Sharing in D

[Jason House]

Steven Schveighoffer Wrote:

> "Walter Bright" wrote
> > Steven Schveighoffer Wrote:
> >> I'm thinking more in the case of functions.  If I have a function foo, 
> >> and I
> >> want to pass my shared data version to it, I need to re-implement foo 
> >> with
> >> the parameters as being shared.  Imagine a function with several 
> >> parameters,
> >> in which you want to pass a combination of shared/unshared.  That's 2^n
> >> variations you have to write.
> >
> > Not necessary, just make them shared. Unshared implicitly converts to 
> > shared, so that works. Putting a fence around an unshared read doesn't 
> > break anything, etc.
> 
> I'm assuming you read my previous example, but I'll rewrite it to 
> demonstrate what I'm trying to show:
> 
> class sharedObj
> {
>    private int *_x;
>    synchronized shared int x() {return *_x;}
>    synchronized shared int x(int y) {*_x = y;}
>    synchronized shared void rebind(shared int *y) {_x = y;}
> }
> 
> shared sharedObj obj;
> 
> static this()
> {
>    obj = new sharedObj;
> }
> 
> int *x2;
> 
> void bar()
> {
>     obj.x = x2;
>     *x2 = 3; // is this supposed to be guaranteed safe?
> }
> 
> Now, Let's say you have thread 1 which calls bar.  So now thread 1's x2 and 
> obj._x point to the exact same data, which is both viewed as shared and 
> unshared at the same time.  Is this something that should be allowed?  Would 
> you not make the assumption that you can do whatever you want with what x2 
> points to without synchronization because it is unshared?  Maybe I don't 
> completely understand what you are intending for shared to mean.
> 
> I was under the impression that 'unshared' means 'lock free', which cannot 
> be enforcable if you can implicitly cast away the 'lock free' aspect of it.
> 
> ???
> 
> -Steve 

Uh oh Steve, you're starting to sound like me! I made the 2^n comment in the context of the const system a while back and now you're looking at how globals might break a transitve data type system... I could have sworn you were the loudest voice against my ideas!