From: "Brian Schröder" <ruby.brian@...>
Date: 2005-08-23T23:09:18+09:00
Subject: Re: Very strange GC behaviour

On 23/08/05, Geert Fannes <Geert.Fannes@ikanconsulting.com> wrote:
> Hello, can someone explain why GC fails to release "a" in the program
> hereunder. When I exchange the lines "a=A.new" and "b=B.new", GC fails
> to release b.
> 
> 
> 
> PROGRAM
> 
> #create some dummy classes
> 
> class A;end
> 
> class B;end
> 
> 
> 
> #create two variables, one for each dummy class
> 
> a=A.new
> 
> b=B.new
> 
> 
> 
> #store the object_id's for these object to check if these are still
> present
> 
> bid=b.object_id
> 
> aid=a.object_id
> 
> 
> 
> #check which objects can still be found
> 
> ObjectSpace.each_object{|obj|puts("a still found") if
> obj.object_id==aid;puts("b still found") if obj.object_id==bid}
> 
> puts("")
> 
> 
> 
> #set both objects to nil and do garbage collection
> 
> b=nil
> 
> a=nil
> 
> GC.start
> 
> 
> 
> #check again which object are still present. Both a and b should be
> gone, but that is not the case
> 
> ObjectSpace.each_object{|obj|puts("a still found") if
> obj.object_id==aid;puts("b still found") if obj.object_id==bid}
> 
> 
> 
> 
> 
> OUTPUT:
> 
> gfannes@ivon gruby $ ruby test.rb
> 
> b still found
> 
> a still found
> 
> 
> 
> a still found
> 
> gfannes@ivon gruby $
> 
> 
> 
> Greetings,
> 
> Geert
> 
> 
> 

The Garbage Collector is conservative. If the address of an object
stays in some register, or even some calculation filled a register
with a number that happens to equal the address, the pointed-to object
will not be gc'ed.

regards,

Brian

-- 
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