multithreading - Haskell forkIO 线程使用 putStrLn 在彼此之上写入

Question

我正在使用 Haskell 轻量级线程 ( forkIO ) 使用以下代码:

import Control.Concurrent

beginTest :: IO ()
beginTest = go
    where
    go = do
        putStrLn "Very interesting string"
        go
        return ()

main = do
    threadID1 <- forkIO $ beginTest
    threadID2 <- forkIO $ beginTest
    threadID3 <- forkIO $ beginTest
    threadID4 <- forkIO $ beginTest
    threadID5 <- forkIO $ beginTest

    let tID1 = show threadID1
    let tID2 = show threadID2
    let tID3 = show threadID3
    let tID4 = show threadID4
    let tID5 = show threadID5

    putStrLn "Main Thread"
    putStrLn $ tID1 ++ ", " ++ tID2 ++ ", " ++ tID3 ++ ", " ++ tID4 ++ ", " ++ tID5
    getLine
    putStrLn "Done"

现在，对此的预期输出将是一大堆:

Very interesting string
Very interesting string
Very interesting string
Very interesting string

其中一个在那里的某个地方:

Main Thread

然而，输出(或前几行)结果是这样的:

Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very interesting string
Very VVVViMeeeenarrrrtiyyyyen    r iiiieTnnnnshtttttreeeeierrrrnaeeeegdssss 
ttttsiiiitTnnnnrhggggir    nessssgatttt
drrrrIiiiiVdnnnne ggggr5



y1 ,VVVVi eeeenTrrrrthyyyyer    reiiiieannnnsdtttttIeeeeidrrrrn eeeeg5ssss 2tttts,iiiit nnnnrTggggih    nrssssgetttt
arrrrdiiiiVInnnnedggggr 



y5 3VVVVi,eeeen rrrrtTyyyyeh    rriiiieennnnsatttttdeeeeiIrrrrndeeeeg ssss 5tttts4iiiit,nnnnr ggggiT    nhssssgrtttt
errrraiiiiVdnnnneIggggrd



y  5VVVVi5eeeen
rrrrtyyyye    riiiiennnnsttttteeeeirrrrneeeegssss ttttsiiiitnnnnrggggi    nssssgtttt
rrrriiiiVnnnneggggr



y VVVVieeeenrrrrtyyyye    riiiiennnnsttttteeeeirrrrneeeegssss ttttsiiiitnnnnrggggi    nssssgtttt
rrrriiiiVnnnneggggr

文本每隔几行就会移动一次，但很明显 Very interesting string s 最终彼此重叠，因为不知何故线程使用 putStrLn同时最终在彼此之上写入标准输出。为什么会这样，以及如何(不求助于消息传递、计时或其他一些过于复杂和令人费解的解决方案)可以克服它？

Answer 1

简单地说，putStrLn不是原子操作。每个字符都可以与来自不同线程的任何其他字符交错。

(我也不确定在多字节编码(如 UTF8)中是否保证多字节字符是原子处理的。)

如果你想要原子性，你可以使用共享互斥锁，例如

do lock <- newMVar ()
   let atomicPutStrLn str = takeMVar lock >> putStrLn str >> putMVar lock ()
   forkIO $ forever (atomicPutStrLn "hello")
   forkIO $ forever (atomicPutStrLn "world")

正如以下评论中所建议的，我们还可以简化上述异常并使其安全，如下所示:

do lock <- newMVar ()
   let atomicPutStrLn str = withMVar lock (\_ -> putStrLn str)
   forkIO $ forever (atomicPutStrLn "hello")
   forkIO $ forever (atomicPutStrLn "world")