If you need to round-trip a pointer through an integer and back to the same pointer, you need to use intptr_t or uintptr_t. (Note that this is true for object pointers but not necessarily function pointers. There’s no integer type guaranteed to be big enough for a function pointer.)

If you want to round-trip an integer through a pointer and back to the same integer, you can use any integer type provided it is not bigger than intptr_t or uintptr_t.