🚀 go-pugleaf

> ... Again, the basic question is still unresolved here: can a conversion
> from one pointer type to another affect bytes other than those
> where the pointer variable is stored?

There is nothing in the standard to justify such an action, so it
would be permissible only under the "'as if' rule".  It could, for
example, modify bytes that the program mustn't read because the
behavior would be undefined, such as the bytes of an object of
indeterminate value.  It could modify the bytes of a volatile object.
But it could NOT modify some arbitrary object with a well-defined
value that the program correctly expects to persist!

In particular, it could not modify padding at the end of a struct,
precisely *because* it would break the struct hack.  Another example
of code that would break is:

	struct st {
		/* anything */
	} *stp;
	union {
		struct st st;
		char chch[sizeof(struct st)];
	} uu;
	void *vp = &uu;
	int i;

	for (i = 0; i < sizeof uu.chch; ++i)
		uu.chch[i] = '#';
	stp = vp;
	for (i = 0; i < sizeof uu.chch; ++i)
		assert (uu.chch[i] == '#');

-- 
Mark Brader            \ "It's not in the slightest bit harder to write Fortran
SoftQuad Inc., Toronto  \ or Basic programs in C++ or Smalltalk than it is
utzoo!sq!msb, msb@sq.com \ to write them in C or Pascal."   -- Peter da Silva

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