[Haskell-cafe] trying to understand runProcess handles

[Matthias Fischmann]

hi there,

I am running some unix command.  I just realized there is
runInteractiveProcess in System.Process, so my problem is solved in
practice:

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

showPlot :: String -> IO (Handle, Handle, Handle, ProcessHandle)
showPlot file = runInteractiveProcess executable arguments wd env
    where
    executable   = "/usr/bin/gnuplot"
    arguments    = ["-geometry", "-19+3", file, "-"]
    wd           = Nothing
    env          = Nothing

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

But first I tried to create the handles myself with createPipe from
System.Posix.IO, and I failed for a reasons that I have no clue how to
learn to understand.  This is what the code looked like:

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

showPlot :: String -> IO (ProcessHandle, Handle, Handle, Handle)
showPlot file = do
                (stdinR,  stdinW)  <- createPipeHandles
                (stdoutR, stdoutW) <- createPipeHandles
                (stderrR, stderrW) <- createPipeHandles
                h <- runProcess
                        executable arguments wd env
                        (Just stdinR) (Just stdoutW) (Just stderr W)
                return (h, stdinW, stdoutR, stderrR)
    where
    executable   = "/usr/bin/gnuplot"
    arguments    = ["-geometry", "-19+3", file, "-"]
    wd           = Nothing
    env          = Nothing

createPipeHandles :: IO (Handle, Handle)
createPipeHandles = do
                    (fR, fW) <- createPipe
                    hR <- fdToHandle fR
                    hW <- fdToHandle fW
                    return (hR, hW)

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

The problem is that gnuplot terminates right away after it tries to
read from stdin (I can see the shadow of a window appear and vanish
immediately).  I tried setFdOption, with no effect.  Is this because
the handles passed to runProcess are closed in the parent process, and
therefore the pipe is teared down and useless?

I don't think so: If I run 'find /' instead of gnuplot, the process
happily starts and I can hGetLine from stdout.

If you have any clue I'd love to learn.  Anyway, as I said, it's
really better to simply use runInteractiveProcess in this simple case.

thanks,
Matthias
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