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Re: [Chicken-users] newbie: foreign pointers that depend on Scheme objec

From: Alejandro Forero Cuervo
Subject: Re: [Chicken-users] newbie: foreign pointers that depend on Scheme objects
Date: Thu, 18 Dec 2008 14:52:52 +0100
User-agent: Mutt/1.5.13 (2006-08-11)

> To retain a safe reference to a Scheme-level object from C
> can only be done via GC-roots, otherwise the reference will be lost on
> the next GC.

What happens if your Scheme-level object is only referenced (through a
GC-root) from the C-level object and the C-level object is *only*
referenced from the Scheme-level object?  Do we not leak memory?

> Of course, any references to C data (which may refer to Scheme data
> via GC-roots) are not seen by the GC or finalization mechanism, so
> will not "loop" into that direction.

You mean "any references inside C data", right?  I see what you're

I think we should have some structure "weaker" than GC-root that
serves this purpose of allowing C-level objects to reference
Scheme-level objects but which doesn't have the "never GC this"
semantics.  The GC-roots should use this structure but also have the
stronger property of never being collected, even if they are not
reachable from Scheme-level objects.

The way I think it would make sense to do this would be the following.
If I receive a c-pointer to some structure that can hold a pointer, I
use a record that wraps the c-pointer and *always* refer to it through
the structure.  Then, when I set something inside of the c-pointer to
point to a Scheme-object foo, I:

1. Create a new instance of the structure-weaker-than-a-GC-root, set
it to foo and store a pointer to that in the C data.

2. Set an attribute in the record to foo, so that the GC code will
know that it can't collect foo yet.

What do you think?


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