[gambit-list] Wills from beyond.

Marc Feeley feeley at iro.umontreal.ca
Mon Dec 23 09:14:43 EST 2013

On Dec 22, 2013, at 9:35 PM, Estevo <euccastro at gmail.com> wrote:

> Thanks for the detailed explanation.
> > The GC only calls the finalizer (release function?) when it reclaims the space of the object.
> > So as long as you can get your hands on o2 (when w1’s action procedure is executed), the
> > finalizer will not have been called.
> After w1 and o2 have been set to #f, is there anything preventing the space for either of them to be reclaimed anytime?  In particular,
>   - is the fact that o1 hasn't been reclaimed yet keeping w1 alive?
>   - will the reference to o2 in w1's action closure prevent it from being reclaimed before that closure is reclaimed?
> A corollary to the latter: if I delay the execution of w1's action closure for long enough (e.g. by holding onto o1) could I ever get it to execute with o2_ pointing to freed/invalid memory?

Lets define some concepts more formally:

1) reachability path of an object: a chain of references from the roots that reaches the object (these references can be a mixture of weak and strong references)

2) weak reachability of an object: all the reachability paths of the object contain at least one weak reference

3) strong reachability of an object: at least one of the reachability paths of the object contain only strong references

4) reachability of an object: an object that is either weakly reachable or strongly reachable

Note that during execution, an object’s reachability may alternate between weakly reachable and strongly reachable (any number of times, as required by the program).

The action procedure of a will is executed when the GC detects that the will’s testator object is weakly reachable.  The GC is *not* taking the decision to reclaim the testator object, in fact it can’t reclaim the object since it is (at least) required to be passed as a parameter to the action procedure.

The GC reclaims objects that are not reachable.  When a foreign object is reclaimed by the GC, its release function is called (which may cause other “finalization” actions, such as reclaiming space on the C heap).  Note that it is possible to force the execution of the release function by explicitly calling (foreign-release! obj).  Those are the only situations when the release function is called.  So if you don’t use foreign-release!, the Scheme code manipulating a foreign object has the guarantee that the release function hasn’t been called (regardless of whether the object is the testator of a will or not).


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