Hello It seems that opening the same file multiple times (one writer and multiple readers) is not supported at least on *nix with GHC. I want to use one Handle to use append data till the end of the file while other Handles perform random access IO with seeks on the file. Sharing the same Handle for all the threads is not possible since they perform seeks and may thus mess each other up. Hiding the Handle behind a mutex would limit concurrency more than I like. Thus I wanted to open multiple Handles to the file, but this seems quite hard. My best guess is to create a function like: #ifdef mingw32_HOST_OS openUnlocked fn mode = openBinaryFile fn mode #else openUnlocked fn mode = withMVar mutex $ do h <- openBinaryFile fn mode fd <- handleToFd h unlockFile $ fromIntegral fd return h {-# NOINLINE mutes #-} mutex = unsafePerformIO $ newMVar () #endif Is there really no simpler solution? - Einar Karttunen