Combines values from all processes and distribute the result back to all processes
int MPI_Allreduce ( sendbuf, recvbuf, count, datatype, op, comm )
- sendbuf - starting address of send buffer (choice)
- count - number of elements in send buffer (integer)
- datatype - data type of elements of send buffer (handle)
- op - operation (handle)
- comm - communicator (handle)
- recvbuf - starting address of receive buffer (choice)
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
- MPI_ERR_BUFFER - Invalid buffer pointer. Usually a null buffer where
one is not valid.
- MPI_ERR_COUNT - Invalid count argument. Count arguments must be
non-negative; a count of zero is often valid.
MPI_Datatype (see MPI_Type_commit).
- MPI_ERR_TYPE - Invalid datatype argument. May be an uncommitted
- 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_COMM - Invalid communicator. A common error is to use a null
communicator in a call (not even allowed in MPI_Comm_rank).