Nobody?
On Tue, Aug 4, 2009 at 10:06 AM, Levi
Greenspan
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