Re: [PATCH] C undefined behavior fix

Florian Weimer
2002-01-05T20:02:20Z

jkl@miacid.net writes:

> An arbitrary integer may be converted to a pointer.
> ^^^^^^^^^

This rule exists so that implementations are not forced to issue a
diagnostic for (char *)1.

> I interpret this to mean that one MAY use integer arithmatic to
> do move a pointer outside the bounds of an array. Specifically, as soon
> as I've cast the pointer to an integer, the compiler has an obligation to
> forget any assumptions it makes about that pointer. This is what casts
> from pointer to integer are for! when i say (int)p I'm saying that I
> understand the address structure of the machine and I want to manipulate
> the address directly.

According to the standard, you say that you want to cast p to type
int. You cannot manipulate machine addresses in C because C is
defined as a high-level language, without backdoors to such low-level
concepts as machine addresses.

The fact that quite a few implementations traditionally provide
such backdoors in some cases does not mean that the C language is a
low-level language, or that all implementations (even future ones)
have to provide these backdoors.

> If the satandard has changed so this is no longer possible, there
> NEEDS to be some other way in the new standard to express the same
> concept, or large application domains where C is currently in use will
> stop working.

I don't think there are fundamental and conceptual changes in C99 in
this area. Even with previous C reversions, you should have read the
compiler manual carefully before doing address arithmetic.