If you want to know how bad it can be, only need to take a look at GCC's implementation of `stddef.h` for `size_t` and `ptrdiff_t` etc.

https://github.com/gcc-mirror/gcc/blob/master/gcc/ginclude/s...

There's a simpler way to define these types in newer versions of C which have `typeof`, we can take advantage of the fact that `sizeof()` always returns a `size_t`, `ptr - ptr` always returns a `ptrdiff_t`, and a literal L'c' has type `wchar_t`, etc.

    typedef typeof(sizeof(0)) size_t;
    typedef typeof(nullptr) nullptr_t;
    typedef typeof((void*)0-(void*)0) ptrdiff_t;
    typedef typeof(L'\0') wchar_t;
    typedef typeof(u8'\0') char8_t;
    typedef typeof(u'\0') char16_t;
    typedef typeof(U'\0') char32_t;

Don't need ifdef soup to test what the sizes of things are on different architectures - we just utilize the compiler's implementation of those types.