Signal: Segmentation fault (11) in MPI C++

Question

I have a code, which counts the average value of integers in MPI:

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

// Average method
int compute_avg(int *array, int num_elements) {
    int sum = 0;
    int i;
    for (i = 0; i < num_elements; i++) {
    sum += array[i];
    }
    return sum / num_elements;
}

int main(int argc, char** argv) {

    if (argc != 2) {
    fprintf(stderr, "Usage: avg num_elements_per_proc\n");
    exit(1);
    }
    int num_elements_per_proc = atoi(argv[1]);

    MPI_Init(NULL, NULL);
    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);

// Create array with integers
    int *nums = NULL;
    if (world_rank == 0) {

        for (int i =0; i<5; i++){
            nums[i] = i;
        }
    }


// Subtable from any processes
    int *sub_nums = (int *)malloc(sizeof(int) * num_elements_per_proc);
    assert(sub_nums != NULL);


// distribution numbers for all processes
    MPI_Scatter(nums, num_elements_per_proc, MPI_INT, sub_nums,
    num_elements_per_proc, MPI_INT, 0, MPI_COMM_WORLD);

// Count avg subtable
    int sub_avg = compute_avg(sub_nums, num_elements_per_proc);

// Collectiong averages
    int *sub_avgs = NULL;
    if (world_rank == 0) {
    sub_avgs = (int *)malloc(sizeof(int) * world_size);
    assert(sub_avgs != NULL);
    }
    MPI_Gather(&sub_avg, 1, MPI_INT, sub_avgs, 1, MPI_INT, 0, MPI_COMM_WORLD);


// Calculates the overall average
    if (world_rank == 0) {
    int avg = compute_avg(sub_avgs, world_size);
    printf("Avg of all elements is %d\n", avg);
// Obliczenie średniej na danych oryginalnych i wyświetlenie.
    int original_data_avg =
    compute_avg(nums, num_elements_per_proc * world_size);
    printf("Avg computed across original data is %d\n", original_data_avg);
}

// free memory
    if (world_rank == 0) {
    free(nums);
    free(sub_avgs);
    }
    free(sub_nums);
    MPI_Barrier(MPI_COMM_WORLD);
    MPI_Finalize();

return 0;
}

When i try to run this (mpirun -c 4 avg 4), i`m getting the error list:

[mangeke-mpi-2431940:03372] * Process received signal *

[mangeke-mpi-2431940:03372] Signal: Segmentation fault (11)

[mangeke-mpi-2431940:03372] Signal code: Address not mapped (1)

[mangeke-mpi-2431940:03372] Failing at address: (nil)

[mangeke-mpi-2431940:03372] * End of error message *

How i can fix this problem?

Anyway, this will segfault if world_rank = 0 because you will try to assign values to a null pointer — loginn, Jan 30 '16 at 23:49
`nums` is initialized to `NULL` in all ranks, hence any assignment to its elements in the loop that follows immediately will fail with a segmentation fault of all ranks. — Hristo Iliev, Feb 01 '16 at 16:38

score 1 · Accepted Answer · answered Feb 03 '16 at 08:52

As Hristo comments, the nums is initialized to NULL. If you explore the core file generated with the debugger, it raises the following statement

Core was generated by `./m 4'. Program terminated with signal SIGSEGV, Segmentation fault. #0 0x0000000000408809 in main (argc=2, argv=0x7ffd4fc87e68) at m.cxx:36 36 nums[i] = i;

if you change the following code as shown below you'll get to make it run without segfaulting.

    ....

    // Create array with integers
    int nums[num_elements_per_proc]; // <<-- change here
    if (world_rank == 0) {

        for (int i =0; i<5; i++){
            nums[i] = i;
        }
    }

    ....
    // free memory
    if (world_rank == 0) {
    // free(nums);                   // <<-- change here, free not needed
    free(sub_avgs);
    }

Signal: Segmentation fault (11) in MPI C++

1 Answers1

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