Memmove is frequently cited as an example of a standard library function that can't be implemented using only standard C. It's incorrect, though: the way to do memmove using standard C is to use malloc to allocate a temporary buffer.

But the whole question is not terribly relevant. When memmove is provided as part of the C implementation it can rely on non-portable platform behaviour just fine. There's no rule that you have to implement libc using only standard C facilities.

And how do you implement malloc() in standard C?

Another example of arbitrary pointer comparison I'm familiar with is for resolving lock ordering problems. If I have a big set of locks that I need to take to complete a particular operation, a simple way to prevent deadlocking is to acquire those locks in the order of their pointers.

E.g. if I have three locks I need to take,

  lock_t *a = (lock_t *)0x100;
  lock_t *b = (lock_t *)0x300;
  lock_t *c = (lock_t *)0x200;

then I'll take them in order a, c, b;

If order of locks is important maybe it's worth considering putting them in an array.

Or maybe keeping a global atomic counter, and reading a unique (to the process) value out of it every time you create a mutex?