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