Control.Concurrent.STM.TMVar's combine the best of MVars and TVars:
-Unlike TVars, they permit blocking/early retry when the TMVar is
being used by other process, so that complete processes are not
retried at the end when happens that the TVars have been modified
in the meantime.
- Unlike MVars, they have no deadlocks, specially when a process is
trying to adquire exclusive access to more than one TMVar.
however, It happens that fails in my windows box with ghc 6.10.1 ,
single core
here is the code and the results:
-----------begin code:
module Main where
import Control.Concurrent.STM
import Control.Concurrent
import System.IO.Unsafe
import GHC.Conc
mtxs= unsafePerformIO $ mapM newTMVarIO $ take 5 $ repeat 0
proc i= atomically $ do
unsafeIOToSTM $ putStr $ "init of process "++ show i++"\n"
xs<- mapM takeTMVar mtxs
mapM (\(mtx,x) ->putTMVar mtx (x+1)) $ zip mtxs xs
xs' <- mapM readTMVar mtxs
unsafeIOToSTM $ putStr $ "End of processs "++show i ++ " result= "+
+ show xs'++"\n"
main=do
mapM (forkIO . proc) [1..5]
threadDelay 100000000
-------------end code
the rigth result must be (occasionally with some "init of process
x" repeated)
init of process 1
init of process 2
End of processs 1 result= [1,1,1,1,1]
init of process 2 -- retried
End of processs 2 result= [2,2,2,2,2]
init of process 3
End of processs 3 result= [3,3,3,3,3]
init of process 4
End of processs 4 result= [4,4,4,4,4]
init of process 5
End of processs 5 result= [5,5,5,5,5]
under windows the program produces strange results for example
init of process 1
init of process 2
init of process 3
init of process 4
init of process 5
End of processs 1 result= [1,1,1,1,1]
End of processs 2 result= [1,1,1,1,1]
(deadlock)
or this other:
init of process 1
init of process 2
init of process 3
init of process 4
init of process 5
End of processs 1 result= [1,1,1,1,1]
End of processs 2 result= [1,1,1,1,1]
init of process 3
init of process 4
init of process 5
init of process 2
End of processs 4 result= [2,2,2,2,2]
End of processs 3 result= [2,2,2,2,2]
End of processs 5 result= [2,2,2,2,2]
End of processs 2 result= [2,2,2,2,2]
init of process 3
init of process 5
init of process 2
End of processs 3 result= [3,3,3,3,3]
End of processs 5 result= [3,3,3,3,3]
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