It's strict aliasing that's the issue, not the mapping of memory to a address space. You run into the same issues with a global segment.

Both are a problem, because they can only be implemented outside ISO C semantics.

That is why I tend to always write ISO C instead of C.

One's not a blocker to a malloc implementation since you could otherwise just cut up a static char buffer. Coordinating with the OS is a nice to have. Strict aliasing is hard blocker. Hence why that's the issue under discussion.

And I'm not sure I understand why you're using the inability to describe certain actions as the reason why you use ISO C.

Implementations of C have more functionality than C itself - like inline assembly or syscalls or machine-specific intrinsics, so it can do more. ISO C only has what's written in the standard.

(A syscall is an example of "something you can only do because the implementation isn't visible to the caller" - it can violate aliasing that way.)

Also, my argument isn't about type aliasing, it's about UB on out of bounds pointers. Could be some other aliasing issues though.