Hi all! I have found a simple program on the web: --code begin: copierer.hs module Main (main) where main = interact id --code end I compiled it with ghc -threaded --make copierer.hs If i start it from a terminal,it behaves like the "cat" program without arguments: simply copies the stdin to stdout line by line. I wanted it to use from another Haskell program as subprocess: --code begin: twowaysubprocesscomm.hs module Main where import System.IO import System.Process import Control.Concurrent main :: IO () main = do (hin,hout,p) <- start_subprocess send_and_receive (hin,hout) "boo" send_and_receive (hin,hout) "foo" terminateProcess p start_subprocess :: IO (Handle,Handle,ProcessHandle) start_subprocess = do -- (hin, hout, _, p) <- runInteractiveProcess "cat" [] Nothing Nothing -- This line works as expected (hin, hout, _, p) <- runInteractiveProcess "copierer" [] Nothing Nothing -- This line doesn't work hSetBuffering hin LineBuffering hSetBuffering hout LineBuffering return (hin, hout, p) send_and_receive :: (Handle,Handle) -> String -> IO () send_and_receive (hin,hout) indata = do forkIO $ hPutStrLn hin indata outdata <- hGetLine hout putStrLn $ "outdata: " ++ outdata --code end I compiled it with: ghc -threaded --make twowaysubprocesscomm.hs then ran: ./twowaysubprocesscomm twowaysubprocesscomm: fd:7: hGetLine: end of file twowaysubprocesscomm: fd:6: hPutChar: resource vanished (Broken pipe) Copierer doesn't work as subprocess! If i compile twowaysubprocesscomm.hs using the original "cat", it works as expected: ./twowaysubprocesscomm outdata: boo outdata: foo What is wrong here? The "copierer.hs", its usage in "twowaysubprocesscomm.hs" or both? I use GHC 6.12.3 on a 64 bit linux. Thanks, Árpád