From: Christer Sandberg <sillen@...>
Date: 2006-09-21T19:00:16+09:00
Subject: Re: Creating a reference to a ruby variable in a C extension

David Balmain wrote:
> 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
> 

Alright...

Didn't know about that method (rb_gc_unregister_address(&value);), 
thanks Dave!

Christer