Reduces values on all processes to a single value
int MPI_Reduce ( void *sendbuf, void *recvbuf, int count,
MPI_Datatype datatype, MPI_Op op, int root, MPI_Comm comm )
|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)
- address of receive buffer (choice,
significant only at root)
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
All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER
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.
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 predefined error handler
MPI_ERRORS_RETURN may be used to cause error values to be returned.
Note that MPI does not guarentee that an MPI program can continue past
- 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 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).