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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  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
          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’).  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.

              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

       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).

              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 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.


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

              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.

              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.

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

       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
              SIGPIPE 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
              vs.  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
       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 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


       See bind(2).


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


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