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

Hi Simon, Many thanks for your reply. I am not actually using sleep in my code. I only used it for here for highlighting the problem. It will be the same when using poll(2) for instance. Does this mean that because of SIGVTALRM I can always get an EINTR when calling a foreign function that blocks on a system call? Cheers, Levi On Thu, Aug 6, 2009 at 12:17 PM, Simon Marlow wrote:

The SIGVTALRM signal is delivered to one (random) thread in the program, so I imagine it just isn't being delivered to the thread that runs your second call to sleep.  (the main Haskell thread is a "bound thread" and hence gets an OS thread to itself).

Is there some reason you can't use threadDelay?  threadDelay is much more friendly: it doesn't require another OS thread for each sleeping Haskell thread.

Cheers,        Simon

On 05/08/2009 17:01, Levi Greenspan wrote:

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Nobody?

On Tue, Aug 4, 2009 at 10:06 AM, Levi Greenspan  wrote:

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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