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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 communicate between processes on the same
       machine efficiently.  Traditionally, UNIX domain sockets can be either unnamed, or bound to a  filesystem
       pathname  (marked  as  being  of  type  socket).   Linux  also  supports  an  abstract namespace which is
       independent of the filesystem.

       Valid socket types in the UNIX domain are: SOCK_STREAM, for a stream-oriented socket; SOCK_DGRAM,  for  a
       datagram-oriented  socket that preserves message boundaries (as on most UNIX implementations, UNIX domain
       datagram  sockets  are  always  reliable  and  don't  reorder  datagrams);  and   (since   Linux   2.6.4)
       SOCK_SEQPACKET,  for a sequenced-packet socket that is connection-oriented, preserves message boundaries,
       and delivers messages in the order that they were sent.

       UNIX domain sockets support passing file descriptors or process  credentials  to  other  processes  using
       ancillary data.

   Address format
       A UNIX domain socket address is represented in the following structure:

           struct sockaddr_un {
               sa_family_t sun_family;               /* AF_UNIX */
               char        sun_path[108];            /* pathname */
           };

       The  sun_family  field  always contains AF_UNIX.  On Linux sun_path is 108 bytes in size; see also NOTES,
       below.

       Various systems calls (for example, bind(2), connect(2), and sendto(2)) take a  sockaddr_un  argument  as
       input.  Some other system calls (for example, getsockname(2), getpeername(2), recvfrom(2), and accept(2))
       return an argument of this type.

       Three types of address are distinguished in the sockaddr_un structure:

       *  pathname: a UNIX domain socket can be bound to a null-terminated filesystem  pathname  using  bind(2).
          When the address of a pathname socket is returned (by one of the system calls noted above), its length
          is

              offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1

          and sun_path contains the null-terminated  pathname.   (On  Linux,  the  above  offsetof()  expression
          equates  to the same value as sizeof(sa_family_t), but some other implementations include other fields
          before sun_path, so the offsetof()  expression  more  portably  describes  the  size  of  the  address
          structure.)

          For further details of pathname sockets, see below.

       *  unnamed:  A  stream socket that has not been bound to a pathname using bind(2) has no name.  Likewise,
          the two sockets created by socketpair(2) are unnamed.  When  the  address  of  an  unnamed  socket  is
          returned, its length is sizeof(sa_family_t), and sun_path should not be inspected.

       *  abstract:  an  abstract  socket  address  is  distinguished  (from a pathname socket) by the fact that
          sun_path[0] is a null byte ('\0').  The socket's address in this namespace is given by the  additional
          bytes  in  sun_path that are covered by the specified length of the address structure.  (Null bytes in
          the name have no special significance.)  The name has no connection with filesystem  pathnames.   When
          the   address   of   an   abstract   socket   is  returned,  the  returned  addrlen  is  greater  than
          sizeof(sa_family_t) (i.e., greater than 2), and the name of the  socket  is  contained  in  the  first
          (addrlen - sizeof(sa_family_t)) bytes of sun_path.

   Pathname sockets
       When  binding  a socket to a pathname, a few rules should be observed for maximum portability and ease of
       coding:

       *  The pathname in sun_path should be null-terminated.

       *  The length of the pathname, including the terminating  null  byte,  should  not  exceed  the  size  of
          sun_path.

       *  The  addrlen  argument  that  describes  the enclosing sockaddr_un structure should have a value of at
          least:

              offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1

          or, more simply, addrlen can be specified as sizeof(struct sockaddr_un).

       There is some variation in how implementations handle UNIX domain socket addresses that do not follow the
       above rules.  For example, some (but not all) implementations append a null terminator if none is present
       in the supplied sun_path.

       When coding portable applications, keep in mind that some implementations have sun_path as  short  as  92
       bytes.

       Various  system  calls  (accept(2),  recvfrom(2),  getsockname(2),  getpeername(2)) return socket address
       structures.  When applied to UNIX domain sockets, the value-result addrlen argument supplied to the  call
       should  be  initialized  as  above.   Upon return, the argument is set to indicate the actual size of the
       address structure.  The caller should check the value returned in this  argument:  if  the  output  value
       exceeds  the input value, then there is no guarantee that a null terminator is present in sun_path.  (See
       BUGS.)

   Pathname socket ownership and permissions
       In the Linux implementation, pathname sockets honor  the  permissions  of  the  directory  they  are  in.
       Creation  of a new socket fails if the process does not have write and search (execute) permission on the
       directory in which the socket is created.

       On Linux, connecting to a stream socket object requires  write  permission  on  that  socket;  sending  a
       datagram to a datagram socket likewise requires write permission on that socket.  POSIX does not make any
       statement about the effect of the permissions on a socket file, and on some systems (e.g.,  older  BSDs),
       the socket permissions are ignored.  Portable programs should not rely on this feature for security.

       When  creating a new socket, the owner and group of the socket file are set according to the usual rules.
       The socket file has all permissions enabled, other  than  those  that  are  turned  off  by  the  process
       umask(2).

       The owner, group, and permissions of a pathname socket can be changed (using chown(2) and 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.

       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.

       Before Linux 3.4, the use of MSG_TRUNC in the flags argument of recv(2) was not supported by UNIX  domain
       sockets.

       The  SO_SNDBUF  socket  option does have an effect for UNIX domain sockets, but the SO_RCVBUF option does
       not.  For datagram sockets, the SO_SNDBUF value imposes an upper limit on the size of outgoing datagrams.
       This limit is calculated as the doubled (see socket(7)) option value less 32 bytes used for overhead.

   Ancillary messages
       Ancillary  data  is  sent  and  received  using  sendmsg(2)  and  recvmsg(2).  For historical reasons the
       ancillary message types listed below are specified with a SOL_SOCKET type even though  they  are  AF_UNIX
       specific.   To  send  them set the cmsg_level field of the struct cmsghdr to SOL_SOCKET and the cmsg_type
       field to the type.  For more information see 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 real user ID, effective user ID, or
              saved set-user-ID (unless it has CAP_SETUID), and its real group ID, effective group ID, or  saved
              set-group-ID (unless it has CAP_SETGID).  To receive a struct ucred message the SO_PASSCRED option
              must be enabled on the socket.

   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 SIGPIPE 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.

       ETOOMANYREFS
              This  error  can occur for sendmsg(2) when sending a file descriptor as ancillary data over a UNIX
              domain socket (see the description of SCM_RIGHTS, above).  It occurs if the number of  "in-flight"
              file  descriptors  exceeds  the  RLIMIT_NOFILE  resource  limit  and  the caller does not have the
              CAP_SYS_RESOURCE capability.  An in-flight file  descriptor  is  one  that  has  been  sent  using
              sendmsg(2) but has not yet been accepted in the recipient process using recvmsg(2).

              This  error  is  diagnosed since mainline Linux 4.5 (and in some earlier kernel versions where the
              fix has been backported).  In earlier kernel versions, it  was  possible  to  place  an  unlimited
              number  of  file  descriptors  in flight, by sending each file descriptor with sendmsg(2) and then
              closing the file descriptor so that it was not accounted against the RLIMIT_NOFILE resource limit.

       Other errors can be generated by the generic socket  layer  or  by  the  filesystem  while  generating  a
       filesystem socket object.  See the appropriate manual pages for more 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 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

       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 (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),  capabilities(7),  credentials(7),  socket(7),
       udp(7)

COLOPHON

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.