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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 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

   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

       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.

       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

              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 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.

       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  send(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.

              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


       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.)


       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.


       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"

       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.


           /* Create local socket. */

           connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
           if (connection_socket == -1) {

            * 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) {

            * 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) {

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

           for (;;) {

               /* Wait for incoming connection. */

               data_socket = accept(connection_socket, NULL, NULL);
               if (data_socket == -1) {

               result = 0;
               for(;;) {

                   /* Wait for next data packet. */

                   ret = read(data_socket, buffer, BUFFER_SIZE);
                   if (ret == -1) {

                   /* Ensure buffer is 0-terminated. */

                   buffer[BUFFER_SIZE - 1] = 0;

                   /* Handle commands. */

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

                   if (!strncmp(buffer, "END", BUFFER_SIZE)) {

                   /* Add received summand. */

                   result += atoi(buffer);

               /* Send result. */

               sprintf(buffer, "%d", result);
               ret = write(data_socket, buffer, BUFFER_SIZE);

               if (ret == -1) {

               /* Close socket. */


               /* Quit on DOWN command. */

               if (down_flag) {


           /* Unlink the socket. */



        * 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"

       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) {

            * 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");

           /* Send arguments. */

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

           /* Request result. */

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

           /* Receive result. */

           ret = read(data_socket, buffer, BUFFER_SIZE);
           if (ret == -1) {

           /* Ensure buffer is 0-terminated. */

           buffer[BUFFER_SIZE - 1] = 0;

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

           /* Close socket. */



       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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