Some problems writing a threaded program

10 Feb 2008

      I am running my program in WinXP with ghc 2.6.8

If you install netstat and change the parameters it should still work in
linux.

Why does thread # 3 dominate over the over threads int the output?
Why does thread # 4 never seem to run?

I can't use the sleep function in System.Process.Win32 since it puts all the

threads asleep at the same time.  Is there a way to put only one thread
asleep?

That would allow more of a chance for thread #4 to run.

The simplified program:
---------------------------------------------------------------

module Main where

import Data.IORef
import Data.List
import System.IO
import System.Process

import Control.Concurrent
import Control.Concurrent.Chan

data Connection = Null | Last | Active deriving (Eq)

instance Show Connection where
show Null = "Null"
show Last = "Last"
show Active = "Active"

instance Read Connection where
readsPrec _ s = case take 5 s of
" UDP" -> [(Active, "")]
" TCP" -> [(Active, "")]
"Last" -> [(Last,"")]
_ -> [(Null,"")]

-- ptrints one 0 and 1
main = do
stop <- newIORef False
cbuffer <- newChan :: IO (Chan Connection)
putStr "0"
(_,output,_,ph) <- runInteractiveCommand "netstat -noa 5"
sequence $ map forkIO $ [(processConnections ph output cbuffer),
(stopNetstat ph stop False), (printChan cbuffer),(checkStop stop "xxxx")]
putStr "1"
_ <- waitForProcess ph
--mapM killThread ts
putStrLn "\nDone"

-- thread # 2
processConnections :: ProcessHandle -> Handle -> (Chan Connection) -> IO ()
processConnections ph hout chan = do
h <- hReady hout
e <- getProcessExitCode ph
putStr "2"
if (not h && e /= Nothing) then do writeChan chan Last >> return () else do
if h then do readConnection hout >>= writeChan chan else do
processConnections ph hout chan

readConnection :: Handle -> IO Connection
readConnection hout = do
l <- hGetLine hout
let c = (read l :: Connection)
if (c == Null)
then do (readConnection hout)
else do (return c)

-- thread number 3
stopNetstat :: ProcessHandle -> (IORef Bool) -> Bool -> IO ()
stopNetstat netstat _ True = terminateProcess netstat
stopNetstat netstat gref False = putStr "3" >> yield >> readIORef gref >>=
stopNetstat netstat gref

--thread 4
printChan :: (Chan Connection) -> IO ()
printChan chan = do
putStr "4"
c <- readChan chan
printConnection c
printChan chan

checkStop :: (IORef Bool) -> String -> IO ()
checkStop ref s = do
if (take 4 s == "stop")
then do (writeIORef ref True)
else do (getChar >>= (\x -> checkStop ref ((tail s) ++ [x])))

printConnection :: Connection -> IO ()
printConnection c = case c of
Null -> putStr "N"
Last -> putStr "L"
_ -> putStr "A"

Some problems writing a threaded program

John Vogel