stack and heap demo

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

// initialize array so each index contains its value,
// e.g. [0, 1, 2, 3, ...]
void initialize(unsigned int* arr, unsigned int size) {
  for (unsigned int i = 0; i < size; i++) {
    *(arr + i) = i; // arr[i] = i;
  }
}

void display(unsigned int* arr, unsigned int size) {
  printf("[ ");
  for (unsigned int i = 0; i < size; i++) {
    printf("%d ", *(arr + i)); // printf("%d ", arr[i]);
  }
  printf("]\n");
}

unsigned int* init_array_on_stack() {
  unsigned int arr[10];
  initialize(arr, 10); //memory is still on the stack, address still valid.
  return arr; // bad! do not return address of stack memory!
}

unsigned int* init_array_on_heap() {
  unsigned int* arr = (unsigned int*)malloc(10 * sizeof(unsigned int));
  initialize(arr, 10);
  return arr;
}

int main() {
  unsigned int* arr1 = init_array_on_stack();
  unsigned int* arr2 = init_array_on_heap();
  display(arr1, 10); // segmentation fault! stack address invalid.
  display(arr2, 10); // memory was allocated on heap, address still valid.
  free(arr2); // need to free arr2 because it was allocated on heap.
  display(arr2, 10); // segmentation fault! heap address invalid (already freed).
}