unix

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  sockets can be either unnamed, or bound to a file system pathname
       (marked as being of type socket).   Linux  also  supports  an  abstract
       namespace which is independent of the file system.

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

       Unix 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:

           #define UNIX_PATH_MAX    108

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

       sun_family always contains AF_UNIX.

       Three types of address are distinguished in this structure:

       *  pathname: a Unix domain socket can be  bound  to  a  null-terminated
          file  system pathname using bind(2).  When the address of the socket
          is returned by getsockname(2), getpeername(2),  and  accept(2),  its
          length  is  sizeof(sa_family_t) + strlen(sun_path) + 1, and sun_path
          contains the null-terminated pathname.

       *  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  by  getsockname(2),  getpeername(2),  and  accept(2),  its
          length is sizeof(sa_family_t), and sun_path should not be inspected.

       *  abstract: an abstract socket address is distinguished  by  the  fact
          that  sun_path[0] is a null byte ('\0').  All of the remaining bytes

       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
              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.

   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

ERRORS
       EADDRINUSE
              Selected local address is already taken or  file  system  socket
              object already exists.

       ECONNREFUSED
              connect(2)  called  with  a  socket object that isn't listening.
              This can happen when the remote socket does  not  exist  or  the
              filename 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 the missing setting
              of AF_UNIX in the sun_type field  of  passed  addresses  or  the
              socket being 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.

       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 sendmsg(2) or recvmsg(2).

       EPROTONOSUPPORT
              Passed protocol is not AF_UNIX.

       EPROTOTYPE
              Remote socket does not match the local socket  type  (SOCK_DGRAM
              vs.  SOCK_STREAM)

       ESOCKTNOSUPPORT
              Unknown socket type.

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

VERSIONS
       from many BSD-derived systems which ignore permissions for  Unix  sock-
       ets.  Portable programs should not rely on this feature for security.

       Binding to a socket with a filename creates a socket in the file system
       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  file
       system 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 non-ancillary  data  in  the  same
       sendmsg(2) or recvmsg(2) call.

       Unix  domain  stream  sockets  do not support the notion of out-of-band
       data.

EXAMPLE
       See bind(2).

SEE ALSO
       recvmsg(2), sendmsg(2), socket(2),  socketpair(2),  cmsg(3),  capabili-
       ties(7), credentials(7), socket(7)

COLOPHON
       This  page  is  part of release 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-12-01                           UNIX(7)