Const-ness of atomic_load parameter in C

Hi cfe-dev,

C++11 onwards specs the non-member functions atomic_load and atomic_load_explicit as taking the atomic by const (potentially volatile) pointer. C11, in its infinite wisdom, decided to drop the const, and C17 will fix this with DR459 (the current draft forgot to fix B.16, but that’s not the normative part).

I suggest we allow passing const _Atomic(T)* in all versions of C. It seems silly to special-case C11’s semantics because they allow strictly less code to work. I’d usually just post these patches, but the not-quite-correct C11 semantics made me think twice.

This doesn’t affect C++ (unless you try to use stdatomic.h in C++ which… you shouldn’t…).

I would change the following (independently, they don’t need to synchronize-with):

  • clang’s lib/Headers/stdatomic.h implements these as #define to the _c11* equivalent, which are builtins with custom typecheck. Fix the typecheck.
  • Additionally, libc++ defines the _c11* functions in include/atomic for the sake of GCC compatibility. Fix their declaration.

It seems GCC already doesn’t require const:


Your plan sounds fine to me. We generally apply DR resolutions