test.c

#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>

void f(double * b, double * e)
{
    while(b != e) {
        *b += 2;
        ++b;
    }
}

double h(double * data)
{
    double acc_rcv;
    MPI_Reduce(data, &acc_rcv, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
    return acc_rcv;
}

int main(int argc, char ** argv)
{
    MPI_Init(&argc, &argv);
    int size = atoi(argv[1]);
    int ranks, rank_id;
    MPI_Comm_size(MPI_COMM_WORLD, &ranks);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank_id);

    size /= ranks;
    int start = size * rank_id;
    double * data = malloc(sizeof(double)*size);
    f(data, data + size);
    double acc_rcv = h(data);
    if(rank_id == 0)
        printf("%f\n", acc_rcv);
    free(data);
    MPI_Finalize();
    return 0;
}