I'd expect the following program (compiled with ghc and without any
specieal flags) to produce
Just (Exited ExitSuccess)
True
but it produces
Just (Exited ExitSuccess)
False
on Debian Lenny (ghc-6.8), OpenBSD-current (ghc-6.12.3), OpenBSD-current
(ghc=7.0 from the 7.0 branch).
module Main where
import Data.IORef
import System.Posix.Process
import System.Posix.Signals
import System.Posix.Unistd
main = do
caughtCHLD - newIORef False
installHandler sigCHLD (Catch $ writeIORef caughtCHLD True) Nothing
pid - forkProcess $ sleep 2 return ()
s - sleep 8
getProcessStatus False False pid = print
readIORef caughtCHLD = print
The sigCHLD handler is never called in this program. Is this expected
behaviour? If so, why?
If you change the sleep to threadDelay, you get the expected answer.
The reason is that sleep is a foreign call, and in the unthreaded RTS
no other threads can run while the foreign call is in progress, and that
includes threads created to handle signals. When sleep returns, there
isn't enough time before the main thread exits for the sigCHLD handler
thread to run.
threadDelay is more friendly and lets the RTS run other threads.
You could also use -threaded, but then the sleep call will be
interrupted by SIGVTALRM, so using threadDelay is better.
Cheers,
Simon
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