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

Answer 1

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);
}