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NAME

       unix - sockets for local interprocess communication

SYNOPSIS

       #include <sys/socket.h>
       #include <sys/un.h>

       unix_socket = socket(AF_UNIX, type, 0);
       error = socketpair(AF_UNIX, type, 0, int *sv);

DESCRIPTION

       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 sequenced-packet socket that is connection-oriented, 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.

   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.)

   Pathname socket ownership and permissions
       In the Linux implementation, pathname sockets honor  the  permissions  of  the  directory  they  are  in.
       Creation  of a new socket fails if the process does not have write and search (execute) permission on the
       directory in which the socket is created.

       On Linux, connecting to a stream socket object requires  write  permission  on  that  socket;  sending  a
       datagram to a datagram socket likewise requires write permission on that socket.  POSIX does not make any
       statement about the effect of the permissions on a socket file, and on some systems (e.g.,  older  BSDs),
       the socket permissions are ignored.  Portable programs should not rely on this feature for security.

       When  creating a new socket, the owner and group of the socket file are set according to the usual rules.
       The socket file has all permissions enabled, other  than  those  that  are  turned  off  by  the  process
       umask(2).

       The owner, group, and permissions of a pathname socket can be changed (using chown(2) and chmod(2)).

   Abstract sockets
       Socket  permissions have no meaning for abstract sockets: the process umask(2) has no effect when binding
       an abstract socket, and changing  the  ownership  and  permissions  of  the  object  (via  fchown(2)  and
       fchmod(2)) has no effect on the accessibility of the socket.

       Abstract sockets automatically disappear when all open references to the socket are closed.

       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.

       SO_PASSCRED
              Enabling  this  socket  option  causes  receipt  of  the  credentials of the sending process in an
              SCM_CREDENTIALS ancillary message in each subsequently received message.  The returned credentials
              are  those  specified by the sender using SCM_CREDENTIALS, or a default that includes the sender's
              PID, real user ID, and real group ID, if the sender  did  not  specify  SCM_CREDENTIALS  ancillary
              data.

              When  this  option  is  set  and  the  socket  is not yet connected, a unique name in the abstract
              namespace will be generated automatically.

              The value given as an argument to setsockopt(2) and returned as the result of getsockopt(2) is  an
              integer boolean flag.

       SO_PASSSEC
              Enables receiving of the SELinux security label of the peer socket in an ancillary message of type
              SCM_SECURITY (see below).

              The value given as an argument to setsockopt(2) and returned as the result of getsockopt(2) is  an
              integer boolean flag.

              The  SO_PASSSEC  option  is supported for UNIX domain datagram sockets since Linux 2.6.18; support
              for UNIX domain stream sockets was added in Linux 4.2.

       SO_PEEK_OFF
              See socket(7).

       SO_PEERCRED
              This read-only socket option returns the credentials of the peer process connected to this socket.
              The  returned  credentials  are those that were in effect at the time of the call to connect(2) or
              socketpair(2).

              The argument to getsockopt(2) is a pointer to a ucred structure; define  the  _GNU_SOURCE  feature
              test macro to obtain the definition of that structure from <sys/socket.h>.

              The  use  of  this  option  is  possible only for connected AF_UNIX stream sockets and for AF_UNIX
              stream and datagram socket pairs created using socketpair(2).

   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 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.

       Before  Linux 3.4, the use of MSG_TRUNC in the flags argument of recv(2) was 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).

       SCM_RIGHTS
              Send or receive a set of open file descriptors from another process.  The data portion contains an
              integer array of the file descriptors.

              Commonly, this operation is referred to  as  "passing  a  file  descriptor"  to  another  process.
              However,  more  accurately,  what  is being passed is a reference to an open file description (see
              open(2)), and in the receiving process it is likely that a different file descriptor  number  will
              be  used.   Semantically,  this  operation is equivalent to duplicating (dup(2)) a file descriptor
              into the file descriptor table of another process.

              If the buffer used to receive the ancillary data containing file descriptors is too small  (or  is
              absent),  then  the ancillary data is truncated (or discarded) and the excess file descriptors are
              automatically closed in the receiving process.

              If the number of file descriptors received in the ancillary data would cause the process to exceed
              its RLIMIT_NOFILE resource limit (see getrlimit(2)), the excess file descriptors are automatically
              closed in the receiving process.

              The kernel constant SCM_MAX_FD defines a limit on the number of file  descriptors  in  the  array.
              Attempting  to  send  an  array  larger  than  this limit causes sendmsg(2) to fail with the error
              EINVAL.  SCM_MAX_FD has the value 253 (or 255 in kernels before 2.6.38).

       SCM_CREDENTIALS
              Send or receive UNIX credentials.  This can be  used  for  authentication.   The  credentials  are
              passed  as  a  struct  ucred  ancillary  message.   This 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 privileged process 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, in which case the PID of any existing process may be
              specified), its real user ID, effective user ID, or saved set-user-ID (unless it has  CAP_SETUID),
              and its real 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.

       SCM_SECURITY
              Receive  the  SELinux  security  context  (the  security  label) of the peer socket.  The received
              ancillary data is a null-terminated string containing the security context.  The  receiver  should
              allocate at least NAME_MAX bytes in the data portion of the ancillary message for this data.

              To receive the security context, the SO_PASSSEC option must be enabled on the socket (see above).

       When  sending ancillary data with sendmsg(2), only one item of each of the above types may be included in
       the sent message.

       At least one byte of real data should be sent when sending ancillary data.  On Linux, this is required to
       successfully  send  ancillary  data over a UNIX domain stream socket.  When sending ancillary data over a
       UNIX domain datagram socket, it is not necessary on Linux to send any accompanying real  data.   However,
       portable applications should also include at least one byte of real data when sending ancillary data over
       a datagram socket.

       When receiving from a stream socket, ancillary data forms a kind of barrier for the received  data.   For
       example, suppose that the sender transmits as follows:

              1. sendmsg(2) of four bytes, with no ancillary data.
              2. sendmsg(2) of one byte, with ancillary data.
              3. sendmsg(2) of four bytes, with no ancillary data.

       Suppose  that  the receiver now performs recvmsg(2) calls each with a buffer size of 20 bytes.  The first
       call will receive five bytes of data, along with the ancillary data sent by the second  sendmsg(2)  call.
       The next call will receive the remaining four bytes of data.

       If  the  space  allocated  for  receiving incoming ancillary data is too small then the ancillary data is
       truncated to the number of headers that will fit in the supplied buffer (or, in the case of an SCM_RIGHTS
       file  descriptor  list,  the  list  of  file descriptors may be truncated).  If no buffer is provided for
       incoming ancillary data (i.e., the msg_control field of the msghdr structure supplied  to  recvmsg(2)  is
       NULL),  then  the incoming ancillary data is discarded.  In both of these cases, the MSG_CTRUNC flag will
       be set in the msg.msg_flags value returned by recvmsg(2).

   Ioctls
       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:

       SIOCINQ
              For SOCK_STREAM sockets, this call returns the number of unread bytes 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  sockets,  the  returned value is the same as for Internet domain
              datagram sockets; see udp(7).

ERRORS

       EADDRINUSE
              The specified local address is already in use or the filesystem socket object already exists.

       EBADF  This error can occur for sendmsg(2) when sending a file descriptor as ancillary data over  a  UNIX
              domain  socket  (see the description of SCM_RIGHTS, above), and indicates that the file descriptor
              number that is being sent is not valid (e.g., it is not an open file descriptor).

       ECONNREFUSED
              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.

       ECONNRESET
              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.

       EISCONN
              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.

       ENOTCONN
              Socket operation needs a target address, but the socket is not connected.

       EOPNOTSUPP
              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 send(2) or sendmsg(2).

       EPROTONOSUPPORT
              Passed protocol is not AF_UNIX.

       EPROTOTYPE
              Remote socket does not match the local socket type (SOCK_DGRAM versus SOCK_STREAM).

       ESOCKTNOSUPPORT
              Unknown socket type.

       ESRCH  While  sending an ancillary message containing credentials (SCM_CREDENTIALS), the caller specified
              a PID that does not match any existing process.

       ETOOMANYREFS
              This error can occur for sendmsg(2) when sending a file descriptor as ancillary data over  a  UNIX
              domain  socket (see the description of SCM_RIGHTS, above).  It occurs if the number of "in-flight"
              file descriptors exceeds the RLIMIT_NOFILE resource  limit  and  the  caller  does  not  have  the
              CAP_SYS_RESOURCE  capability.   An  in-flight  file  descriptor  is  one  that has been sent using
              sendmsg(2) but has not yet been accepted in the recipient process using recvmsg(2).

              This error is diagnosed since mainline Linux 4.5 (and in some earlier kernel  versions  where  the
              fix  has  been  backported).   In  earlier  kernel versions, it was possible to place an unlimited
              number of file descriptors in flight, by sending each file descriptor  with  sendmsg(2)  and  then
              closing the file descriptor so that it was not accounted against the RLIMIT_NOFILE resource limit.

       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.

VERSIONS

       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.)

NOTES

       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 socket, you must 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.

BUGS

       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.

EXAMPLE

       The following code demonstrates the use of sequenced-packet sockets for local interprocess communication.
       It  consists  of  two  programs.  The server program waits for a connection from the client program.  The
       client sends each of its command-line arguments in separate messages.  The  server  treats  the  incoming
       messages as integers and adds them up.  The client sends the command string "END".  The server sends back
       a message containing the sum of the client's integers.  The client prints the sum and exits.  The  server
       waits  for  the  next  client to connect.  To stop the server, the client is called with the command-line
       argument "DOWN".

       The following output was recorded while running the server in the background and repeatedly executing the
       client.  Execution of the server program ends when it receives the "DOWN" command.

   Example output
           $ ./server &
           [1] 25887
           $ ./client 3 4
           Result = 7
           $ ./client 11 -5
           Result = 6
           $ ./client DOWN
           Result = 0
           [1]+  Done                    ./server
           $

   Program source

       /*
        * File connection.h
        */

       #define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
       #define BUFFER_SIZE 12

       /*
        * File server.c
        */

       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/socket.h>
       #include <sys/un.h>
       #include <unistd.h>
       #include "connection.h"

       int
       main(int argc, char *argv[])
       {
           struct sockaddr_un name;
           int down_flag = 0;
           int ret;
           int connection_socket;
           int data_socket;
           int result;
           char buffer[BUFFER_SIZE];

           /*
            * In case the program exited inadvertently on the last run,
            * remove the socket.
            */

           unlink(SOCKET_NAME);

           /* Create local socket. */

           connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
           if (connection_socket == -1) {
               perror("socket");
               exit(EXIT_FAILURE);
           }

           /*
            * For portability clear the whole structure, since some
            * implementations have additional (nonstandard) fields in
            * the structure.
            */

           memset(&name, 0, sizeof(struct sockaddr_un));

           /* Bind socket to socket name. */

           name.sun_family = AF_UNIX;
           strncpy(name.sun_path, SOCKET_NAME, sizeof(name.sun_path) - 1);

           ret = bind(connection_socket, (const struct sockaddr *) &name,
                      sizeof(struct sockaddr_un));
           if (ret == -1) {
               perror("bind");
               exit(EXIT_FAILURE);
           }

           /*
            * Prepare for accepting connections. The backlog size is set
            * to 20. So while one request is being processed other requests
            * can be waiting.
            */

           ret = listen(connection_socket, 20);
           if (ret == -1) {
               perror("listen");
               exit(EXIT_FAILURE);
           }

           /* This is the main loop for handling connections. */

           for (;;) {

               /* Wait for incoming connection. */

               data_socket = accept(connection_socket, NULL, NULL);
               if (data_socket == -1) {
                   perror("accept");
                   exit(EXIT_FAILURE);
               }

               result = 0;
               for (;;) {

                   /* Wait for next data packet. */

                   ret = read(data_socket, buffer, BUFFER_SIZE);
                   if (ret == -1) {
                       perror("read");
                       exit(EXIT_FAILURE);
                   }

                   /* Ensure buffer is 0-terminated. */

                   buffer[BUFFER_SIZE - 1] = 0;

                   /* Handle commands. */

                   if (!strncmp(buffer, "DOWN", BUFFER_SIZE)) {
                       down_flag = 1;
                       break;
                   }

                   if (!strncmp(buffer, "END", BUFFER_SIZE)) {
                       break;
                   }

                   /* Add received summand. */

                   result += atoi(buffer);
               }

               /* Send result. */

               sprintf(buffer, "%d", result);
               ret = write(data_socket, buffer, BUFFER_SIZE);
               if (ret == -1) {
                   perror("write");
                   exit(EXIT_FAILURE);
               }

               /* Close socket. */

               close(data_socket);

               /* Quit on DOWN command. */

               if (down_flag) {
                   break;
               }
           }

           close(connection_socket);

           /* Unlink the socket. */

           unlink(SOCKET_NAME);

           exit(EXIT_SUCCESS);
       }

       /*
        * File client.c
        */

       #include <errno.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/socket.h>
       #include <sys/un.h>
       #include <unistd.h>
       #include "connection.h"

       int
       main(int argc, char *argv[])
       {
           struct sockaddr_un addr;
           int i;
           int ret;
           int data_socket;
           char buffer[BUFFER_SIZE];

           /* Create local socket. */

           data_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
           if (data_socket == -1) {
               perror("socket");
               exit(EXIT_FAILURE);
           }

           /*
            * For portability clear the whole structure, since some
            * implementations have additional (nonstandard) fields in
            * the structure.
            */

           memset(&addr, 0, sizeof(struct sockaddr_un));

           /* Connect socket to socket address */

           addr.sun_family = AF_UNIX;
           strncpy(addr.sun_path, SOCKET_NAME, sizeof(addr.sun_path) - 1);

           ret = connect (data_socket, (const struct sockaddr *) &addr,
                          sizeof(struct sockaddr_un));
           if (ret == -1) {
               fprintf(stderr, "The server is down.\n");
               exit(EXIT_FAILURE);
           }

           /* Send arguments. */

           for (i = 1; i < argc; ++i) {
               ret = write(data_socket, argv[i], strlen(argv[i]) + 1);
               if (ret == -1) {
                   perror("write");
                   break;
               }
           }

           /* Request result. */

           strcpy (buffer, "END");
           ret = write(data_socket, buffer, strlen(buffer) + 1);
           if (ret == -1) {
               perror("write");
               exit(EXIT_FAILURE);
           }

           /* Receive result. */

           ret = read(data_socket, buffer, BUFFER_SIZE);
           if (ret == -1) {
               perror("read");
               exit(EXIT_FAILURE);
           }

           /* Ensure buffer is 0-terminated. */

           buffer[BUFFER_SIZE - 1] = 0;

           printf("Result = %s\n", buffer);

           /* Close socket. */

           close(data_socket);

           exit(EXIT_SUCCESS);
       }

       For an example of the use of SCM_RIGHTS see cmsg(3).

SEE ALSO

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

COLOPHON

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