null dereference exception vs. segfault?

[Jonathan M Davis]

On Monday 02 August 2010 00:05:40 Jeffrey Yasskin wrote:
> Even better, you can annotate fail_sometimes with @safe, and it'll
> still access out-of-bounds memory.
> 
> Take the following with a grain of salt since I'm really new to the
> language.
> 
> gdb says:
> Reason: KERN_PROTECTION_FAILURE at address: 0x00000008
> 0x00001e52 in D4test14fail_sometimesFiZv ()
> 
> which indicates that 'a' is getting initialized to null (possibly by
> process startup 0ing out the stack), and then x is being read out of
> it. You can get exactly the same crashes in C++ by reading member
> variables out of null pointers. The D compiler is supposed to catch
> the uninitialized variable ("It is an error to use a local variable
> without first assigning it a value." in
> http://www.digitalmars.com/d/2.0/function.html), but clearly it's
> missing this one.
> 
> I haven't actually found where in the language spec it says that class
> variables are pointers, or what their default values are. I'd expect
> to find this in http://www.digitalmars.com/d/2.0/type.html, but no
> luck.
> 
> Looking through the bug tracker ... Walter's response to
> http://d.puremagic.com/issues/show_bug.cgi?id=671 seems to indicate
> that he isn't serious about uninitialized use being an error. It's
> just undefined behavior like in C++.
> 
> In any case, the fix for your problem will be to initialize 'a' before
> using it.

_All_ variables in D are initialized with a default value. There should be _no_ 
undefined behavior with regards to initializations. D is very concientious about 
avoiding undefined behavior. In the case of references and pointers, they are 
initialized to null. There's not really such a thing as using a variable without 
initializing it, because variables are default initialized if you don't 
initialize them yourself. The _one_ exception would be if you explicitly 
initialized a variable to void:

int[] a = void;

In that case, you are _explicitly_ telling the compiler not to default 
initialize the variable. That _can_ lead to undefined behavior and is definitely 
unsafe. As such, it is intended solely for the purposes of optimizing code where 
absolutely necessary. So, you really shouldn't have any variables in your code 
that weren't initialized, even if you didn't initialize them explicitly.

The pages that you're looking at there need to be updated for clarity.

- Jonathan M Davis