Could I ever want to access the address zero?

Question

The constant 0 is used as the null pointer in C and C++. But as in the question \"Pointer to a specific fixed address\" there seems to be some possible use of assigning fixe

Answer 1

You're starting from a mistaken premise. When you assign an integer constant with the value 0 to a pointer, that becomes a null pointer constant. This does not, however, mean that a null pointer necessarily refers to address 0. Quite the contrary, the C and C++ standards are both very clear that a null pointer may refer to some address other than zero.

What it comes down to is this: you do have to set aside an address that a null pointer would refer to -- but it can be essentially any address you choose. When you convert zero to a pointer, it has to refer to that chosen address -- but that's all that's really required. Just for example, if you decided that converting an integer to a point would mean adding 0x8000 to the integer, then the null pointer to would actually refer to address 0x8000 instead of address 0.

It's also worth noting that dereferencing a null pointer results in undefined behavior. That means you can't do it in portable code, but it does not mean you can't do it at all. When you're writing code for small microcontrollers and such, it's fairly common to include some bits and pieces of code that aren't portable at all. Reading from one address may give you the value from some sensor, while writing to the same address could activate a stepper motor (just for example). The next device (even using exactly the same processor) might be connected up so both of those addresses referred to normal RAM instead.

Even if a null pointer does refer to address 0, that doesn't prevent you from using it to read and/or write whatever happens to be at that address -- it just prevents you from doing so portably -- but that doesn't really matter a whole lot. The only reason address zero would normally be important would be if it was decoded to connect to something other than normal storage, so you probably can't use it entirely portably anyway.