Hello There seems to be a bug in the IO libraries. I'm using the following procedure to call an external program and send it data through a pipe. pipeto :: String -> String -> [String] -> IO () pipeto txt prog par = do catch (do -- create pipe (zu, von) <- createPipe vonh <- fdToHandle von hSetBuffering vonh NoBuffering mpid <- forkProcess case mpid of Nothing -> do -- child -- connect pipe's read end to stdin -- and close its write end dupTo zu (intToFd 0) fdClose zu hClose vonh executeFile prog True par Nothing ... -- (print error message) Just pid -> do -- parent fdClose zu -- close pipe's read end -- ** here ** hPutStr vonh txt -- write text to forked process hClose vonh -- close pipe's write end -- wait for child process to finish (Just ps) <- getProcessStatus True True pid if ps == Exited ExitSuccess then return () else ...) -- (error message) (\err -> ...) -- print error message The problem is that the child process doesn't receive all the data which the parent sends. It's as if "hPutStr vonh txt" sends the data lazily somehow, and "hClose vonh" closes the pipe prematurely. It varies from run to run exactly which data gets through. If I cause the child process to read all its input immediately, the problem doesn't seem to occur. Normally, it does so gradually, which takes a few seconds. I'm using GHC 5.02.2 Volker