Re: [Gc] How to do enable gc of prolog programs using bdw-gc

Why do you need to indicate when they can't be reached from normal code?
That's EXACTLY the job the GC does for you.

If you need to refer to them from somewhere else, but not prevent them
being GCd, then you use a "disappearing link". That's a pointer that the GC
automatically sets to null when the object is collected.


On Thu, May 8, 2014 at 9:28 PM, Stefan Israelsson Tampe <
[email protected]> wrote:

> Yep, this looks like using finaliser is not suitable to model gc of
> logical variables.
>
> The interesting with this application is:
> ==============================
> Logical variables need to backtrack:
> They are the located on a datastructure
> They needed to be available when on the list hence protect from gc
> One need to indicate when they are not reaced from normal code e.g. ready
> for gc.
> Then one carefully need to clean the information off the datstructure and
> compress it.
> Then normal gc can be done of th elogical variables (or they can be
> recycled)
>
> So currently bdw-gc cannot support gc of logical variables. I have a patch
> for it to do so, but it's not anything sanctioned.
>
> Cheers
>
>
> On Thu, May 8, 2014 at 12:57 AM, Bruce Hoult <[email protected]> wrote:
>
>> On Thu, May 8, 2014 at 7:31 AM, Stefan Israelsson Tampe <
>> [email protected]> wrote:
>>
>>> Wingo or anyone else. In a finalizer, if the object should not be gc'ed
>>> what do I need to do? just leave it?
>>> and in the case when it should be gc'ed what do I do scm_gc_free?
>>>
>>
>> If an object should not be gc'd then how did you get into the finalizer
>> for it?
>>
>>  In bdwgc objects with finalizers are not actually collected or
>> overwritten until the GC after the GC in which the finalizer is run.
>> Therefore, technically, the finalizer can prevent the object from being
>> collected by putting a pointer to the object into a global or another
>> gc'able object.
>>
>> But if you still wanted the object then why didn't you already have a
>> reference to it somewhere? If there is no reference to it then it can't
>> take any further part in the computation. Unless maybe you have a reference
>> to it from somewhere the GC doesn't look, such as an object allocated using
>> malloc instead of GC_malloc.
>>
>> I've never seen a situation in which resurrection-by-finalizer was the
>> best solution. I'd be curious to see one. And, remember that by the time
>> the finalizer is run the finalizer has been de-registered, so the
>> resurrected object no longer has a finalizer.
>>
>> In general, very very few objects should have finalizers. Typically the
>> reason is that you need to release some external, non-gc'd resource e.g.
>> close a file or delete an object allocated using some foreign API from a
>> library. Things like that.
>>
>>
>
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