UNIX(7)                    Linux Programmer's Manual                   UNIX(7)

       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 communi-
       cate 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  ab-
       stract 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  pre-
       serves 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 mes-
       sages in the order that they were sent.

       UNIX domain sockets support passing file descriptors or process creden-
       tials 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 ex-
       ample,  getsockname(2), getpeername(2), recvfrom(2), and accept(2)) re-
       turn 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  socket-
          pair(2)  are  unnamed.  When the address of an unnamed socket is re-
          turned, its length is sizeof(sa_family_t), and sun_path  should  not
          be inspected.

       *  abstract:  an abstract socket address is distinguished (from a path-
          name 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 signifi-
          cance.)  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 struc-
          ture 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 sock-

       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 implementa-
       tions have sun_path as short as 92 bytes.

       Various system calls (accept(2), recvfrom(2), getsockname(2),  getpeer-
       name(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 di-
       rectory in which the socket is created.

       On Linux, connecting to a stream socket object requires  write  permis-
       sion  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  fch-
       mod(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  ex-
       plicitly  bound  to  an address, then the socket is autobound to an ab-
       stract 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 ad-
       dresses.  The change to 5 bytes came in Linux 2.3.15.)

   Sockets API
       The following paragraphs describe domain-specific  details  and  unsup-
       ported 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 sock-
       ets, but the SO_RCVBUF option does  not.   For  datagram  sockets,  the
       SO_SNDBUF  value  imposes  an upper limit on the size of outgoing data-
       grams.  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 authen-
              tication.  The credentials are passed as a struct  ucred  ancil-
              lary  message.   Thus  structure is defined in <sys/socket.h> as

                  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  de-
              fined (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 spec-
              ify 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  (un-
              less  it has CAP_SETUID), and its real group ID, effective group
              ID, or saved set-group-ID (unless it has  CAP_SETGID).   To  re-
              ceive  a struct ucred message the SO_PASSCRED option must be en-
              abled 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.

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

              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 ad-
              dresses, or the socket was in an invalid state for  the  applied

              connect(2) called on an already connected socket or a target ad-
              dress 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 SIG-
              PIPE 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.

              This error can occur for sendmsg(2) when sending a file descrip-
              tor  as  ancillary  data  over a UNIX domain socket (see the de-
              scription 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 capabil-
              ity.  An in-flight file descriptor is one that has been sent us-
              ing 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  de-
              scriptor with sendmsg(2) and then closing the file descriptor so
              that it was not accounted  against  the  RLIMIT_NOFILE  resource

       Other  errors  can  be  generated by the generic socket layer or by the
       filesystem while generating a filesystem socket object.  See the appro-
       priate 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 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

       When  binding  a  socket to an address, Linux is one of the implementa-
       tions that appends a null terminator if none is supplied  in  sun_path.
       In  most  cases  this  is unproblematic: when the socket address is re-
       trieved, it will be one byte longer than that supplied when the  socket
       was bound.  However, there is one case where confusing behavior can re-
       sult: 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  ad-
       dress  (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

       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  imple-
       mentations, there is no null terminator in sun_path.

       Applications that retrieve socket addresses can (portably) code to han-
       dle the possibility that there is no null terminator in sun_path by re-
       specting 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 allo-
       cating 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 ap-
       plications 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 back-
       ground  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),  capabili-
       ties(7), credentials(7), socket(7), udp(7)

       This  page  is  part of release 4.16 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

Linux                             2018-04-30                           UNIX(7)

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