From: Peter Zotov <whitequark@...>
Date: 2009-06-14T22:41:00+09:00
Subject: Re: C extension: VALUEs being collected too early

eden li wrote:
> I'm writing a C extension which passes around structs that refer to
> Ruby VALUEs.  During the course of execution, these VALUEs are
> assigned new Ruby strings, hashes or arrays (via the rb_{str,hash,ary)
> _new functions) through a series of callbacks.  The problem is that it
> seems they're getting collected too early before I have a chance to
> use the result.
> 
> I tried a few things to keep these VALUEs around (including calling
> rb_gc_mark directly on these objects), but the only thing that seems
> to prevent this early collection is by disabling the GC while my code
> is executing.
> 
> Is there something I can use to prevent these VALUEs from getting re-
> used without resorting to turning off the GC altogether?
> 
> Here's some pseudo code to help further illustrate the issue:
> 
> typedef struct {
>   VALUE value;
>   // some other fields
> } MyStruct;
> 
> typedef struct {
>   MyStruct **structs;
> } MyOtherStruct;
> 
> VALUE my_rb_func(VALUE self) {
>   MyOtherStruct *other;
>   Data_Get_Struct(rb_iv_get(self, "@other"), MyOtherStruct, other);
>   other->structs[0]->value = rb_hash_new();
> 
>   // This chain will allocate new MyStruct references and assign
>   // new Ruby objects to MyStruct->value entries.  The rb_hash_new()
>   // won't be affected other than using rb_hash_aset().
>   start_callback_chain(other);
> 
>    // without disabling gc, other->structs[0]->value is *not* the
> original
>    // hash this is true even if we don't assign anything to subsequent
>    // MyStruct->value
> }
> 

Why do not use ruby array (rb_array_new) as MyOtherStruct?
This may help (just need to keep one object instead of bunch of them...)

-- 
   WBR, Peter Zotov