as a struct ucred ancillary mes-
	      sage.  Thus structure is defined in <sys/socket.h> as follows:

		  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 defi-
	      nition of this structure.

	      The credentials which the sender specifies are checked by the kernel.  A process with effective user ID 0 is allowed to specify val-
	      ues  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 socket.

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

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

       ioctl_type can be:

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

ERRORS

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

       ECONNREFUSED
	      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.

       ECONNRESET
	      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.

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

       ENOMEM Out of memory.

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

       EOPNOTSUPP
	      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 SIGPIPE is sent as well.  This can be avoided by passing the MSG_NOSIG-
	      NAL flag to sendmsg(2) or recvmsg(2).

       EPROTONOSUPPORT
	      Passed protocol is not AF_UNIX.

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

       ESOCKTNOSUPPORT
	      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.

VERSIONS

       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 differ.)

NOTES

       In the Linux implementation, sockets which are visible in the file system 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)
       permission on the directory the socket is created in.  Connecting to the socket object requires read/write permission.  This behavior  dif-
       fers  from many BSD-derived systems which ignore permissions for Unix sockets.  Portable programs should not rely on this feature for secu-
       rity.

       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 data.

EXAMPLE

       See bind(2).

SEE ALSO

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

COLOPHON

       This  page is part of release 3.27 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								    2010-09-10								   UNIX(7)