STACK

                |==============================|   
  arg_modifier: |                              |
                |        ---------             |
                |      y |   *-- |---------------------
                |        ---------             |      |
                |         -------              |      |
                |      x  | 10  |              |      |
                |         -------              |      |
                |==============================|      |
      main:     |                              |      |
                |                              |      | 
                |         ---------------------------- 
                |         |                    | 
                |        \/                    |
                |        ------                |
                |      b | 5  |                |
                |        ------                |
                |         -----                |
                |      a | 2  |                |
                |         -----                |
                |==============================| 

1. a and b are located in main's stack frame
2. in arg_modifier's stack from
3. y gets the address of the second argument, x gets the value of the first
4. the value of the first argument cannot be modified by the function. the value of the second argument can.

// do you understand why swap is a void function?
void swap(int *v1, int *v2) {
  int temp;
	temp = *v1;
	*v1 = *v2;
	*v2 = temp;
}

int main() {
  int x, y;
	x = 10;
	y = 50;
	printf("x = %d y = %d\n", x, y);  // prints x = 10 y = 50
	swap(&x, &y);
	printf("x = %d y = %d\n", x, y);  // prints x = 50 y = 10
	return 0;
}