Why can't you do bitwise operations on pointer in C, and is there a way around this?

Question

I read that you can't do bitmasks on pointers, how come you can't do bitwise operations on pointers?

Is there any way to achieve the same effect?

Does the same apply to C++?

Answer 1

The reason you can't do bitwise pointer operations is because the standard says you can't. I suppose the reason why the standard says so is because bitwise pointer operations would almost universally result in undefined or (at best) implementation-defined behavior. So there would be nothing you could do that is both useful and portable, unlike simpler operations like addition.

But you can get around it with casting:

#include <stdint.h>  void *ptr1; // Find page start void *ptr2 = (void *) ((uintptr_t) ptr1 & ~(uintptr_t) 0xfff) 

As for C++, just use reinterpret_cast instead of the C-style casts.

Answer 2

It's disallowed because the semantics aren't really well defined. You can certainly do it, though. Just cast to uintptr_t, do the operations and then cast back into a pointer type. That will work in C++, too.