pointers

[Henry Spencer]

In article <1988Sep26.182431.7946 at sq.uucp> msb at sq.com (Mark Brader) writes:
>Harrumph.  The cast not only can, but *does* involve a conversion.

Picky, picky, picky...

>The question is whether the conversion changes any bits.  Most of the
>examples you talk about also involve conversions, and the same thing
>applies.  Inter-module compatibility doesn't, but there are very few
>requirements here.

Inter-module compatibility's main effect is that it puts limits on how
tricky the compiler can be within a single module, because certain aspects
of the trickery have to be consistent across modules.

>Now, as a counterexample to the original assertion, consider a word-
>addressed machine where an "int" occupies one word.  Do you seriously
>want to say that an "int *" points to a particular byte of the word?

The original discussion was about aggregate objects ("aggregate" is used
loosely, possibly not in the precise sense X3J11 uses) in which the
internal structure is programmer-visible.  An int is not a relevant example.

However...  I'm coming to think that Mark is right (although it does come
under the heading of "something no sensible implementor would do").  I was
misled by a related problem that I'd studied at some length.  So long as
the compiler knows about all the conversions being done, which it should
for a portable program, it can fool around as it pleases internally.  So
the implications for out-of-range pointers result from common practice
rather than logical necessity.
-- 
The meek can have the Earth;    |    Henry Spencer at U of Toronto Zoology
the rest of us have other plans.|uunet!attcan!utzoo!henry henry at zoo.toronto.edu