MPI_Reduce(3)                         MPI                        MPI_Reduce(3)



NAME
       MPI_Reduce -  Reduces values on all processes to a single value

SYNOPSIS
       #include "mpi.h"
       int MPI_Reduce ( void *sendbuf, void *recvbuf, int count,
                       MPI_Datatype datatype, MPI_Op op, int root, MPI_Comm comm )

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


OUTPUT PARAMETER
       recvbuf
              - address of receive buffer (choice, significant only at root )


ALGORITHM
       This implementation currently uses a simple tree algorithm.


NOTES FOR FORTRAN
       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
       Fortran.


NOTES ON COLLECTIVE OPERATIONS
       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, for example,
       MPI_ERRORS_RETURN , then no error may be indicated.

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


ERRORS
       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.  Before the value is returned, the current MPI error
       handler is called.  By default, this error handler aborts the MPI  job.
       The  error  handler may be changed with MPI_Errhandler_set ; the prede-
       fined 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.

       MPI_SUCCESS
              - No error; MPI routine completed successfully.
       MPI_ERR_COMM
              - Invalid communicator.  A common error is to use a null  commu-
              nicator in a call (not even allowed in MPI_Comm_rank ).
       MPI_ERR_COUNT
              - Invalid count argument.  Count arguments must be non-negative;
              a count of zero is often valid.
       MPI_ERR_TYPE
              - Invalid datatype argument.  May be an uncommitted MPI_Datatype
              (see MPI_Type_commit ).
       MPI_ERR_BUFFER
              -  Invalid  buffer  pointer.  Usually a null buffer where one is
              not valid.
       MPI_ERR_BUFFER
              -  This error class is assoicated with an error code that  indi-
              cates  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 For-
              tran standard, and rather than have a separate case  for  C  and
              Fortran, the MPI Forum adopted the more restrictive requirements
              of Fortran).

LOCATION
       reduce.c



                                   2/19/2002                     MPI_Reduce(3)