What exactly shared means?

[Jonathan M Davis via Digitalmars-d-learn]

On Friday, January 02, 2015 19:47:50 John Colvin via Digitalmars-d-learn wrote:
> On Friday, 2 January 2015 at 13:14:14 UTC, Jonathan M Davis via
> Digitalmars-d-learn wrote:
> > Objects in D default to being thread-local. __gshared and
> > shared both make
> > it so that they're not thread-local. __gshared does it without
> > actually
> > changing the type, making it easier to use but also dangerous
> > to use,
> > because it makes it easy to violate the compiler's guarantees,
> > because it'll
> > treat it like a thread-local variable with regards to
> > optimizations and
> > whatnot.
>
> I'm pretty sure that's not true. __gshared corresponds to C-style
> globals, which are *not* assumed to be thread-local (see below).

No, the type system will treat __gshared like a thread-local variable. It
gets put in shared memory like a C global would be, but __gshared isn't
actually part of the type, so the compiler has no way of knowing that it's
anything other than a thread-local variable - which is precisely why it's so
dangerous to use it instead of shared. For instance,

__gshared int* foo;

void main()
{
    foo = new int;
    int* bar = foo;
}

will compile just fine, whereas if you used shared, it wouldn't.

- Jonathan M Davis