From: Peter Zotov <whitequark@...>
Date: 2011-02-06T08:34:31+09:00
Subject: Re: C extension: How to check if a VALUE is still alive (not being GC'ed)?

On Sun, 6 Feb 2011 08:02:39 +0900
Iñaki Baz Castillo <ibc@aliax.net> wrote:

> Hi, I'm coding an async DNS resolver for EventMachine based on udns (a
> C library).
> 
> In the Ruby C extension, when submitting a DNS query I pass the
> current object (VALUE pointer) as an argument of the DNS query C
> function.
> Later on, when the DNS server replies, EventMachine (which watches on
> the UDP socket of udns) invokes the C callback function belonging to
> such query, and that callback includes the given VALUE as one of its
> arguments. So I invoke a Ruby method for such VALUE (rb_funcall).
> 
> The problem is that the VALUE could be GC'ed (or being marked for so)
> if the programmer assigns it to "nil" before the callback is executed,
> so we get a coredump.
> 
> So when the callback is executed I need to check that the given VALUE
> is still alive (not GC'ed neither marked for being GC'ed). How can I
> inspect that? (I mean in C).
> 

The easiest way would be to assign this pointer to the resolver object
as an instance variable (with rb_iv_set), or perhaps store it inside a
hash which maps request ID's to pointers, if you want to be able to use
the same resolver for several simultaneous callbacks.

-- 
  WBR, Peter Zotov