NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       unix - sockets for local interprocess communication

SYNOPSIS         top

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

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

DESCRIPTION         top

       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.  The
          abstract socket namespace is a nonportable Linux extension.

   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.)

   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
       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.

       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).

       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
              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.

   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         top

       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
              SIGPIPE is sent as well.  This can be avoided by passing the
              MSG_NOSIGNAL flag to sendmsg(2) or recvmsg(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
       appropriate manual pages for more information.

VERSIONS         top

       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         top

       In the Linux implementation, sockets which are visible in the
       filesystem honor the permissions of the directory they are in.  Their
       owner, group, and permissions can be changed.  Creation of a new
       socket will fail if the process does not have write and search
       (execute) permission on the directory the socket is created in.
       Connecting to the socket object requires read/write permission.  This
       behavior differs from many BSD-derived systems which ignore
       permissions for UNIX domain sockets.  Portable programs should not
       rely on this feature for security.

       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         top

       When binding a socket to an address, Linux is one of the
       implementations that appends a null terminator if none is supplied in
       sun_path.  In most cases this is unproblematic: when the socket
       address is retrieved, it will be one byte longer than that supplied
       when the socket was bound.  However, there is one case where
       confusing behavior can result: if 108 non-null bytes are supplied
       when a socket is bound, then the addition of the null terminator
       takes the length of the pathname beyond sizeof(sun_path).
       Consequently, when retrieving the socket address (for example, via
       accept(2)), if the input addrlen argument for the retrieving call is
       specified as sizeof(struct sockaddr_un), then the returned address
       structure won't have a null terminator in sun_path.

       In addition, some implementations don't require a null terminator
       when binding a socket (the addrlen argument is used to determine the
       length of sun_path) and when the socket address is retrieved on these
       implementations, there is no null terminator in sun_path.

       Applications that retrieve socket addresses can (portably) code to
       handle the possibility that there is no null terminator in sun_path
       by respecting the fact that the number of valid bytes in the pathname
       is:

           strnlen(addr.sun_path, addrlen - offsetof(sockaddr_un, sun_path))

       Alternatively, an application can retrieve the socket address by
       allocating a buffer of size sizeof(struct sockaddr_un)+1 that is
       zeroed out before the retrieval.  The retrieving call can specify
       addrlen as sizeof(struct sockaddr_un), and the extra zero byte
       ensures that there will be a null terminator for the string returned
       in sun_path:

          void *addrp;

          addrlen = sizeof(struct sockaddr_un);
          addrp = malloc(addrlen + 1);
          if (addrp == NULL)
              /* Handle error */ ;
          memset(addrp, 0, addrlen + 1);

          if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == -1)
              /* handle error */ ;

          printf("sun_path = %s\n", ((struct sockaddr_un *) addrp)->sun_path);

       This sort of messiness can be avoided if it is guaranteed that the
       applications that create pathname sockets follow the rules outlined
       above under Pathname sockets.

EXAMPLE         top

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

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

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

   Program source
       /*
        * File connection.h
        */

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

       /*
        * File server.c
        */

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

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

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

           unlink(SOCKET_NAME);

           /* Create local socket. */

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

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

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

           /* Bind socket to socket name. */

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

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

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

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

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

           for (;;) {

               /* Wait for incoming connection. */

               data_socket = accept(connection_socket, NULL, NULL);
               if (ret == -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         top

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

COLOPHON         top

       This page is part of release 4.06 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                            2016-03-15                          UNIX(7)