From: "Iñaki Baz Castillo" <ibc@...>
Date: 2012-05-18T00:07:25+09:00
Subject: rb_gc_register_address() or rb_gc_mark()?

Hi, I've bad experiences with rb_gc_register_address(), it does never
work for me so I end using Ruby hashes and so.

Well, for my Ruby class defined in C I set a "alloc" method in which I
malloc and wrap a C struct:


------------------------------------
  struct my_cdata {
    int active;
    uv_udp_t *uv_handle;
  }


  VALUE MyClass_alloc(VALUE klass)
  {
    // Fill C struct:
    struct my_cdata *cdata = ALLOC(struct my_cdata);
    cdata->active = 0;
    cdata->uv_handle = ALLOC(uv_udp_t);

    // Get the created MyClass instance:
    VALUE instance = Data_Wrap_Struct(klass, NULL, MyClass_free, cdata);

    // Store the instance within the uv_handle->data field:
    cdata->uv_handle->data = instace;

    return instance;
  }
-----------------------------------------


When the UV UDP handle object receive data, UV async library calls a
callback passin gthe UV UDP handle, so in that callback I need to
retrieve the Ruby MyClass instance from the uv_handle:

  VALUE instance = uv_handle->data;
  // Run the Ruby callback and so...
  rb_funcall(instance, xxxxxx, xxxxxx, xxxxxx);


Assuming that the created MyClass instance is not referenced in Ruby
land (it's not in a hash, array, variable...) then this code would
crash since probably GC has freed "instance". So there are (or could
be) 3 solutions:


1) Storing the instance in a Ruby hash (and yes, it works, but it's ugly).

2) In the MyClass_alloc() function, before the "return" add:
      rb_gc_register_address(&instance);
    But this DOES NOT work for me, I get segmentfauls !

3) Setting a mark function in Data_Wrap_Struct, and within such a mark
function do:
      rb_gc_mark(uv_handle->data);


I've read point 3 in this thread:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/147514


So, is the choice 2 wrong and choice 3 is correct?

Thanks a lot.



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