Segmentation fault/access violation in generated code

[Bas van Dijk]

On 23 June 2012 02:40, Ian Lynagh <igloo at earth.li> wrote:
>
> Hi Bas,
>
> On Sun, Jun 17, 2012 at 05:11:35PM +0200, Bas van Dijk wrote:
>>
>> module Main where
>>
>> import Foreign
>> import qualified Foreign.Concurrent as FC
>> import Control.Concurrent
>> import Bindings.Libusb.InitializationDeinitialization
>>
>> main :: IO ()
>> main = do
>>   ctxPtr <- alloca $ \ctxPtrPtr -> do
>>               _ <- c'libusb_init ctxPtrPtr
>>               peek ctxPtrPtr
>>
>>   fp <- newForeignPtr p'libusb_exit ctxPtr
>>   -- fp <- FC.newForeignPtr ctxPtr $ c'libusb_exit ctxPtr
>>
>>   threadDelay 3000000
>>   print $ fp == fp
>
> What happens if you just call
>    c'libusb_exit ctxPtr
> at the end, instead of using a finalizer?

Then I don't get an error. So executing the following program with the
argument "fp" gives an error (although this time I don't get an access
violation but instead Windows pops up a dialog box indicating an
error) and without an argument I get no error:

{-# LANGUAGE ForeignFunctionInterface #-}

module Main where

import Foreign
import Foreign.C.Types
import Control.Concurrent
import System.Environment

main :: IO ()
main = do
  ctxPtr <- alloca $ \ctxPtrPtr -> do
              _ <- c'libusb_init ctxPtrPtr
              peek ctxPtrPtr

  args <- getArgs
  case args of
    ["fp"] -> do
      fp <- newForeignPtr p'libusb_exit ctxPtr
      threadDelay 1000000
      print $ fp == fp

    _ -> c'libusb_exit ctxPtr

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 ())

foreign import stdcall "libusb_exit" c'libusb_exit
  :: Ptr C'libusb_context -> IO ()

> Are you able to reproduce this with just a .c file that is compiled and
> linked with the program, rather than needing libusb? That would make it
> easier to reproduce, to understand, and to add a test.

I'm not sure how to do this. I guess just copying the libusb .c and .h
files to the same directory as the .hs file is not enough since libusb
requires a configure phase.

Regards,

Bas