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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 socket types in the UNIX domain are:  SOCK_STREAM,  for  a  stream-oriented  socket;
       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:

           struct sockaddr_un {
               sa_family_t sun_family;               /* AF_UNIX */
               char        sun_path[108];            /* pathname */

       The sun_family field always contains AF_UNIX.  On Linux sun_path is 108 bytes in size; see
       also NOTES, below.

       Various systems calls (for example, bind(2), connect(2), and sendto(2)) take a sockaddr_un
       argument  as input.  Some other system calls (for example, getsockname(2), getpeername(2),
       recvfrom(2), and accept(2)) return an argument of this type.

       Three types of address are distinguished in the sockaddr_un structure:

       *  pathname: a UNIX domain socket can be bound to a  null-terminated  filesystem  pathname
          using bind(2).  When the address of a pathname socket is returned (by one of the system
          calls noted above), its length is

              offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1

          and sun_path contains the null-terminated pathname.  (On Linux,  the  above  offsetof()
          expression   equates   to  the  same  value  as  sizeof(sa_family_t),  but  some  other
          implementations include other fields before sun_path, so the offsetof() expression more
          portably describes the size of the address structure.)

          For further details of pathname sockets, see below.

       *  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,  its length is sizeof(sa_family_t), and
          sun_path should not be inspected.

       *  abstract: an abstract socket address is distinguished (from a pathname socket)  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,  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.

   Pathname sockets
       When binding a socket  to  a  pathname,  a  few  rules  should  be  observed  for  maximum
       portability and ease of coding:

       *  The pathname in sun_path should be null-terminated.

       *  The  length of the pathname, including the terminating null byte, should not exceed the
          size of sun_path.

       *  The addrlen argument that describes the enclosing sockaddr_un structure should  have  a
          value of at least:

              offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1

          or, more simply, addrlen can be specified as sizeof(struct sockaddr_un).

       There is some variation in how implementations handle UNIX domain socket addresses that do
       not follow the above rules.  For example, some (but not all) implementations append a null
       terminator if none is present in the supplied sun_path.

       When coding portable applications, keep in mind that some implementations have sun_path as
       short as 92 bytes.

       Various system  calls  (accept(2),  recvfrom(2),  getsockname(2),  getpeername(2))  return
       socket  address structures.  When applied to UNIX domain sockets, the value-result addrlen
       argument supplied to the call should be initialized as above.  Upon return,  the  argument
       is  set to indicate the actual size of the address structure.  The caller should check the
       value returned in this argument: if the output value exceeds the input value,  then  there
       is no guarantee that a null terminator is present in sun_path.  (See BUGS.)

   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:

              For SOCK_STREAM socket the function 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>.  For SOCK_DGRAM  socket,
              the returned value is the same as for Internet domain datagram socket; see udp(7).


              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 pathname 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  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.


       When  binding  a  socket to an address, Linux is one of the implementations that appends a
       null terminator if none is supplied in sun_path.  In most  cases  this  is  unproblematic:
       when  the  socket address is retrieved, it will be one byte longer than that supplied when
       the socket was bound.  However, there is one case where confusing behavior can result:  if
       108  non-null  bytes  are  supplied  when a socket is bound, then the addition of the null
       terminator takes the length of the pathname beyond sizeof(sun_path).   Consequently,  when
       retrieving  the socket address (for example, via accept(2)), if the input addrlen argument
       for the retrieving call is specified as  sizeof(struct  sockaddr_un),  then  the  returned
       address structure won't have a null terminator in sun_path.

       In  addition,  some  implementations don't require a null terminator when binding a socket
       (the addrlen argument is used to determine the length of sun_path)  and  when  the  socket
       address is retrieved on these implementations, there is no null terminator in sun_path.

       Applications  that retrieve socket addresses can (portably) code to handle the possibility
       that there is no null terminator in sun_path by respecting the fact  that  the  number  of
       valid bytes in the pathname is:

           strnlen(addr.sun_path, addrlen - offsetof(sockaddr_un, sun_path))

       Alternatively,  an  application  can retrieve the socket address by allocating a buffer of
       size sizeof(struct sockaddr_un)+1 that is zeroed out before the retrieval.  The retrieving
       call  can  specify  addrlen as sizeof(struct sockaddr_un), and the extra zero byte ensures
       that there will be a null terminator for the string returned in sun_path:

          void *addrp;

          addrlen = sizeof(struct sockaddr_un);
          addrp = malloc(addrlen + 1);
          if (addrp == NULL)
              /* Handle error */ ;
          memset(addrp, 0, addrlen + 1);

          if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == -1)
              /* handle error */ ;

          printf("sun_path = %s\n", ((struct sockaddr_un *) addrp)->sun_path);

       This sort of messiness can be avoided if it  is  guaranteed  that  the  applications  that
       create pathname sockets follow the rules outlined above under Pathname sockets.


       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), udp(7)


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