Question

I feel this might be a weird/stupid question, but here goes…

In the question Is NULL in C required/defined to be zero?, it has been established that the NULL pointer points to an unaddressable memory location, and also that NULL is 0.

Now, supposedly a 32-bit processor can address 2^32 memory locations.

2^32 is only the number of distinct numbers that can be represented using 32 bits. Among those numbers is 0. But since 0, that is, NULL, is supposed to point to nothing, shouldn’t we say that a 32-bit processor can only address 2^32 - 1 memory locations (because the 0 is not supposed to be a valid address)?

Answer 1

the NULL pointer points to an unaddressable memory location

This is not true. From the accepted answer in the question you linked:

Notice that, because of how the rules for null pointers are formulated, the value you use to assign/compare null pointers is guaranteed to be zero, but the bit pattern actually stored inside the pointer can be any other thing

Most platforms of which I am aware do in fact handle this by marking the first few pages of address space as invalid. That doesn’t mean the processor can’t address such things; it’s just a convenient way of making low values a non valid pointer. For instance, several Windows APIs use this to distinguish between a resource ID and a pointer to actual data; everything below a certain value (65k if I recall correctly) is not a valid pointer, but is a valid resource ID.

Finally, just because C says something doesn’t mean that the CPU needs to be restricted that way. Sure, C says accessing the null pattern is undefined — but there’s no reason someone writing in assembly need be subject to such limitations. Real machines typically can do much more than the C standard says they have to. Virtual memory, SIMD instructions, and hardware IO are some simple examples.