From: Eric Hodel <drbrain@...7.net>
Date: 2012-05-18T08:09:59+09:00
Subject: Re: rb_gc_register_address() or rb_gc_mark()?

On May 17, 2012, at 15:34, Iņaki Baz Castillo wrote:
> 2012/5/18 Eric Hodel <drbrain@segment7.net>:
>> To do this you use rb_gc_mark(the_struct->my_string).  Without calling rb_gc_mark the garbage collector may reclaim your string which can lead to a crash the next time you try to use it.
> 
> Sure, it's fully understood now, and in fact, without the mark crashes occur.
> 
> Now my other question is about the undocumented
> rb_gc_register_address(VALUE object). As per my experience, this is
> only valid for VALUE created within ruby code, and not for Ruby
> objects that run in Ruby land. For example, in my C extension
> XXXXX_init() function I create a Ruby String which I use within my C
> code:
> 
>  VALUE string_aaa = rb_str_new2("aaa");
> 
> So I must add:
> 
>  rb_gc_register_address(&string_aaa);
> 
> Otherwhise it's GC'd. But if I try to do the same with a previously
> existing Ruby object (created in Ruby land or referenced in Ruby land)
> then rb_gc_register_address does nothing. For example:
> 
> VALUE MyClass_do_domething(VALUE self)
> {
>  rb_gc_register_address(&self);
>  return self;
> }
> 
> This does NOTHING. The object (self) will be GC'd as usual.

rb_gc_register_address is for use with fixed memory addresses like:

  static VALUE string_aaa;

Using it on pointers to the heap or stack is going to result in undefined behavior.