[Haskell-cafe] System.Process and -threaded

Karsten Gebbert k at ioctl.it
Fri May 16 23:44:11 UTC 2014


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:


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.



✉ k at ioctl.it
☎ +49(0)176 / 61995110

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