Segmentation fault on big array

Question

I was using quicksort to see how fast it could order big arrays. I ordered one million no problem.. 2 million no problem. 3 million gives a segfault. Some testing reveals it crashes somewhere between 2 million and 2.1 million, and GDB reveals the crash is from the "fill" function.

This is the code:

int partition(int v[], int N){

  int i, j;
  int x = v[N-1];

  for(i = 0, j = 0; i < N - 1; i++){
    if (v[i] < x) swap(v, j++, i);
  }
  swap(v, j, N - 1);
  return j;
}


void quickSort(int v[], int N){
  int p;

  if (N > 1){
    p = partition(v, N);
    quickSort(v, p);
    quickSort(v + p + 1, N - p - 1);
  }
}


void fill(int v[], int N){
  srand(clock());

  for(int i = 0; i < N; i++){
    v[i] = rand();
  }
}


int main() {

  int size = 3000000;
  int v[size];

  fill(v, size);
  printf("ready\n\n");
  quickSort(v, size);

  for(int i = 0; i < size; i++)
    printf("%d ", v[i]);

  printf("\n\nDone\n\n");

  return 0;
}

Is there any particular reason the code starts crashing at that particular number? Thank you for any answers

Answer 1

With int size = 3000000;int v[size] , you are creating a local variable on the "stack". The stack is - compared to the heap - rather limited; so 3000000 may exceed the available stack size, whereas 3000 , for example, might not.

To create v on the heap, write

int *v = malloc(size);

and afterwards, check if you could allocate the desired space:

if (!v) {
   printf("not enough space.");
   return 1;
}
....