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

   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 will fail  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.

          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 send(2) or sendmsg(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


   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.


   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

   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)


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at


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