From: Eric Hodel <drbrain@...7.net>
Date: 2012-05-18T07:23:51+09:00
Subject: Re: rb_gc_register_address() or rb_gc_mark()?

On May 17, 2012, at 12:24, Iņaki Baz Castillo wrote:
> 2012/5/17 Iņaki Baz Castillo <ibc@aliax.net>:
>> Ok, after some experiment I'm confused:
>> 
>> - I run GC.startt every 0.1 seconds.
>> 
>> - While my_class instance (which wraps the C struct referencing
>> my_string VALUE) remains in the scope (or stored in a global variable)
>> my_string is NOT GC'd. I know it because I've stored its object_id and
>> used it in ObjectSpace._id2ref(id) and it returns my_string.
>> 
>> So... even if I've seen the usage of rb_gc_mark() in some C
>> extensions, in my experiments it does nothing. Any help would be
>> appreciated.
> 
> Forget it please! I was storing my_string in a class attribute :(
> 
> Yes, rb_gc_mark() stuf is fully understood. Now I will investigate
> what rb_gc_(un)register does, since in my experiments it does NOTHING.

Correct.

You use rb_gc_mark() to mark ruby objects pointed-to by C structs that are wrapped as ruby objects.

If you have

struct my_struct {
  VALUE my_string;
}

And wrap that with Data_Wrap_Struct, then you must provide a mark function that tells ruby about your string inside your struct (because ruby does not otherwise know it exists).

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.