Re: [PATCH] C undefined behavior fix

From: Joseph S. Myers (
Date: Sat Jan 05 2002 - 14:37:48 EST

On Sat, 5 Jan 2002 wrote:

> Wat this last bit added to the standard after ANSI/ISO 9899-1990? I'm
> looking through my copy and I can't find it. All I can find is that

I was quoting from the GCC manual (providing the documentation
implementations are required to provide of implementation-defined
behaviour); of course the subclause numbers are different in C99 (from
which the subclause numbers in the GCC manual are given) from those in
C90. Perhaps once all the documentation for implementation-defined
behaviour in C99 is present I'll go over what's required to document it
all relative to C90 as well.

> 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.

Just because you've created a pointer P, and it compares bitwise equal to
a valid pointer Q you can use to access an object, does not mean that P
can be used to access that object. Look at DR#260, discussing the
provenance of pointer values, which arose from extensive discussion in the
UK C Panel, and if you think it should be resolved otherwise from how we
(UK C Panel) proposed then raise your position on it at a meeting of your
National Body before the next WG14 meeting.

Joseph S. Myers

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