Re: [Haskell-cafe] Race Condition in threads

18 Dec 2012

      Hi Serguey
Thank you for reply. I tried with IORef but I am missing a  function which
modify it.

In this case every thread just write value 10 to variable n.
incr_count :: MVar () -> IORef Int  -> IO ()
incr_count m n = ( forM_ [ 1 .. 10000 ] $ \_ -> writeIORef n 10  ) >>
putMVar m ()

main :: IO ()
main = do
      count <- newIORef 0
      list <- forM [1..10] $ \_ -> newEmptyMVar
      forM_ list $ \var -> forkIO . incr_count var $ count
      forM_ list $ \var ->  takeMVar var
      val <- readIORef count
      print val

ghci>:t atomicModifyIORef
atomicModifyIORef :: IORef a -> (a -> (a, b)) -> IO b
ghci>:t readIORef
readIORef :: IORef a -> IO a
ghci>:t writeIORef
writeIORef :: IORef a -> a -> IO ()

I have atomicModifyIORef but it puts it into IO. I am missing some thing
like this
ghci>:t modifyMVar_
modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()

modifyIORef_:: IORef a -> ( a -> IO a ) -> IO ()

Mukesh Tiwari

On Tue, Dec 18, 2012 at 11:50 PM, Serguey Zefirov wrote:
...
...
Hello All
I have two questions.
1. I wrote this code to create 10  simultaneous threads. Could some one
please tell me if this is correct or not ?
incr_count :: MVar () -> MVar Int -> IO ()
incr_count m n = ( forM_ [ 1..10000 ] $ \_ -> modifyMVar_ n ( return . (
2012/12/18 mukesh tiwari :
+
...
10 ) ) ) >> putMVar m ()
main :: IO()
main = do
      count <- newMVar 0
      list <- forM [1..10] $ \_ -> newEmptyMVar
      forM_ list $ \var -> forkIO . incr_count var $ count
      forM_ list $ \var ->  takeMVar var
      val <- takeMVar count
      print val
It is pretty much correct (some comments would be nice, though).
...
2. I am trying to create  race condition which makes the variable in
inconsistent state. Could some one please tell me how to achieve this ? I
look at IORef but it does not have function like modifyMVar_.
MVars are "atomic" in the sense that they have empty state and mutator
can wait until variable will have a value. So you have to operate with
something else, either IORef or with readMVar instead of takeMVar or
modifyMVar_.
...
Mukesh Tiwari
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