Why doesn't the OS crash if I dereference a null pointer?

Question

Dereferencing a null pointer results in undefined behavior. In practice it usually means that my program will crash. But why doesn\'t the OS crash? Because if my progra

Answer 1

There are no rules of logical transitivity when it comes to UB. Your assumption is wrong.

UB does mean that anything can happen, so on a poorly written OS, your program might actually crash the OS. Don't rule it out.

Also, your program doesn't crash because you dereference a NULL pointer. It crashes because the OS tells it to crash.

Answer 2

Because if my program dereferences a null pointer, and my program is run by the OS, then, according to the rules of logical transitivity, this means the OS tried to dereference a null pointer. Why doesn't the OS enter a state of "undefined behavior"?

This is wrong. There's something called memory protection and thats WHY your program is terminated. Is the OS that is protecting itself (in the terms of memory use).

Answer 3

Sorry, what rules of 'logical transitivity'? One of the things an operating system is designed to do is to protect programs from the misbehaviour of other programs. In particular, the O/S should not crash just because your program tries to do something silly.

On operating systems without memory protection, accessing via a null (or any invalid) pointer could indeed cause the O/S to crash (if the O/S happened to use location 0 for something interesting).

But that has nothing to do with logical transitivity. That has to do with your program accessing memory that belongs to another program. Either programs could crash in those circumstances.