From: "Iñaki Baz Castillo" <ibc@...>
Date: 2012-05-31T05:51:19+09:00
Subject: [C exten] The last question about rb_gc_register_address(): a case in which in works with dynamic memory

Hi, what I understand (after some useful threads in this mailist)
about rb_gc_register_address() is that it's only valid for fixed
memory (in the stack). So if for example I have a global/static VALUE
in my MyLibrary_init() function, I should use it to avoid GC (it's not
referenced by any other Ruby object in Ruby land):

-------------------------------------------
  static VALUE string_Via;

  void MyLibrary_init(void) {
      some_string = rb_str_new2("HELLO WORLS");

      // If I don't do this, it will be GC'd:
      rb_gc_register_address(&some_string);
  }
-----------------------------------------


But I cannot use rb_global_variable(), for example, to avoid GC for a
VALUE object passed as function parameter since I'm trying to register
its *memory address*, which would get invalidated after the function
and when the variable gets out of the Ruby scope.

Ok, it makes sense. But I've found a way to make it to work and would
like to share it (to know if it's a risk or it just works in certain
cases):

-----------------------------------------
struct data_struct {
    VALUE data;
    int lalala;
    long lololo;
}

VALUE MyLibrary_some_method(VALUE self, VALUE data)
{
  struct data_struct* my_data_struct = ALLOC(struct data_struct);

  my_data_struct->data = data;

  // This WONT avoid GC:
  //  rb_gc_register_address(&data);

  // But this WILL avoid GC:
  rb_gc_register_address(&(my_data_struct->data));
}
---------------------------------------

The memory address for which I'm calling rb_register_address() is
allocated by me, and I will not free it for now. And before freeing it
I will call rb_gc_unregister_address(&(my_data_struct->data)), so it
"should" be safe.


So, could somebody confirm me that the above is a *correct* usage of
rb_gc_register_address()? As said before, in my tests it works (it
avoids GC).


BTW I would also like to know whether both rb_gc_register_address()
and rb_gc_unregister_address() can be called without the GVL.


Really thanks a lot.


PS: I'm already using rb_gc_mark() when the VALUE is attached to the C
struct wrapped within a Ruby object, that's ok. But I "need" the above
usage for the case of C data holding Ruby objects, without such a C
data being "mapped" to a Ruby object.


-- 
Iñaki Baz Castillo
<ibc@aliax.net>