On Tue, Oct 20, 2015 at 10:10:44PM +0000, Joseph Myers wrote:
> > typedef struct FA5_7 {
> > int i;
> > char a5_7 [5][7];
> > } FA5_7;
> >
> > __builtin_offsetof (FA5_7, a5_7 [0][7]), // { dg-warning
> > "index" }
> > __builtin_offsetof (FA5_7, a5_7 [1][7]), // { dg-warning
> > "index" }
> > __builtin_offsetof (FA5_7, a5_7 [5][0]), // { dg-warning
> > "index" }
> > __builtin_offsetof (FA5_7, a5_7 [5][7]), // { dg-warning
> > "index" }
> >
> > Here I think the last one of these is most likely invalid (being 8 bytes
> > past
> > the end of the object, rather than just one) and the others valid. Can you
> > confirm this? (If the &a.v[2].a example is considered invalid, then I think
> > the a5_7[5][0] test would be the equivalent and ought to also be considered
> > invalid).
>
> The last one is certainly invalid. The one before is arguably invalid as
> well (in the unary '&' equivalent, &a5_7[5][0] which is equivalent to
> a5_7[5] + 0, the questionable operation is implicit conversion of a5_7[5]
> from array to pointer - an array expression gets converted to an
> expression "that points to the initial element of the array object", but
> there is no array object a5_7[5] here).
C11, 6.5.2.1/3:
Successive subscript operators designate an element of a
multidimensional array object. If E is an n-dimensional array (n >= 2)
with dimensions i x j x . . . x k, then E (used as other than an lvalue)
is converted to a pointer to an (n - 1)-dimensional array with
dimensions j x . . . x k. If the unary * operator is applied to this
pointer explicitly, or implicitly as a result of subscripting, the
result is the referenced (n - 1)-dimensional array, which itself is
converted into a pointer if used as other than an lvalue. It follows
from this that arrays are stored in row-major order (last subscript
varies fastest).
As far as I see, a5_7[5] here is never treated as an array, just as a
pointer, and &a5_7[5][0] is valid.
Segher
