Am Montag, dem 18.08.2025 um 23:19 +0000 schrieb Joseph Myers: > On Fri, 15 Aug 2025, Alejandro Colomar wrote: > > > Hi Joseph, > > > > On Thu, Aug 14, 2025 at 10:03:00PM +0000, Joseph Myers wrote: > > > On Thu, 14 Aug 2025, Alejandro Colomar wrote: > > > > > > > And I'm proposing it as a GNU extension, which means we don't even need > > > > to care about what ISO C says about [n]. We, as a quality > > > > implementation, treat it with stronger semantics, which this patch uses. > > > > > > As a GNU extension, it's also necessary to define semantics in the > > > presence of parameter forward declarations. > > > > I'd say that if there are conflicting declarations the behavior is > > undefined. So, if you have [2] and [3], think again what you're doing. > > We shouldn't introduce undefined behavior like that. And the declarations > *aren't* conflicting under current language semantics. It's the > introduction of this _Countof extension that has significant implications > for the conceptual model of how parameter declarations are handled in C > and introduces such a conflict.
Taking the size of the definition in _Countof seems largely orthogonal to the question on how one deals with inconsistencies with forward declarations of the function or parameters. Forming a composite type could allow this in even more cases later, but this seems unproblematic to add later. The only possible change in semantics I can see would be where a VLA becomes a fixed size array when forming a composite type, so the _Countof becomes a constant where it would now still be a run-time value. So while I agree in general that we should be careful, I would not see this as a risky change for GCC even when the other papers are subject to change or not adopted at all. But I agree that it would make sense to have a complete discussion of this topic before moving forward. In the meantime, there are plenty of other things to do. A step forward could be to add warnings for the case when thesize of an array parameter is inconsistent with a forward declaration. Such warnings seem to be desirable in any case. There are many other cases related to this general areas where we still miss "obvious" warnings. Martin
