What happens in OS when we dereference a NULL pointer in C?

Question

Let\'s say there is a pointer and we initialize it with NULL.

int* ptr = NULL;
*ptr = 10;

Now , the program will crash since ptr

Answer 1

In a typical case, int *ptr = NULL; will set ptr to point to address 0. The C standard (and the C++ standard) is very careful to not require that, but it's extremely common nonetheless.

When you do *ptr = 10;, the CPU would normally generate 0 on the address lines, and 10 on the data lines, while setting a R/W line to indicate a write (and, if the bus has such a thing, assert the memory vs. I/O line to indicate a write to memory, not I/O).

Assuming the CPU supports memory protection (and you're using an OS that enables it), the CPU will check that (attempted) access before it happens though. For example, a modern Intel/AMD CPU will use paging tables that map virtual addresses to physical addresses. In a typical case, address 0 won't be mapped to any physical address. In this case, the CPU will generate an access violation exception. For one fairly typical example, Microsoft Windows leaves the first 4 megabytes un-mapped, so any address in that range will normally result in an access violation.

On an older CPU (or an older operating system that doesn't enable the CPUs protection features) the attempted write will often succeed. For example, under MS-DOS, writing through a NULL pointer would simply write to address zero. In small or medium model (with 16-bit addresses for data) most compilers would write some known pattern to the first few bytes of the data segment, and when the program ended, they'd check to see if that pattern remained intact (and do something to indicate that you'd written via a NULL pointer if it failed). In compact or large model (20-bit data addresses) they'd generally just write to address zero without warning.