Hi - When I use -O2 to compile, gcc gives a compilation error: too few arguments to function "duma_releaseFont" When I look at the ghc output with -ddump-cmm, this seems to be where the bug is: r2MP_entry() { // SNIP - all kinds of stuff here call "ccall" suspendThread((BaseReg, NoHint))[(_c36U, NoHint)]; call "ccall" duma_releaseFont()[(_c36V, PtrHint)]; -- @@@@ MISSING ARG call "ccall" resumeThread((_c36U, NoHint)); // SNIP more stuff jump stg_gc_enter_1; } So ghc is not using supplying an argument when it should be supplying duma_releaseFont with the raw pointer contained inside the ForeignPtr being garbage collected. This also explains why the program crashes even with native compilation (debug build). So the bug is not that it is supplying a NULL arg but that it is not supplying any arg in the first place. I've also tried using addForeignPtrFinalizer instead but this doesn't help: GHC is determined to not give the finalizer any argument. createFont name pointSize bold italic underline = block $ do fRaw <- (withCString name $ \cname -> duma_createFont cname pointSize bold italic underline) f <- newForeignPtr_ fRaw addForeignPtrFinalizer duma_releaseFont f -- no luck either return $ Font f Regards, Brian.