Bcast function with array

Question

Why the Bcast function works with my array incorrectly? I did according to the following example (only there is not an array, but a number).

#include <iostream>
#include <mpi.h>
#include <vector>

void test(int rnk) {
    int n = 5;

    vector<int> array;
    array.reserve(n);

    if (rnk == 0) {
        for (int i = 0; i < n; i++) {
            array[i] = i;
        }
    }

    cout << "Process " << rnk << " before Bcast:" << endl;
    for (auto x: array) {
        cout << x << " ";
    }
    cout << endl;

    MPI_Bcast(&array[0], n, MPI_INT, 0, MPI_COMM_WORLD);

    cout << "Process " << rnk << " after Bcast:" << endl;
    for (auto x: array) {
        cout << x << " ";
    }
    cout << endl;
}


int main(int argc, char **argv) {
    int num, rank;

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &num);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    test(rank);

    MPI_Finalize();
    return 0;
}

Result - empty rows without data:

Process 2 before Bcast:

Process 0 before Bcast:

Process 0 after Bcast:

Process 2 after Bcast:

Process 1 before Bcast:

Process 1 after Bcast:

Process 3 before Bcast:

Process 3 after Bcast:

And what if not one array, but for example 3? Will there be as much MPI_Bcast? Need MPI_Barrier then?

Make sure `n` is valid on all ranks, and try using `resize()` instead of `reserve()`. — Gilles Gouaillardet, May 25 '20 at 22:47
@GillesGouaillardet, I checked and all the ranks see "n"; replaced reserve with resize, now instead of empty lines 25 zeros are displayed (if n = 5) — Samuel, May 25 '20 at 23:12
Please edit your question with a [MCVE] and I will have a look. — Gilles Gouaillardet, May 25 '20 at 23:18
In the edited question (`#include` are missing btw), replacing `reserve()` with `resize()` does the trick. In the previous version, only use `resize()` on non root rank. — Gilles Gouaillardet, May 25 '20 at 23:57
@GillesGouaillardet, Here the replacement helped, but in the previous version did not help — Samuel, May 26 '20 at 00:03
in the previous version, only `resize()` on **non** root ranks. If it does not work, edit your question with a [MCVE] — Gilles Gouaillardet, May 26 '20 at 00:18
@GillesGouaillardet, why are zeros displayed in the previous version? — Samuel, May 26 '20 at 00:18
@GillesGouaillardet, `reserve` for root rank and `resize` for non-root ranks helped me. Why is that? — Samuel, May 26 '20 at 00:27
Is your problem fixed? If not, read carefully all the output (you only need 2 tasks). — Gilles Gouaillardet, May 26 '20 at 00:31
@GillesGouaillardet, The problem is fixed, but I wonder why it arose. Also answer the last question in the topic? — Samuel, May 26 '20 at 00:35
Run your program with two (or even one) MPI task and carefully read the output, you should notice something fishy that will answer your question. — Gilles Gouaillardet, May 26 '20 at 00:57
@GillesGouaillardet, to be honest, I don’t quite understand what you mean by MPI task and what output? — Samuel, May 26 '20 at 03:03
@GillesGouaillardet, task is just a process, ok, I thought it was something complicated — Samuel, May 26 '20 at 10:34
@GillesGouaillardet, ok, thank you for your help. If you write an answer, I will mark it — Samuel, May 26 '20 at 10:37

Bcast function with array

0 Answers0