I just tried building the following program with the new GHC win64_alpha1 and apart from warnings from using the unsupported stdcall calling convention running the program doesn't give a segmentation fault as it does when building the program with GHC-7.4.2: {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Foreign.C.Types main :: IO () main = do ctxPtr <- alloca $ \ctxPtrPtr -> do _ <- c'libusb_init ctxPtrPtr peek ctxPtrPtr fp <- newForeignPtr p'libusb_exit ctxPtr threadDelay 1000000 print $ fp == fp data C'libusb_context = C'libusb_context foreign import stdcall "libusb_init" c'libusb_init :: Ptr (Ptr C'libusb_context) -> IO CInt foreign import stdcall "&libusb_exit" p'libusb_exit :: FunPtr (Ptr C'libusb_context -> IO ()) Regards, Bas