On Tue, Jun 5, 2018 at 1:39 AM Martin Sebor <[email protected]> wrote:
>
> GCC silently (without -Wpedantic) accepts declarations of zero
> length arrays that are followed by other members in the same
> struct, such as in:
>
> struct A { char a, b[0], c; };
>
> Is it intended that accesses to elements of such arrays that
> alias other members be well-defined?
The middle-end assumes that fields in a structure do not overlap.
For overlaps you have to use a union.
In C++ I guess the rule that sizeof() of anything is at least 1 saves
you here so IMHO this is a C FE bug and we should probably simply
reject non-trailing empty arrays.
Note since b has size zero there isn't any real overlap, so ...
> In my tests, GCC assumes that neither read nor write accesses
> to elements of internal zero-length arrays alias other members,
> so assuming those aren't bugs I wonder if the documentation
> should be updated to make that clear and a warning added for
> such declarations (and perhaps also accesses).
>
> For example, the test in the following function is eliminated,
> implying that GCC assumes that the access to p->b does not modify
> p->c, even though with i set to 0 it would:
>
> void f (struct A *p, int i)
> {
> int x = p->c;
> p->b[i] = 1;
> if (x != p->c)
> __builtin_abort ();
... your testcase simply invokes undefined behavior by accessing
b out of bounds.
Richard.
> }
>
> Martin
