NAME

       MPI_Intercomm_create - Creates an intercommunicator from two intracommunicators.

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Intercomm_create(MPI_Comm local_comm, int local_leader,
            MPI_Comm peer_comm, int remote_leader, int tag, MPI_Comm *newintercomm)

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_INTERCOMM_CREATE(LOCAL_COMM, LOCAL_LEADER, PEER_COMM,
                 REMOTE_LEADER, TAG, NEWINTERCOMM, IERROR)
            INTEGER   LOCAL_COMM, LOCAL_LEADER, PEER_COMM, REMOTE_LEADER
            INTEGER   TAG, NEWINTERCOMM, IERROR

C++ Syntax

       #include <mpi.h>
       Intercomm Intracomm::Create_intercomm(int local_leader, const
            Comm& peer_comm, int remote_leader, int tag) const

INPUT PARAMETERS

       local_comm
                 The  communicator  containing the process that initiates the inter-communication
                 (handle).

       local_leader
                 Rank of local group leader in local_comm (integer).

       peer_comm "Peer" communicator; significant only at the local_leader (handle).

       remote_leader
                 Rank of remote group leader in peer_comm; significant only at  the  local_leader
                 (integer).

       tag       Message tag used to identify new intercommunicator (integer).

OUTPUT PARAMETERS

       newintercomm
                 Created intercommunicator (handle).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       This  call  creates an intercommunicator. It is collective over the union of the local and
       remote groups. Processes should provide identical local_comm  and  local_leader  arguments
       within each group. Wildcards are not permitted for remote_leader, local_leader, and tag.

       This  call uses point-to-point communication with communicator peer_comm, and with tag tag
       between the leaders. Thus, care must be taken that there be no  pending  communication  on
       peer_comm that could interfere with this communication.

       If  multiple  MPI_Intercomm_creates  are  being made, they should use different tags (more
       precisely, they should ensure that the local and remote leaders are using  different  tags
       for each MPI_intercomm_create).

NOTES

       We  recommend  using a dedicated peer communicator, such as a duplicate of MPI_COMM_WORLD,
       to avoid trouble with peer communicators.

       The  MPI  1.1  Standard  contains  two  mutually   exclusive   comments   on   the   input
       intracommunicators.   One  says  that  their respective groups must be disjoint; the other
       that the leaders can be the same process.  After some discussion by the MPI Forum, it  has
       been decided that the groups must be disjoint.  Note that the reason given for this in the
       standard  is  not  the  reason  for  this  choice;  rather,  the   other   operations   on
       intercommunicators  (like  MPI_Intercomm_merge  )  do not make sense if the groups are not
       disjoint.

ERRORS

       Almost all MPI routines return an error value; C routines as the value of the function and
       Fortran  routines in the last argument. C++ functions do not return errors. If the default
       error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then  on  error  the  C++  exception
       mechanism will be used to throw an MPI:Exception object.

       Before  the  error value is returned, the current MPI error handler is called. By default,
       this error handler aborts the MPI job, except for I/O function errors. The  error  handler
       may    be   changed   with   MPI_Comm_set_errhandler;   the   predefined   error   handler
       MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not
       guarantee that an MPI program can continue past an error.

SEE ALSO

       MPI_Intercomm_merge
       MPI_Comm_free
       MPI_Comm_remote_group
       MPI_Comm_remote_size