Should atomic_xxx() functions reject not-_Atomic() arguments ?

Jonathan Wakely
Sat Mar 7 19:27:35 GMT 2020

On Sat, 7 Mar 2020 at 11:00, Andrew Haley <> wrote:
> On 3/6/20 5:45 PM, Jonathan Wakely wrote:
> > On Fri, 6 Mar 2020 at 16:17, Chris Hall <> wrote:
> >
> >> FWIW: clang gets this right, and where the Standard says a parameter
> >> must be an _Atomic(foo_t)* [for a standard atomic_xxx()], clang rejects
> >> foo_t* arguments.
> >
> > It's not clear to me that C actually requires it to be rejected, or if
> > it's just undefined (in which case GCC's decision to accept it and do
> > the obvious thing is OK).
> Except in pedantic mode.

If the standard doesn't actually forbid it (which I don't think it
does) then it's up to the implementation if and when it diagnoses it.
Doing so for -pedantic might be reasonable though.

> I remember there was a move to actually permit
> this in C++:
> This seems obviously right to me...

Yeah. The feature is in C++20, via the std::atomic_ref<T> template.
You wrap an ordinary non-atomic variable in an atomic_ref and can
perform atomic operations on it.

For this to work, the implementation has to be able to accept
non-atomic variables in its __atomic_xxx functions (though not
necessarily in the public atomic_xxx ones).

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