[kaffe] Weak references/Class destruction

Helmer Krämer hkraemer at freenet.de
Wed Dec 22 10:20:51 PST 2004

Guilhem Lavaux <guilhem at kaffe.org> wrote:

> Here is a patch which changes the way the classes are destroyed and 
> implements
> weak references for boehm-gc and kaffe-gc. This fixes class concurrent
> destruction issues which was bugging boehm-gc (and maybe also kaffe-gc) and
> should insure that we do not access already freed memory.

If I remember it correctly, the problems with boehm-gc and
garbage collecting classes are:

a) Suppose class A  is the only class implementing interface I. If
   A becomes unreachable, so does I. In this case however, there's
   no guarantee that C is destroyed before I. This means that destroyClass
   has to deal with the fact that I might already have been destroyed
   when C is destroyed. This gets complicated for the implementors
   table and the interface dispatch tables because destroyClass has
   to remove C from some of the structures stored inside I.

b) Same thing when a class A and its super class A' become unreachable
   at the same time. In this case, some of the pointers stored in the
   vtable (and interface dispatch table) may no longer be valid because
   A' was already destroyed.

The solution for a) is to use weak references.

The solution for b) is to use allocation types that are automatically
freed by the gc for everything that might be stored in a vtable (or
interface dispatch table).

Is this correct so far?


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