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Re: in which we present an ugly hack to make sys/queue.h CIRCLEQ work
On Nov 23, 2:16pm, Dennis Ferguson wrote:
} On 22 Nov, 2013, at 21:40 , David Holland
} >> 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.
} It isn't perfectly clear to me that this code has an aliasing problem
} the way it is, though. The only thing that matters in the standard are
} the types of the lvalue expressions used to access object in storage. The
} lvalue expression types used to access the objects in storage in this
} case are 'type **', 'type **' and 'type *', which are the types those
"type **" and "type *" are not the same types.
} objects were stored with and the types that would be used for other
} accesses to the same locations. The structure type used to arrive there
} should only matter if it is the type of an lvalue expression itself,
} e.g. *(struct foo *)ptr(?).
} I would be interested in knowing an actual example of the comparison
} problem with the CIRCLEQ macro, if the concern isn't theoretical. Since
Uh, do you really think people would be doing all this work
for something that was theoretical? The problem is that gcc 4.8
optimises out the comparison as being always false due to the
} the C standard explicitly allows a pointer to a structure type to be
} converted to the type of its first member and back, to another structure
} type and back, or to char * or void * and back, the fact that the two
I rather doubt that you can convert to a different structure type
and back. Those would definitely be different objects.
} pointers point at different structure types is by itself insufficient to
} prove that they would not compare equal when suitably converted. It seems
} like that conclusion would minimally need to depend on proving that there
} was no possible use of the structure pointers which wouldn't violate the
} aliasing requirements, i.e. that that are no structure members at the same
} offsets which have compatible types. That's a rather aggressive optimization,
} and is kind of like throwing you in jail for a crime you haven't actually
} committed yet (though I guess that happens too).
}-- End of excerpt from Dennis Ferguson
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