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Re: smob mark functions in 2.0

From: Andy Wingo
Subject: Re: smob mark functions in 2.0
Date: Sat, 27 Sep 2014 17:29:02 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

On Fri 26 Sep 2014 06:50, Mark H Weaver <address@hidden> writes:

> Mark H Weaver <address@hidden> writes:
>> Andy Wingo <address@hidden> writes:
>>> Specifically, there is a warning in gc/gc_mark.h:
>>>     /* WARNING: Such a mark procedure may be invoked on an unused object    
>>> */
>>>     /* residing on a free list.  Such objects are cleared, except for a     
>>> */
>>>     /* free list link field in the first word.  Thus mark procedures may    
>>> */
>>>     /* not count on the presence of a type descriptor, and must handle this 
>>> */
>>>     /* case correctly somehow.                                              
>>> */
>>> So, your mark function might see freed objects.
>> How can this happen?

This can happen if the object is on a free list.  Free lists are just
normal heap-managed data structures -- traced by the GC, counting as
live.  See e.g. gc-inline.h in master; a variation of this exact code is
used internally in libgc by the allocation API.


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