On Sat, 7 Nov 2015, Segher Boessenkool wrote: > > 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.
As usual, based on taking the address, not offsetof where there's the open question of whether the C standard actually requires support for anything other than a single element name there: a5_7[5] is an expression of array type. The only way for it to be treated as a pointer is for it to be converted implicitly to pointer type. That implicit conversion is what I think is problematic. Only once the implicit conversion has taken place do the special rules about &A[B] meaning A + B take effect. But since the problem I see is with the conversion of A to a pointer, you still have undefined behavior. The paragraph you quote seems to not to add anything to the semantics defined elsewhere in the standard; it's purely descriptive of some consequences of those semantics. Whether we wish to be more permissive about some such cases (depending on -Warray-bounds=N) is a pragmatic matter depending on the extent to which they are used in practice. -- Joseph S. Myers jos...@codesourcery.com