The point is that there are two very fundamentally different things that can be stored in a C pointer: 1. A valid address to an object of some type. 2. Null.

The point is that there are two very fundamentally different things that can be stored in a C pointer: 1. A valid address to an object of some type. 2. Null. One of these can be dereferenced, the other cannot; the valid operations are not the same, because they are not the same "type". Now, C of course will happily store both the address of an object and "NULL" in the same "type", and that's the problem. There are a great many places in code in C where one would like to take a pointer that must contain an object's address; that is, a non-null pointer. The type system offered by C has no way to indicate this, and so the compiler cannot catch passing NULL to such a function. (And I honestly would bet that the kernel limits it more due to C, than C uses 0 because the kernel limits it. That is, yes, the kernel limits allocating address 0, but the arrow of causality is the other way around.)