Annoyingly, the _Atomic quaifier does not follow the usual rules for qualifiers. _Atomic-qualified types are completely new types which are not compatible with the non-atomic type. In particular they might have different size and alignment.
Thus, it is not possible to implicitely convert a pointer to a non-atomic to a routine that wants to use atomic accesses.
Potential solution: Make _Atomic a proper qualifier while _Atomic( ) may also change size/alignment.