Re: Calling Scheme from C and Garbage Collection

[Greg Harvey <Greg.Harvey@thezone.net>]

Mikael Djurfeldt <mdj@nada.kth.se> writes:

> Greg Harvey <Greg.Harvey@thezone.net> writes:
> 
> > I really dislike the current work around, since it is, after all, not
> > much less than explicit marking.
> 
> But it is more efficient, at least given the current mark-stack system.
> Take it for what it is, an optimization internal to Guile.
> 
> > I'm hoping that there's a way to avoid this without declaring every
> > SCM value volatile.
> 
> I haven't followed this discussion.  I just jumped into it when I
> happened to see Chris' message.
> 
> I claim that this is really not a problem.

It isn't really, but at the time I was sufficiently annoyed with the
gc (stop crashing, damnit... better yet, stop putting free cells in
scm_symhash!).

> The conservative GC is safe, regardless of the intelligence of the
> compiler, as long as you follow one simple rule:
> 
> * Always refer to Scheme objects with malloced memory through their
>   SCM value.
> 
> When you aren't working with Scheme objects with malloced memory,
> there are no rules at all: It is safe.
> 
> An example of when the rule above apply is:
> 
> *Don't* write
> 
>   SCM* p = SCM_VELTS (v);
>   x = p[i];
> 
> Instead, write
> 
>   x = SCM_VELTS (v)[i];
> 
> Is this so problematic?

No, a bit annoying though, since it can crop up unexpectedly, and it
may be hard to diagnose if you don't know the details. A mention in
the documentation should be enough.

> /mdj
> 
> P. S. The reason why big2str has a problem is that it refers directly
> to malloced memory through the pointer ds.  It doesn't need to do
> this, but it is slightly more efficient.

Yep, I don't think I was really clear in explaining what was required
to invoke the problem.

-- 
Greg