Re: Releasing Aliased Variables

[Robert A Duff <bobduff@world.std.com>]

Byron Kauffman <byronbkauffman@netscape.net> writes:

> I'm having a problem understanding what defining a variable as aliased
> does. Is there a problem with releasing an aliased pointer? Can or
> should you even alias a pointer?

If you declare X to be aliased, then you may say X'Access or
X'Unchecked_Access to create a pointer to it.  This makes the code
easier to understand, because you know that such pointers might exist.

In code-generation terms, it causes X to allocated at an addressable
location (eg, not in a register).  It is unlikely that "aliased"
controls whether X is allocated on the stack versus the heap (but of
course compilers can do what they like).

If Y = X'Access, then make sure you never call Unchecked_Deallocation on
Y.

> I'm declaring all of the directsound objects as aliased, which solved a
> problem I was having creating the objects ( it looked like some very
> large buffers were being created on the stack). What's happening now is
> that the release procedure is bombing down in the Windows code, so
> control never returns.

Sounds like some sort of dangling-pointer problem.  Declaring things
aliased wouldn't directly cause that, but deallocating things at the
wrong time, or twice, or returning X'Unchecked_Access where X is local
could all cause that sort of trouble.

- Bob