On 05/01/2017 02:31 PM, Daniel Berlin wrote:
So you believe that you can index into an object randomly by
pointer arithmetic and pull out a different field?
For starters, this is illegal because you don't know where the
padding bytes are.
You cannot assume that X.a + 1 == X.b
"Implementation alignment requirements might cause two adjacent
members not to be allocated immediately after each other;"
See 9.2.14
IE at best you'd have to add
&(struct X*(0))->b - &(struct X*(0))->a
I don't think that helps because the structure offsets are an observable
part of the model. The code could easily be:
if (offsetof(X, b) == sizeof(int)) {
// thing we're talking about
} else {
// something safer
}
I don't believe this is legal either.
Let me try to dredge up the long discussions we had about these cases
on the gcc mailing lists.
The conclusion was, i believe:
"if you want to go marching through an object as a char *, that's
fine, if you expect to be able to get at fields by playing pointer
arithmetic games, from other fields, that is not)
I feel like every couple years, a different compiler has the same
aliasing discussions :)
I'm sure that's true :-)
-Hal
--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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