How wrong is it exactly? Did not get a reply on beginners@haskell.org import Control.Exception import Control.Monad.ST import Control.Monad.ST.Unsafe import Data.Word import Foreign.C.String import Foreign.C.String import Foreign.C.Types import Foreign.Marshal import Foreign.Ptr import Foreign.Storable foreign import ccall unsafe "strerror_r_portalbe.c strerror_r_portable" c_strerror_r :: CInt -> Ptr CChar -> CSize -> IO CInt memSet :: Ptr a -> Word8 -> Word32 -> IO (Ptr a) memSet ptr _ 0 = return ptr memSet ptr byte size = do let voidptr = castPtr ptr :: Ptr Word8 acts = map (\i -> pokeByteOff voidptr i byte) [0 .. fromIntegral (size - 1)] mapM_ id acts return ptr strError :: Int -> String strError errnum = runST $ unsafeIOToST ioString where baseSize :: CSize baseSize = 50 ioString = run baseSize run :: CSize -> IO String run size | size > 100000 = throwIO $ userError $ "!!! INTERNAL strError memory leak detected, " ++ "you can not recover !!!" | otherwise = do may <- tryIOString size case may of Just str -> return str Nothing -> run (size + baseSize) tryIOString :: CSize -> IO (Maybe String) tryIOString size = allocaBytes (fromIntegral size) $ \ptr -> do zeroptr <- memSet ptr 0 (fromIntegral size) st <- c_strerror_r (fromIntegral errnum) ptr size -- heuristic case st of 22 -> return . return $ "Unknown error " ++ show errnum 34 -> return Nothing _ -> peekCAString ptr >>= return . return main = mapM_ (putStrLn) $ map strError [1 .. 1000] The C file is #define _POSIX_C_SOURCE 200112L #include int strerror_r (int, char *, size_t); int strerror_r_portable (int e, char *c, size_t s) { return strerror_r (e, c, s); } By default GHC picks up GNU version of strerror_r(), I want to pick up the default portable version. I do not know how to make GHC understand my desires.