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

       unix, AF_UNIX, AF_LOCAL - Sockets for local interprocess communication

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

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

       The  AF_UNIX  (also known as AF_LOCAL) socket family is used to communicate 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  imple-
       mentations,  Unix  domain  datagram  sockets  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  pro-
       cesses 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  getsock-
          name(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 socketpair(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

       *  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  sig-
          nificance.)   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 AF_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.

   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 histori-
       cal 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).

              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 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 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.

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

              connect(2)  called with a socket object that isn't listening.  This can hap-
              pen 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 connected.

              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).

              Passed protocol is not AF_UNIX.

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

              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 differ.)

       In the Linux implementation, sockets which are visible in the file system 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) permission on the directory the socket  is  created  in.
       Connecting to the socket object requires read/write permission.  This behavior dif-
       fers from many BSD-derived systems  which  ignore  permissions  for  Unix  sockets.
       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.

       See bind(2).

       recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), creden-
       tials(7), socket(7)

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2008-12-01                           UNIX(7)

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