signal handling and optimisation

[Simon Marlow]

=20
> I'm trying to write some code that catches unix signals and turns them
> into GHC exceptions, GHC version 6.0, debian linux
>=20
> Heres my code:
>=20
> ------
>    module Main where
>   =20
>    import Control.Concurrent
>    import Control.Exception
>    import System.Posix
>    import IO=20
>   =20
>    catchCtrlC :: IO Handler
>    catchCtrlC
>       =3D do main_thread <- myThreadId
>            installHandler sigINT (Catch (handler main_thread)) Nothing
>            where
>            handler :: ThreadId -> IO ()
>            handler main_thread =3D throwTo main_thread=20
> (ErrorCall "Kaboom")
>   =20
>    main :: IO ()
>    main =3D do catchCtrlC=20
>              print (f 1)
>   =20
>    f :: Int -> Int
>    f x =3D f (x + 1)
> ------  =20
>=20
> The function "f" is intentionally bogus, I want it to loop so=20
> I have enough time to hit cntrl-C.

You've hit a known bug with concurrency, described in the
Control.Concurrent documentation.

The problem is that context switches only happen when some allocation is
going on: in your f function above, the optimiser turns it into a loop
that does no allocation, so no context switches can happen and the
exception is never delivered.

You probably won't run into this bug in a real program ;-)

Cheers,
	Simon