MPI_Reduce - Reduces values on all processes to a single value
int MPI_Reduce(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype,
MPI_Op op, int root, MPI_Comm comm)
- - address of send buffer (choice)
- - number of elements in send buffer (integer)
- - data type of elements of send buffer (handle)
- - reduce operation (handle)
- - rank of root process (integer)
- - communicator (handle)
- - address of receive buffer (choice, significant only at root )
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.
All MPI routines in Fortran (except for MPI_WTIME
have an additional argument ierr
at the end of the argument list.
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
All MPI objects (e.g., MPI_Datatype
) are of type
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
or silently skip the problem. Thus, if you change the error
handler from MPI_ERRORS_ARE_FATAL
to something else, for example,
, 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
) 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 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
guarentee that an MPI program can continue past an error; however,
MPI implementations will attempt to continue whenever possible.
- - 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. Additionally, this error can occur if an
uncommitted MPI_Datatype (see MPI_Type_commit ) is used in a
- - Invalid buffer pointer. Usually a null buffer where one is not
- - 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).