dereferencing null

[Chad J]

On 03/07/2012 07:57 AM, Steven Schveighoffer wrote:
> On Mon, 05 Mar 2012 23:58:48 -0500, Chad J
> <chadjoan at __spam.is.bad__gmail.com> wrote:
>
>>
>> Why is it fatal?
>
> A segmentation fault indicates that a program tried to access memory
> that is not available. Since the 0 page is never allocated, any null
> pointer dereferencing results in a seg fault.
>
> However, there are several causes of seg faults:
>
> 1. You forgot to initialize a variable.
> 2. Your memory has been corrupted, and some corrupted pointer now points
> into no-mem land.
> 3. You are accessing memory that has been deallocated.
>
> Only 1 is benign. 2 and 3 are fatal. Since you cannot know which of
> these three happened, the only valid choice is to terminate.
>
> I think the correct option is to print a stack trace, and abort the
> program.
>

Alright, I think I see where the misunderstanding is coming from.

I have only ever encountered (1).  And I've encountered it a lot.

I didn't even consider (2) and (3) as possibilities.  Those are far from 
my mind.

I still have a nagging doubt though: since the dereference in question 
is null, then there is no way for that particular dereference to corrupt 
other memory.  The only way this happens in (2) and (3) is that related 
code tries to write to invalid memory.  But if we have other measures in 
place to prevent that (bounds checking, other hardware signals, etc), 
then how is it still possible to corrupt memory?

>
> [...]
>
> -Steve