From: David Balmain <dbalmain.ml@...>
Date: 2006-09-21T18:45:08+09:00
Subject: Re: Creating a reference to a ruby variable in a C extension

On 9/21/06, Christer Sandberg <sillen@init.se> wrote:
> David Balmain wrote:
> > On 9/21/06, Christer Sandberg <chrsan@gmail.com> wrote:
> >> Vincent Fourmond wrote:
> >> > Hello !
> >> >
> >> >> Then later on I wan't to use it like this:
> >> >>
> >> >> void call(void)
> >> >> {
> >> >>   rb_funcall(ref, rb_intern("some_method"), 0, 0);
> >> >> }
> >> >
> >> >   How comes this function has no parameters ? Is it being called from
> >> > ruby, or only from C code ?
> >> >
> >>
> >> Thank you for the reply, I'll try to clear things up a bit and explain
> >> what I want to do.
> >>
> >> The function is just an example for the sake of making my question a bit
> >> clearer. The real functions I am using has parameters. Those functions
> >> such as the one above will only be called from C code.
> >>
> >> >
> >> >   I tend to think that using global variables is not a really good
> >> idea.
> >> >  We need a bit more context about what you want to do. Is it a class ?
> >> > Will it have several instances ?
> >> >
> >> >   In the latter case, have a look at rb_iv_get and rb_iv_set to get/set
> >> > instance variables of a class instance, something like
> >> >
> >> >   rb_iv_set(self, rb_intern("@variable"), arg); /* in init(...) */
> >> >
> >> >   and
> >> >
> >> >
> >> >
> >> rb_funcall(rb_iv_get(self,rb_intern("@variable")),rb_intern("some_method"),
> >>
> >> > 0, 0); /* in call */
> >> >
> >>
> >> Yes, I'm writing a tiny Ruby class in C that wraps and uses a bison/flex
> >> generated parser. The saved variable reference will be used for making
> >> calls on the ruby instance. When those calls are made self is long gone
> >> out of scope sice it's the parser who triggers the method call. The C
> >> Ruby class just acts as a proxy or adapter between the parser and the
> >> Ruby object.
> >
> > Hi Christer,
> >
> > Instead of saving it as a global variable, you could save it as a
> > class variable. See rb_cvar_[gs]et (defined in variable.c). Just an
> > idea.
> >
> > Cheers,
> > Dave
> >
>
> Hi Dave!
>
> That would work if the passed in object should be shared among all
> instances of my class, but that's not the case since it's a callback
> object that's supplied for a particular instance of my class.
>
> Correct me if I'm wrong but the variable is only global in my C code
> since I haven't made it visible to the Ruby environment with a
> rb_gv_set(const char *name, VALUE value)?
>
> Thanx
> Christer

Hi Christer,

Right you are. I can't believe I haven't looked at this method before.
rb_global_variable simply registers the object so that it won't be
garbage collected.  Incidentally for those interested,
rb_global_variable is really just an alias for rb_gc_register_address.
If you later want to let the object to be garbage collected you just
need to call rb_gc_unregister_address(&value). I wish I knew about
this a couple of months ago.

Anyway, now that I know this I think the way you are going about
solving the problem is the correct one.

Thanks,
Dave