On Mon, May 1, 2017 at 2:07 PM, John McCall <rjmcc...@gmail.com> wrote:
> On Mon, May 1, 2017 at 3:31 PM, Daniel Berlin <dber...@dberlin.org> 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 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 :) >> > > With the caveat that, in practice, compilers have to support both: > a) "going up a level" by subtracting an offsetof, which is clearly > officially intended and not otherwise supported, > AFAIK, Vtable accesses were supported by doing this, but pretty much nothing else. > and > b) treating a final or flexible array member, possibly at depth > 1, as > contiguous with a trailing array, because this is an extremely common > extension and artificial idiom. > Yes, anything at the end was treated as magically special.
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