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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.