Re: scheme closures: crash during garbage collection

From: Marius Vollmer <mvo@zagadka.de>
Cc: guile-devel@gnu.org
Subject: Re: scheme closures: crash during garbage collection
Date: Sat, 08 Jul 2006 18:06:37 +0300	[thread overview]
Message-ID: <87u05s2l1e.fsf@zagadka.de> (raw)
In-Reply-To: <87odwyt33x.fsf@ossau.uklinux.net> (Neil Jerram's message of "Mon, 12 Jun 2006 23:34:42 +0100")

Neil Jerram <neil@ossau.uklinux.net> writes:

>> Guile wants you to integrate your objects with its mark/sweep
>> approach, by providing appropriate smob marking functions, for
>> example.
>
> If I've understood correctly, this isn't possible in Gregory's
> scenario.
>
> (See
> http://lists.gnu.org/archive/html/guile-gtk-general/2006-06/msg00013.html
> if you didn't see the whole description on guile-gtk-general already.)

I think this (and also the problem of reference loops that easily form
over widgets and signal handlers) has been successfully solved in the
guile-gtk bindings of yore: http://www.gnu.org/software/guile-gtk/

Here is the comment describing the wrapping strategy for GObjects:

/* GtkObjects.

   GtkObjects are wrapped with a smob.  The smob of a GtkObject is
   called its proxy.  The proxy and its GtkObject are strongly
   connected; that is, the GtkObject will stay around as long as the
   proxy is referenced from Scheme, and the proxy will not be
   collected as long as the GtkObject is used from outside of Scheme.

   The lifetime of GtkObjects is controlled by a reference count,
   while Scheme objects are managed by a tracing garbage collector
   (mark/sweep).  These two techniques are made to cooperate like
   this: the pointer from the proxy to the GtkObject is reflected in
   the reference count of the GtkObject.  All proxies are kept in a
   list and those that point to GtkObjects with a reference count
   greater than the number of `internal' references are marked during
   the marking phase of the tracing collector.  An internal reference
   is one that goes from a GtkObject with a proxy to another GtkObject
   with a proxy.  We can only find a subset of the true internal
   references (because Gtk does not yet cooperate), but this should be
   good enough.

   By using this combination of tracing and reference counting it is
   possible to break the cycle that is formed by the proxy pointing to
   the GtkObject and the GtkObject pointing back.  It is
   straightforward to extend this to other kind of cycles that might
   occur.  For example, when connecting a Scheme procedure as a signal
   handler, the procedure is very likely to have the GtkObject that it
   is connected to in its environment.  This cycle can be broken by
   including the procedure in the set of Scheme objects that get
   marked when we are tracing GtkObjects with a reference count
   greater than the number of internal references.

   Therefore, each proxy contains a list of `protects' that are marked
   when the proxy itself is marked.  In addition to this, there is
   also a global list of `protects' that is used for Scheme objects
   that are somewhere in Gtk land but not clearly associated with a
   particular GtkObject (like timeout callbacks).

  */

-- 
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