MPI_Allreduce(3)

NAME
     MPI_Allreduce - Combines values from all processes and distributes the
     result back to all processes

SYNOPSIS
     C:

          #include <mpi.h>

          int MPI_Allreduce ( sendbuf, recvbuf, count,
          datatype, op, comm )
          void             *sendbuf;
          void             *recvbuf;
          int               count;
          MPI_Datatype      datatype;
          MPI_Op            op;
          MPI_Comm          comm;


     C++:

          #include <mpi.h>

          void Intracomm::Allreduce(
          const void*       sendbuf,
          void*             recvbuf,
          int               count,
          const Datatype&   datatype,
          const Op&         op) const


     Fortran:

          INCLUDE "mpif.h" (or USE MPI)

          <type> SENDBUF(*), RECVBUF(*)
          INTEGER count, datatype, op,
          comm, ierror

          CALL MPI_ALLREDUCE(sendbuf, recvbuf, count,
          datatype, op, comm, ierror)


STANDARDS
     This release implements the MPI 1.2 standard, as documented by the MPI
     Forum in the spring 1997 release of MPI:  A Message Passing Interface
     Standard.

     The MPI_IN_PLACE option as defined in the MPI 2.0 standard is also
     supported in this implementation.
DESCRIPTION
     The MPI_Allreduce routine combines values from all processes and
     distributes the result back to all processes.  This routine accepts the
     following parameters:

     sendbuf   Specifies the starting address of the send buffer (choice).
               You may also pass in the argument MPI_IN_PLACE for the value of
               sendbuf at all processes.  In such cases, the input data is
               taken at each process from the receive buffer, where it will be
               replaced by the output data.  Please note that using the
               MPI_IN_PLACE option in Fortran causes the receive buffer to
               become a send-and-receive buffer, so a binding that includes
               INTENT must be marked as INOUT, not OUT.

     count     Specifies the number of elements in the send buffer (integer)

     datatype  Specifies the data type of elements in the send buffer (handle)

     op        Specifies the operation (handle)

     comm      Specifies the communicator (handle)

     recvbuf   Returns the starting address of the receive buffer (choice)

     ierror    Specifies the return code value for successful completion,
               which is in MPI_SUCCESS.  MPI_SUCCESS is defined in the mpif.h
               file.