[Haskell-cafe] Re: FFI: Problem with Signal Handler Interruptions

[Levi Greenspan]

Nobody?

On Tue, Aug 4, 2009 at 10:06 AM, Levi
Greenspan<greenspan.levi at googlemail.com> wrote:
> Dear list members,
>
> In February this year there was a posting "Why does sleep not work?"
> (http://www.haskell.org/pipermail/haskell-cafe/2009-February/055400.html).
> The problem was apparently caused by signal handler interruptions. I
> noticed the same (not with sleep though) when doing some FFI work and
> compiled the following test program:
>
>
> {-# LANGUAGE ForeignFunctionInterface #-}
> module Main where
>
> import Foreign.C.Types
> import Control.Concurrent
>
> sleep :: IO ()
> sleep = c_sleep 3 >>= print
>
> fails :: IO ()
> fails = sleep
>
> works :: IO ()
> works = forkIO sleep >> return ()
>
> main :: IO ()
> main = fails >> works >> threadDelay 3000000
>
> foreign import ccall unsafe "unistd.h sleep"
>    c_sleep :: CUInt -> IO CUInt
>
>
> When compiled with GHC (using --make -threaded), it will print 3
> immediately (from the "fails" function) and after 3 seconds 0 (from
> "works"), before it finally exits. man sleep(3) tells me that sleep
> returns 0 on success and if interrupted by a signal the number of
> seconds left to sleep. Clearly "fails" is interrupted by a signal
> (which seems to be SIGVTALRM). This was mentioned in the discussion
> from February.
>
> I would like to know why "fails" fails and "works" works, i.e. why is
> "sleep" not interrupted when run in a separate thread? And what can be
> done to make "sleep" work in the main thread? It wouldn't be wise to
> block SIGVTALRM, wouldn't it?
>
> Many thanks,
> Levi
>