Data commit

Task/Determine-if-only-one-instance-is-running/Haskell/determine-if-only-one-instance-is-running.hs

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+import Control.Concurrent
+import System.Directory (doesFileExist, getAppUserDataDirectory,
+    removeFile)
+import System.IO (withFile, Handle, IOMode(WriteMode), hPutStr)
+
+oneInstance :: IO ()
+oneInstance = do
+    -- check if file "$HOME/.myapp.lock" exists
+    user <- getAppUserDataDirectory "myapp.lock"
+    locked <- doesFileExist user
+    if locked
+    then print "There is already one instance of this program running."
+    else do
+        t <- myThreadId
+        -- this is the entry point to the main program:
+        -- withFile creates a file, then calls a function,
+        -- then closes the file
+        withFile user WriteMode (do_program t)
+        -- remove the lock when we're done
+        removeFile user
+
+do_program :: ThreadId -> Handle -> IO ()
+do_program t h = do
+    let s = "Locked by thread: " ++ show t
+    -- print what thread has acquired the lock
+    putStrLn s
+    -- write the same message to the file, to show that the
+    -- thread "owns" the file
+    hPutStr h s
+    -- wait for one second
+    threadDelay 1000000
+
+main :: IO ()
+main = do
+    -- launch the first thread, which will create the lock file
+    forkIO oneInstance
+    -- wait for half a second
+    threadDelay 500000
+    -- launch the second thread, which will find the lock file and
+    -- thus will exit immediately
+    forkIO oneInstance
+    return ()