[Haskell-cafe] System.Process and -threaded

16 May 2014

      Hello,

I'm writing a little networking wrapper around a sub-process (mplayer
-idle -slave) and I'm running into some issues with the System.Process
API. This is the program:
...
module Main where
import System.IO
import System.Process
import Network.Socket hiding (recv)
import Network.Socket.ByteString 
import Control.Concurrent
import qualified Data.ByteString.Char8 as C
-- WARNING: when compiled with -threaded, this program is likely not going
-- to work. As soon as one writes to the stdin of the forked process, it 
-- zombifies and any other command with crash this program.
main = withSocketsDo $ do
    -- network stuff
    addrinfos <- getAddrInfo Nothing (Just "localhost") (Just "4000")
    let serveraddr = head addrinfos
    sock <- socket (addrFamily serveraddr) Stream defaultProtocol
    bindSocket sock (addrAddress serveraddr)
    listen sock 1
-- mplayer 
    (hand,o,e,pid) <- runInteractiveProcess "mplayer" ["-fs", "-idle", "-slave"] Nothing Nothing
    hSetBinaryMode hand False
    hSetBuffering hand LineBuffering
putStrLn "listening for commands"
    loop sock hand
-- closing everything down
    sClose sock
    terminateProcess pid
    waitForProcess pid
    return ()
loop sock hand = do 
    (conn, _) <- accept sock
    str <- recv conn 2048
putStr $ "received: " ++ C.unpack str
-- write command to handler
    hPutStr hand $ C.unpack str
sClose conn
    loop sock hand
When compile with -threaded, the mplayer process gets zombified and
hangs until I shut down the program. When compiled with non-threaded RTS
(thats whats its called, correct?) I can successfully send a few
commands, but then mplayer freezes. When I strace mplayer, this error is
what it gets stuck on. 

    ioctl(0, TIOCGWINSZ, 0x7fff2897a070)    = -1 ENOTTY (Inappropriate ioctl for device)

Apparently that means I'm trying to communicate with it as though it
were a type writer. How fitting :)

The commands are all simple strings as docs here:

    http://www.mplayerhq.hu/DOCS/tech/slave.txt

My questions are these: is there anything I need to take care of when
handling sub-processes like this, specifically while writing to stdin of
the process, and with particular regard to -threaded? Does anybody spot
a problem or something I'm overlooking when handling processes like this? 
I have been reading the API docs, but found no mention of potential
caveats pertaining to -threaded.

Thanks!

k

✉ k@ioctl.it
☎ +49(0)176 / 61995110

[Haskell-cafe] System.Process and -threaded

Karsten Gebbert