Haskell forkIO threads writing on top of each other with putStrLn

Question

I was playing around with Haskell lightweight threads (forkIO) with the following code:

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"

Now the expected output to this would be a whole bunch of these:

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

with one of these somewhere in there:

Main Thread

However, the output (or first several lines anyway) turned out to be this:

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

Every few lines the text would shift, though it's pretty clear that the Very interesting strings ended up on top of each other, because somehow the threads using putStrLn at the same time ended up writing to stdout on top of each other. Why is this, and how (without resorting to message passing, timing, or some other overcomplicated and convoluted solution) can it be overcome?

Answer 1

Simply put, putStrLn is not an atomic operation. Every character may be interleaved with any other from a different thread.

(I am also not sure about whether in multi-byte encodings such as UTF8 it is guaranteed that a multi-byte character is atomically handled.)

If you want atomicity, you can use a shared mutex e.g.

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

---

As suggested in the comments below, we can also simplify and make the above exception-safe as follows:

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