#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

       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.)

   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-

              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.

       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).

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