unix(7)



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

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 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
       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  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
       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  socket-
          pair(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 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-
          addr_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 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 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  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.

       SO_PASSCRED
              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 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.

       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 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).

       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.  The passed file descriptors behave as though  they
              have been created with dup(2).

       SCM_CREDENTIALS
              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
              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 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 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.

   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 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).

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

       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 SIG-
              PIPE 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.

       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.

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, 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.

BUGS
       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
       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  path-
       name 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 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
       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  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
       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 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"

       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), capabili-
       ties(7), credentials(7), socket(7), udp(7)

COLOPHON
       This page is part of release 4.08 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2016-07-17                           UNIX(7)

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