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Re: in which we present an ugly hack to make sys/queue.h CIRCLEQ work
On Thu, Nov 21, 2013 at 11:02:24PM -0500, Ken Hornstein wrote:
> >Modulo some administrative details, it's just "no object in memory may
> >be accessed using more than one type".
> Ok ... I _think_ I see it. But doesn't that mean that like 90% of the casts
> used by C programmers are totally wrong? :-)
Yes. That's why -fno-strict-aliasing is very popular, and would be
more popular if people in general were more aware of the issue.
As noted, inspecting the representation of an object using char * or
unsigned char * is explicitly allowed. And some constructions with
unions are allowed. But other stuff isn't.
Note that the bog-standard (struct sockaddr *) cast that one needs and
conventionally uses to call bind(2), connect(2), accept(2), and
similar is, strictly speaking, illegal.
> Alright, I think I understand. If tqe_prev pointed to a queue entry
> that would be the problem, because that memory already has a TAILQ_ENTRY
Yes. The idea in C is that when you first access fresh memory, that
imprints it with a type. You're then supposed to use only the same
type to access it again later, until you free() it.
> So ... looking at this code ... it seems like the core problem is that
> TAILQ_HEAD and TAILQ_ENTRY are two different types (even though they
> literally the same structure layout). So if TAILQ_HEAD and TAILQ_ENTRY
> were the same structure, it wouldn't be an issue. It doesn't quite leap
> out to me how that would be possible without changing the API a bit.
I think it can be done by sticking an anonymous union into TAILQ_HEAD,
but of course anonymous unions aren't supported until C11.
David A. Holland
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