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MPI_Reduce(3)				     LAM/MPI				    MPI_Reduce(3)

       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 distributed).

       All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have an additional argu-
       ment 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 Fortran.

       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  excep-
       tions  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 ).
	      - Invalid communicator.  LAM/MPI does  not  yet  support	invoking  collectives  on
	      - A communicator that contains some non-local IMPI procs was used for some function
	      which has not yet had the IMPI extensions implemented yet.  For example, most  col-
	      lectives on IMPI communicators have not been implemented yet.
	      - 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_com-
	      mit ).
	      - 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 ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz ), the initial for-
       matting, and some initial text for most of the MPI-1 man pages.


LAM/MPI 6.5.8				    11/10/2002				    MPI_Reduce(3)
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