[wish] Flexible array members in unions

Kees Cook keescook@chromium.org
Thu May 11 22:16:43 GMT 2023


On Thu, May 11, 2023 at 09:43:49PM +0000, Joseph Myers wrote:
> On Thu, 11 May 2023, Kees Cook via Gcc wrote:
> 
> > Why are zero-sized objects missing in Standard C? Or, perhaps, the better
> > question is: what's needed to support the idea of a zero-sized object?
> 
> Zero-sized objects break the principle that different objects have 
> different addresses, and the principle of being able to subtract pointers 
> to different elements of an array.  There would also be serious C++ 
> compatibility concerns, since C++ allows a struct with no members but it 
> has nonzero size, unlike the GNU C extension where a struct with no 
> members has size zero.

Okay, understood. If this is a C-only thing, we can ignore the C++
impact. What depends on the "different objects have different addresses"
principle? And why do unions not break this -- they could point to the
same locations within the object? And don't flexible arrays already need
special handling in this regard?

-- 
Kees Cook


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