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       MPI_Reduce -  Reduces values on all processes to a single value


       #include <mpi.h>
       int MPI_Reduce(void *sbuf, void* rbuf, int count,
                      MPI_Datatype dtype, MPI_Op op, int root,
                      MPI_Comm comm)


       sbuf   - address of send buffer (choice)
       count  - number of elements in send buffer (integer)
       dtype  - data type of elements of send buffer (handle)
       op     - reduce operation (handle)
       root   - rank of root process (integer)
       comm   - communicator (handle)


       rbuf   - address of receive buffer (choice, significant only at root )


       This  function has had the IMPI extensions implemented.  It is legal to
       call this function on IMPI communicators.


       If there are 4 or less ranks involved, the root  loops  over  receiving
       from each rank, and then performs the final reduction locally.

       If there are more than 4 ranks involved, a tree-based algorithm is used
       to collate the reduced data at the root (the data is  reduced  at  each
       parent  in  the  tree  so  that  the  reduction operations are actaully


       All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK )  have
       an  additional  argument ierr at the end of the argument list.  ierr is
       an integer and has the same meaning as the return value of the  routine
       in  C.   In Fortran, MPI routines are subroutines, and are invoked with
       the call statement.

       All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in


       The  reduction functions ( MPI_Op ) do not return an error value.  As a
       result, if the functions detect an error, all they  can  do  is  either
       call  MPI_Abort  or silently skip the problem.  Thus, if you change the
       error  handler  from  MPI_ERRORS_ARE_FATAL  to  something  else  (e.g.,
       MPI_ERRORS_RETURN ), then no error may be indicated.

       The  reason for this is the performance problems that arise in ensuring
       that all collective routines return the same error value.


       If an error occurs in an MPI function, the current MPI error handler is
       called  to  handle  it.   By default, this error handler aborts the MPI
       job.  The error handler may be changed with  MPI_Errhandler_set  ;  the
       predefined  error  handler MPI_ERRORS_RETURN may be used to cause error
       values to be returned (in C and Fortran; this  error  handler  is  less
       useful  in  with  the  C++  MPI bindings.  The predefined error handler
       MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the  error  value
       needs  to  be recovered).  Note that MPI does not guarantee that an MPI
       program can continue past an error.

       All MPI routines (except MPI_Wtime and  MPI_Wtick  )  return  an  error
       value;  C routines as the value of the function and Fortran routines in
       the last argument.  The C++  bindings  for  MPI  do  not  return  error
       values;  instead,  error values are communicated by throwing exceptions
       of type MPI::Exception (but  not  by  default).   Exceptions  are  only
       thrown if the error value is not MPI::SUCCESS .

       Note  that  if  the MPI::ERRORS_RETURN handler is set in C++, while MPI
       functions will return upon an error, there will be no  way  to  recover
       what the actual error value was.
              - No error; MPI routine completed successfully.
              -  Invalid  communicator.   A  common  error  is  to  use a null
              communicator in a call (not even allowed in MPI_Comm_rank ).
              - A collective implementation was not able to be located at run-
              time for this communicator.
              - Invalid count argument.  Count arguments must be non-negative;
              a count of zero is often valid.
              - Invalid datatype argument.  May be an uncommitted MPI_Datatype
              (see MPI_Type_commit ).
              -  Invalid  buffer  pointer.  Usually a null buffer where one is
              not valid.
              - This error class  is  associcated  with  an  error  code  that
              indicates  that  two buffer arguments are aliased ; that is, the
              describe overlapping storage (often  the  exact  same  storage).
              This  is  prohibited  in  MPI  (because  it is prohibited by the
              Fortran standard, and rather than have a separate case for C and
              Fortran, the MPI Forum adopted the more restrictive requirements
              of Fortran).
              - Invalid root.  The root must be specified as  a  rank  in  the
              communicator.   Ranks  must  be between zero and the size of the
              communicator minus one.


       For more information, please see the official MPI Forum web site, which
       contains  the  text  of  both  the  MPI-1  and  MPI-2 standards.  These
       documents contain detailed information about each MPI function (most of
       which is not duplicated in these man pages).


       The  LAM Team would like the thank the MPICH Team for the handy program
       to       generate        man        pages        ("doctext"        from     ),    the    initial
       formatting, and some initial text for most of the MPI-1 man pages.