Enforcing checks

As far as possible:

• The union discriminator should be checked before accessing a field of the union.
• The index should be compared against the bounds of the array should be checked before the array is indexed.

We have already seen, in previous sections, how the checks are made part of the accessors when the unions and arrays are members of a structure.

To enable us to enforce array bounds checking, whenever an array is passed to a function, we also pass the bounds of the array. (Alternatively, we pass enough information that the bounds of the array can be computed). For instance:

	int
	copy_int_stack(
		int_stack	stack,
		int		nels,	/* max array size */
		int		els[]
		)
	{
	  int	i;
	
	  i = 0;
	  while( !is_empty_int_stack(stack) ) {
	    assert(i < nels);	/* bounds check */
	    els[i] = top_int_stack(stack);
	    i++;
	    pop_int_stack(stack);
	  }
	  return i;
	}