Function mpi::ffi::PMPI_Allreduce

pub unsafe extern fn PMPI_Allreduce(sendbuf: *const c_void,
                                    recvbuf: *mut c_void,
                                    count: c_int,
                                    datatype: MPI_Datatype,
                                    op: MPI_Op,
                                    comm: MPI_Comm)
                                    -> c_int