Guarantees regarding finalizers
marlowsd at gmail.com
Tue Dec 1 07:13:03 EST 2009
On 01/12/2009 11:25, Mietek Bąk wrote:
>> The docs are rather backward. Foreign.ForeignPtr finalizers are guaranteed
>> to run, Foreign.Concurrent finalizers are not. I'll fix the docs, thanks
>> for the report.
> Thanks for the reply. Unfortunately, I'm running into the following
> error message when trying to use a Foreign.ForeignPtr finalizer:
> "foo: error: a C finalizer called back into Haskell.
> This was previously allowed, but is disallowed in GHC 6.10.2 and later.
> To create finalizers that may call back into Haskll, use
> Foreign.Concurrent.newForeignPtr instead of Foreign.newForeignPtr."
> Is there a technical reason why the Foreign.Concurrent finalizers
> aren't guaranteed to run?
It's hard to make it work reliably, and arguably finalizers shouldn't be
used in this way .
Haskell finalizers are run in separate threads, but separate threads
don't normally prevent the Haskell program from exiting, so there would
be an inconsistency if we made a special case for finalizers. Another
problem is that you often find you need to express dependencies between
finalizers, but that is not supported (the finalizers all run
concurrently), and even if it were supported, it would probably be
necessary to run the GC repeatedly until we run out of finalizers.
 "Destructors, Finalizers, and Synchronization", Boehm, Hans-J.
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