From: "Sean O'Dell" <sean@...[REMOVE_THE_SPAM]>
Date: 2003-09-04T07:43:57+09:00
Subject: Re: Ruby GC eating my VALUE on the stack

Mauricio Fern�ndez wrote:
> On Thu, Sep 04, 2003 at 03:43:21AM +0900, Sean O'Dell wrote:
> 
>>I thought I had read everything I needed about the Ruby GC, but 
>>something just popped up that took me by surprise.
>>
>>I am allocating a structure using malloc in QuiXML (in C), then wrapping 
>>it in Data_Wrap_Struct, providing mark and free functions.
>>
>>Data_Wrap_Struct returns a VALUE, which is stored in a variable on the 
>>stack.
>>
>>I never leave this function; I call other functions and pass the pointer 
>>to the structure around, but the variable holding the VALUE stays on the 
>>stack the entire time.
>>
>>I am stress-testing QuiXML, so I am deliberately dogging memory to get 
>>the GC to invoke naturally.
>>
>>At some point, a rb_str_new() call causes the GC to kick in because 
>>right then, the free callback for my structure gets called.
>>
>>Now, from what I understand, since the VALUE associated with the 
>>structure is still on the stack, the GC was supposed to see it and 
>>automatically mark it so my structure won't get freed prematurely.
>>
>>So why did it not get marked?  Why was free called?
> 
> 
> Is there any difference if you declare the VALUE to be volatile?

That solved the problem!  I guess because volatile forces the variable 
onto the stack, eh?  Hmm...didn't think of that.

Thanks Mauricio!

	Sean O'Dell