[Haskell-cafe] FFI woes!

Sebastian Sylvan sebastian.sylvan at gmail.com
Sat Dec 18 04:32:38 EST 2004

On Thu, 16 Dec 2004 11:08:13 -0500, Robert Dockins
<robdockins at fastmail.fm> wrote:

> > -- play a sample
> > samplePlay :: SoundSample -> IO SamplePlayback
> > samplePlay sample = do ch <- withForeignPtr sample (fsound_PlaySound (-1))
> >                      let spb = SP ch sample -- SamplePlaybackRaw
> >                        mkWeakPtr spb (Just (samplePlaybackFinalizer spb))
> Ahh... look.  You are returning a weak pointer!  Those specifically
> allow the referenced object to be garbage collected.  So the only
> references left to your SamplePlaybackRaw are (a) inside a weak pointer
> and (b) inside a finalizer closure.  Such references are NOT considered
> reachable from the root set, and can thus be GCed (I am about 90% sure
> this is true about finalizers).  You have arranged it so that the RTS
> can garbage collect spb, and thus the ch and sample, whenever it likes.

Well that shouldn't affect the functionality. The weak pointer was
only a way of attatching a finalizer to the Playback object. It is
true that I should probably wrap up the the SoundPlaybackRaw inside
the SoundPlayback as well, to save CPU, but it shouldn't matter for
the core functionality.

It appears weak pointers don't really run the finalizer when I want
them to run (they run too late!). Or maybe it's the finalizer for the
ForeignPtr to the sound data which runs to early (I create it using
Nevertheless, i couldn't get it to work when doing "strong" pointers
either (StablePtr, cast to a Ptr, remove StablePtr, create ForeignPtr
from Ptr and attach a finalizer).

I really don't like the idea of having a background loop running all
the time (I'd rather spawn one just before the GC tries to clean up
the Playback). If I were okay with that I'd just spawn a new one for
each Playback, which would be considerably cleaner I think than having
a single loop with some sort of master database of playbacks...


Sebastian Sylvan
UIN: 44640862

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