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

       unix,  PF_UNIX,  AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local inter-
       process communication

       #include <&lt;sys/socket.h>&gt;
       #include <&lt;sys/un.h>&gt;

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

       The PF_UNIX (also known as PF_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

       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
          in  sun_path  define  the  "name" of the socket.  (Null bytes in the
          name have no special significance.)  The name has no connection with
          file  system  pathnames.   The socket's address in this namespace is
          given by the rest of the bytes in sun_path.  When the address of  an
          abstract  socket  is returned by getsockname(2), getpeername(2), and
          accept(2), its length is sizeof(struct  sockaddr_un),  and  sun_path
          contains the abstract name.  The abstract socket namespace is a non-
          portable Linux extension.

   Socket Options
       For historical reasons  these  socket  options  are  specified  with  a
       SOL_SOCKET type even though they are PF_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
              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.

   (Un)supported Features
       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 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 PF_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.

                  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 */

              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

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

              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.

              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.

              connect(2)  called  on  an  already connected socket or a target
              address was specified on a connected socket.

       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 sendmsg(2) or recvmsg(2).

              Passed protocol is not PF_UNIX.

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

              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.

       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

       In the Linux implementation, sockets which are visible in the file sys-
       tem  honor  the permissions of the directory they are in.  Their owner,
       group and their permissions can be changed.  Creation of a  new  socket
       will  fail if the process does not have write and search (execute) per-
       mission 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  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

       See bind(2).

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

       This  page  is  part of release 3.05 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                             2008-06-17                           UNIX(7)