Re: C calling Scheme and garbage collection

[Isaac Jurado <diptongo@gmail.com>]

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On Sat, Jun 29, 2019 at 7:44 PM Greg Troxel <gdt@lexort.com> wrote:

> Isaac Jurado <diptongo@gmail.com> writes:
>
> > On Thu, Jun 27, 2019 at 9:52 PM Greg Troxel <gdt@lexort.com> wrote:
> >
> >> I have been down this path before, with guile and with lua.  Basically,
> >> if C (or non-scheme) has a pointer to a scheme object, then you need to
> >> hold a logical reference for it and protect the scheme object, and when
> >> the C pointer is dropped decrease the refcnt.
> >>
> >> I am unclear on the details of how you have a ref that gc is made aware
> >> of.  One way is to have a scheme array of the object and a count, and
> >> have the code null out the object when the count goes to zero or
> >> something like that.  But the point is that you need to have  a proxy in
> >> the scheme world, visible to gc, when a pointer to a scheme object is
> >> held outside of the scheme world.
> >>
> >
> > That's more or less what I had in mind, although instead of an array I
> > would use a hash table indexed by a fundamental type (e.g. integer) which
> > can be converted painlessly between Scheme and C.
>
> Sure - it just needs to be something the gc will find.
>

My intention was quite the opposite, use something that the GC will NOT
find, like 32/64 bit integers.  I assumed that (pointer->procedure) and its
inverse would convert from/to uint32, uint64, etc. natively, without
creating GC objects.  This way I can safely give a way such a value to the
C code, while using it as a key to a Scheme hashmap.


> Just because something wasn't collected doesn't mean it is safe.  You
> don't actually know that the closure wasn't garbage collected, just that
> when you used it the bits were still there.  You might try running under
> valgrind.  Or modify guile to clear all objects that are garbage
> collected, which I'm guessing it doesn't do.
>

Agreed, just like the old use after free().


> In my experience, it's definitely not ok to capture a pointer to a
> scheme object and store it for later use without protecting the scheme
> object from gc by holding a reference.
>

It's the sort of advice I was seeking to confirm, so thanks :-)

Best regards.

-- 
Isaac Jurado

"The noblest pleasure is the joy of understanding"
Leonardo da Vinci

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