problems with semaphores in ghc-6.10.4 unix-2.3.2.0

module Main ( main ) where

import System ( getArgs )
import Control.Monad
import Data.List
import System.Posix.Semaphore
import System.Posix.Files

main = do
  c <- liftM (head . head) getArgs
  putStrLn $ "Writing " ++ [c] ++ " to file foo"
  s <- semOpen "fooSemaphor" (OpenSemFlags True False) ownerModes 1
  semWait s
  appendFile "foo" $ replicate 99999999 c
  semPost s
  semUnlink "fooSemaphor"
  putStrLn $ "Done writing " ++ [c]

When called like this "./locking2 a & ./locking2 b" I get:

locking2: fooSemaphor: semUnlink: does not exist (No such file or directory)
locking2: error: a C finalizer called back into Haskell.
   This was previously allowed, but is disallowed in GHC 6.10.2 and later.
   To create finalizers that may call back into Haskll, use
   Foreign.Concurrent.newForeignPtr instead of Foreign.newForeignPtr.

I dont know what this means, but it looks as if some things are going wrong internally.