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Re: C calling Scheme and garbage collection

From: Greg Troxel
Subject: Re: C calling Scheme and garbage collection
Date: Thu, 27 Jun 2019 15:52:24 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (berkeley-unix)

Isaac Jurado <address@hidden> writes:

> Hello,
> I'm playing with event loop libraries implemented in C (libev, libevent,
> etc... in my case libsystemd), but configuring them from Guile.
> The qsort example in the documentation [1] seems safe because the qsort C
> function directly calls back, so the callback Scheme bindings stay
> referenced (by the Scheme code calling qsort) during all the C code
> execution.
> Now, in C event loops the situation is different.  There is one call to
> configure the event callback, in which the function and data pointers are
> lent to the loop; and then there is the main loop or the single iteration
> call.
> The way I see it, suppose I add a timer.  I call one C function passing a
> (proceudre->pointer) and an (scm->pointer).  In a future time, those
> pointers will be used by the C event loop.  If a garbage collection happens
> in the middle, the results of (procedure->pointer) and (scm->pointer) may
> have been reclaimed by the time the C event loop calls back.

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.

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.

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