Quoth Claude Heiland-Allen
The conclusion I drew was that "unsafe" foreign functions block the current "capability" (OS thread) and any "threads" (Haskell forkIO etc) currently scheduled on that capability, but other capabilities and threads continue executing as normal.
If a trivial test program would help, here I call the sleep() function, which I believe on a POSIX platform suspends the thread until receipt of a SIGALRM. If "unsafe", during the execution of sleep() in one thread, Haskell execution will be blocked in the other, so they will alternate. If "safe", the two sleep intervals will overlap. I believe we all now expect that, but if it does come as a surprise, I hope someone will test it on a more common platform. +RTS -N2 makes no difference. Donn Cave, donn@avvanta.com ----------------------------------- {-# LANGUAGE ForeignFunctionInterface #-} module Main (main) where import Control.Concurrent (forkOS) import Foreign import Foreign.C foreign import ccall unsafe "sleep" sleep :: CInt -> IO CInt rep :: (CInt -> IO CInt) -> CInt -> IO () rep f s = do print s f s rep f s main = do forkOS $ rep sleep 3 rep sleep 1