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

Levi Greenspan greenspan.levi at googlemail.com
Tue Aug 4 04:06:16 EDT 2009

Dear list members,

In February this year there was a posting "Why does sleep not work?"
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,

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