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       MPI_Reduce_scatter -  Combines values and scatters the results


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


       sbuf   - starting address of send buffer (choice)
              -  integer  array  specifying  the  number of elements in result
              distributed to each process.  Array must  be  identical  on  all
              calling processes.
       dtype  - data type of elements of input buffer (handle)
       op     - operation (handle)
       comm   - communicator (handle)


       rbuf   - starting address of receive buffer (choice)


       LAM/MPI  does  not yet support invoking this function on a communicator
       that contains ranks that are non-local IMPI procs.


       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.
              -  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  collectives  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_commit ).
              - Invalid buffer pointer.  Usually a null buffer  where  one  is
              not valid.
              -  Invalid  operation.  MPI operations (objects of type MPI_Op )
              must either be one of the predefined operations (e.g., MPI_SUM )
              or  created  with  MPI_Op_create  .   Additionally, only certain
              datatypes are alloed  with  given  predefined  operations.   See
              MPI-1, section 4.9.2.
              -  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).


       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.