MPI_Reduce_scatter

Combines values and scatters the results

Synopsis



int MPI_Reduce_scatter(const void *sendbuf, void *recvbuf,
                       const int recvcounts[], MPI_Datatype datatype, MPI_Op op,
                       MPI_Comm comm)




int MPI_Reduce_scatter_c(const void *sendbuf, void *recvbuf,
                         const MPI_Count recvcounts[], MPI_Datatype datatype,
                         MPI_Op op, MPI_Comm comm)






Input Parameters




sendbuf  
 starting address of send buffer (choice)


recvcounts  
 non-negative integer array (of length group size) specifying the number of elements of the result distributed to each process. (non-negative integer)


datatype  
 data type of elements of send and receive buffers (handle)


op  
 operation (handle)


comm  
 communicator (handle)





Output Parameters




recvbuf  
 starting address of receive buffer (choice)





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.



Thread and Interrupt Safety



This routine is thread-safe.  This means that this routine may be
safely used by multiple threads without the need for any user-provided
thread locks.  However, the routine is not interrupt safe.  Typically,
this is due to the use of memory allocation routines such as malloc
or other non-MPICH runtime routines that are themselves not interrupt-safe.



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.



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_Comm_set_errhandler (for communicators),
MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for
RMA windows).  The MPI-1 routine MPI_Errhandler_set may be used but
its use is deprecated.  The predefined 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; however, MPI implementations will attempt to continue whenever
possible.





MPI_SUCCESS  
 No error; MPI routine completed successfully.





MPI_ERR_BUFFER  
 Invalid buffer pointer.  Usually a null buffer where
one is not valid.





MPI_ERR_COMM  
 Invalid communicator.  A common error is to use a null
communicator 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_OP  
 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.





MPI_ERR_TYPE  
 Invalid datatype argument.  Additionally, this error can
occur if an uncommitted MPI_Datatype (see MPI_Type_commit) is used
in a communication call.





MPI_ERR_OTHER  
 Other error; use MPI_Error_string to get more information
about this error code.