2012/12/18 mukesh tiwari
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 . ( + 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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