From: nobu.nokada@...
Date: 2002-10-22T10:02:50+09:00
Subject: Re: rb_gc_register_address problem

Hi,

At Tue, 22 Oct 2002 03:54:24 +0900,
Paul Brannan wrote:
> We ran into a problem today with the garbage collector (caused by our
> own bug).  We had a function that did something like this:
>   std::vector<char *> foo;
>   void func(int argc, VALUE * argv, VALUE self) {
>     foo.push_back(STR2CSTR(argv[0]));
>     rb_global_variable(&argv[0]);
>   }
> 
> This is a bug, because once the function returns, argv[0] is invalid,
> and global_List contains an invalid VALUE* that may or may not point to
> the VALUE we want to keep global.  When the GC tries to mark this
> VALUE*, it ends up marking an invalid object.

If this "func" will be called from ruby as a method, you don't
need to call rb_global_variable() here.

> So my question is: why does rb_gc_register_address() store VALUE*'s
> instead of VALUE's?  I suppose that if it stored a VALUE, then it would
> be possible to register an object as a global but not keep a reference
> to that object; this may not be desirable.  In our case, though, we just
> wanted the string to remain valid.

And introduce memory bugs?  Just store the VALUE in a local
variable instead.

-- 
Nobu Nakada