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Re: C calling Scheme and garbage collection


From: Isaac Jurado
Subject: Re: C calling Scheme and garbage collection
Date: Fri, 28 Jun 2019 13:07:53 +0200


On Thu, Jun 27, 2019 at 9:52 PM Greg Troxel <address@hidden> 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.
 
Forcing gc is not going to be reliable.   If you have a reliable scheme,
gc can happen at any random time and things will be ok.

I prepared a minimal case of the kind of C interactions that I'm trying.  I'm attaching the files, the C code has to be compiled with:

gcc -shared -fPIC -o mysalsa.so mysalsa.c

Running the Scheme script yields something like the following:

Captured: Closure without collection
Argument: noitcelloc tuohtiw erusolC
Captured: Closure with garbate collected
Argument: 

So primitive values seem to be garbage collected, but closures are treated slightly differently.  This is very interesting, since working with closures eliminates the need of those common "void *userdata" extra arguments.

Testing continuation is going to be interesting too.

Best regards.

--
Isaac Jurado

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

Attachment: mysalsa.c
Description: Text Data

Attachment: mysalsa.scm
Description: Text Data


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