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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

       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

   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 2.3.15.)

   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 (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

              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

              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
              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 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
       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

       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


       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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