incorrect_mpi_reduce.c

#include "mpi.h"
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>

/*
 * Each process computes random number
 * Root process finds and prints the maximum generated value
 * Have each process print its value
 *
 * Let each process scale its value by this maximum
 */

int main(int argc, char*argv[]) {
    MPI_Init(&argc, &argv);

    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    int root = 0;
    double local_max_send;
    double global_max_recv;

    MPI_Reduce(&local_max_send, &global_max_recv, 1,
            MPI_DOUBLE, MPI_MAX, root, MPI_COMM_WORLD);
    srand((int) (rank * (double) RAND_MAX / size));
    double randomfraction = (rand() / (double) RAND_MAX);
    printf("Process %d generated number %lf\n", rank, randomfraction);

    if (rank == root) {
        MPI_Reduce(&local_max_send, &global_max_recv, 1,
                MPI_DOUBLE, MPI_MAX, root, MPI_COMM_WORLD);
        printf("Found the max! %lf\n", global_max_recv);
    }

    MPI_Barrier(MPI_COMM_WORLD);
    MPI_Finalize();
}