[gclist] Boehm's GC and multi-threaded libraries

Dean Brettle brettle@picard.jmb.bah.com
Wed, 22 Jan 1997 11:40:10 -0500

Hans Boehm wrote:
<stuff deleted>
> 2. Arrange that the collector is only invoked from the one active thread that
> might have pointers to garbage collected memory.  It sounds to me like
> Dean's observation to avoid REDIRECT_MALLOC is close to sufficient in this
> case.
> You do have to be careful that you do not pass essential pointers to garbage
> collected data to the library, if the library expects the data to outlive
> the call.  I would guess that's easy in this case.
<stuff deleted>

Thanks for the help.

I am still somewhat confused by the warning concerning passing essential
pointers to the library.  Assuming:

1. for each call to the library, the threads created do not outlive the
call (i.e. there is only one thread when the call completes).  For
ImageVision, I _think_ this is the case for all but the methods
beginning with q.

2. the GC is not called within the library (because I haven't done a

Why would there be a problem with passing pointers to GC'd memory into
the library, even if the library expects the data to outlive the call? 
What is it about multi-threading that makes this a problem?  Or, is the
problem due to the fact that we want to do _incremental_ GC?  

It seems like, with the above assumptions, the GC shouldn't be able to
tell that the library is multi-threaded.  Am I missing something, or was
your response based on different assumptions?  If I am missing
something, is there a simple example or reference that would clear
things up?

Thanks again,