Help needed: GHC.Conc.threadWaitRead behavior on MacOS
Greetings,
I'm trying to understand why threadWaitRead
seems to be behaving differently on MacOS than it does on Linux (for various versions of GHC 8 and 9 that I have tried) for the following demonstration program. Here's the source:
module Main where
import Control.Monad (forever)
import GHC.Conc (threadWaitRead)
import System.IO (openFile, stdin, IOMode(..), BufferMode(..), hSetBuffering)
import System.Posix (FdOption(..), setFdOption, fdRead, handleToFd)
import System.Exit (exitFailure)
import System.Environment (getArgs)
main :: IO ()
main = do
args <- getArgs
handle <- case args of
["stdin"] -> return stdin
["tty"] -> openFile "/dev/tty" ReadMode
_ -> putStrLn "Usage: minimal <stdin|tty>" >> exitFailure
hSetBuffering handle NoBuffering
ttyFd <- handleToFd handle
setFdOption ttyFd NonBlockingRead False
forever $ do
putStrLn "Calling threadWaitRead"
threadWaitRead ttyFd
putStrLn "Calling fdRead"
(b, _) <- fdRead ttyFd 16
putStrLn $ "read: " <> show b
- On Linux, running this program with either argument (
stdin
ortty
) shows the same behavior: the program is initially blocked atthreadWaitRead
, and as soon as a keypress is available onstdin
, the program wakes up, callsfdRead
, and immediately reads and prints the available character. This is the behavior I expected to see, namely thatthreadWaitRead
would block until data is available (as documented) and the subsequent call tofdRead
would always return immediately. - On MacOS, running this program with the argument
stdin
has the exact same behavior as both arguments produce on Linux. However, running this program with the argumenttty
results in a situation I don't understand: the initial call tothreadWaitRead
returns immediately despite there being no data to read, and then the program blocks atfdRead
.
What am I missing here?
Thanks!