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