C - malloc array in function and then access array from outside

Question

Here is how I malloc an int var and then access this var outside of the function

int f1(int ** b) {
  *b = malloc(sizeof(int)); 
  **b = 5;
}

int main() {
          


        

Answer 1

In exactly the same way but with some different arithmetic. You can think of what you are doing now as allocating an array with one element. Just multiply sizeof(int) by the number of elements you want your array to have:

int f1(int ** b, int arrsize) {
  *b = malloc(sizeof(int) * arrsize);

  // then to assign items:
  (*b)[0] = 0;
  (*b)[1] = 1;
  // etc, or you can do it in a loop
}

int main() {
  int * a, i;
  int size = 20;

  f1(&a, size); // array of 20 ints

  for (i = 0; i < size; ++i)
      printf("%d\n", a[i]); // a[i] is the same as *(a + i)
                            // so a[0] is the same as *a

  // keep it clean : 
  free(a);

  return 0;
}