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

The OS sets up a fault handler which is called if a memory access violates rules imposed by the OS - such as an access to the null address. If your program is about to dereference a null pointer this fault handler is called and the program will be ended before it accesses the disallowed memory region. So your program does actually never dereference a null pointer, it is catched while trying.

The mechanism for detecting forbidden memory accesses is often done with hardware support like page tables or memory segmentation.

If the OS kernel itself dereferences a null pointer, it usually halted while trying to do so. You will get a blue screen, kernel oops or similar. If it keeps going, that may actually result in "undefined behaviour".

Note that the term "undefined behaviour" is only exactly defined in C or similar languages, the processor doesn't really care - usually what happens if you try to access a memory region for which you don't have sufficient rights is very well defined in the context of the architecture.