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If you have a page mapped at address 0, accessing address 0 is valid.

NULL isn't always the integer 0 in C. It's implementation-defined.

In every real world implementation anyone cares about, it's zero. Also I believe it is defined to compare equal to zero in the standard, but don't quote me on that.

> Also I believe it is defined to compare equal to zero in the standard, but don't quote me on that.

That's true for the literal constant 0. For 0 in a variable it is not necessarily true. Basically when a literal 0 is assigned to a pointer or compared to a pointer the compiler takes that 0 to mean whatever bit pattern represents the null pointer on the target system.

What? If you have a null pointer to a class, and try to reference the member that starts 156 bytes from the start of the class, you’ll deference 0x9c (0 + 156)

Strangely, not necessarily on every implementation on every processor.

It's not guaranteed that NULL is 0.

Still, I don't think you'd find a counterexample in the wild these days.