> 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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