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       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 communicate 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 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
          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 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
       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

   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 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 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 filesystem while
       generating a filesystem  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 filesystem 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 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 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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