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       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 communicate 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 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 domain 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 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  socketpair(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 flag.

   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

   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

       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

              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  authentication.   The
              credentials  are  passed  as  a  struct ucred ancillary message.  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 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 specify 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 socket.

       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  (EINVAL)  is  returned.   SIOCINQ  is
              defined  in <linux/sockios.h>.  Alternatively, you can use the synonymous FIONREAD,
              defined in <sys/ioctl.h>.


              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 operation.

              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 connected.

              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_NOSIGNAL flag to sendmsg(2) or

              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


       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.)


       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  differs  from  many  BSD-derived
       systems  which  ignore  permissions for UNIX domain sockets.  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 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.


       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),    capabilities(7),
       credentials(7), socket(7)


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