I use the Posix library since I have to communicate via a pipe with another UNIX process. Therefore I have to use fdRead :: Fd -> ByteCount -> IO (String,ByteCount) The problem is that "fdRead" , as a non Haskell-IO sub-system function, seems to block the entire STG system. The rest of the other threads are blocked too. BTW, "String" stands for non-printable bytes I can be sent for a system, '\0' included ? Alternatives: 1 .- Polling. Sleep and ask. ================================ You have to use the setFdOption fd NonblockingRead True fdReadNonBlockingSTG :: Fd -> Int -> IO (String,ByteCount) fdReadNonBlockingSTG mypipe nobytes = do (string,nobytesread) -> fdRead mypipe nobytes if (nobytesread == -1) then do threadDelay 10000 fdReadNonBlockingSTG mypipe nobytes else return (string,nobytesread) myprogram :: IO () ... setFdOption mypipe NonBlockingRead True (a,b) <- fdReadNonBlockingSTG mypipe 2 ... Problem: As you see, "1000" es an arbitrary constant maybe too short, may be too long and this affects throughput. 2.- Desired approach ======================= Ideally I would like to use the threadWaitRead :: Int -> IO() from Concurrent Haskell as ... -- setFdOption mypipe NonBlockingRead True -- should be called? threadWaitRead (fdToInt mypipe) (string,nobytes) <- fdRead mypipe 2 ... But... QUESTIONS ========== A ) is the GHC-RTS ready to dispatch the signal send by the O.S on a "Posix Fd" and make "threadWaitRead" to run as expected ? The Posix library doc warns: intToFd :: Int -> Fd -- use with care fdToInt :: Fd -> Int -- ditto B) Other approaches, like Handles... Is it possible to convert a Fd into a Handle or bypassing it to the RTS another way ?...