Ivan Tomac wrote:
It appears that if I add
import Control.Concurrent
and call yield just after performGC then the finalizer does get called.
But it only seems to work if I call both performGC and yield and in that
order.
There is no guarantee that a finalizer will be run before your program exits.
The only thing you can be sure of is that it will *not* run while the ForeignPtr
is still reachable by the garbage collector. In practice GHC will schedule the
finalizer to run as soon as the GC detects that the ForeignPtr is unreachable.
There will be a lag (possibly a very long lag) between the time at which the
ForeignPtr becomes unreachable and the time at which a major GC is performed,
and a further lag between the GC scheduling the finalizer and the finalizer
actually being run. Furthermore, GHC will allow the program to exit without
running all the outstanding finalizers, which is why your finalizer might not
necessarily run.
Is this normal and if so is it documented anywhere?
It's documented in GHC's System.Mem.Weak module, which is used to implement
ForeignPtrs in GHC. I'll add some documentation to ForeignPtr too.
Can this behavior be relied upon in future versions of GHC?
You can rely on the lack of guarantees for as long as you like :-)
Cheers,
Simon
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