Haskell snippet that demonstates that the GHC RTS spawns an unlimited number of FFI threads for safe FFI calls; see https://ghc.haskell.org/trac/ghc/ticket/13296

safe-blocking.hs

-- From:
-- slyfox: yeah, ghc's RTS does not limit amount of outstanding safe FFIs. example: http://dpaste.com/2YT058D
-- slyfox: 'ghc --make safe-blocking.hs -threaded -debug && time ./safe-blocking +RTS -Ds' this runs in 6 seconds and nicely prints full list of blocked threads on FFI
import Control.Concurrent
import Control.Concurrent.MVar
import Control.Monad
import Foreign.C.Types

foreign import ccall safe "unistd.h sleep" sleep :: CInt -> IO CInt

main :: IO ()
main = do
    ts <- forM [1..100] $ \n -> do
              lock <- newEmptyMVar
              forkIO $ do
                  sleep 5
                  putStrLn $ show n ++ " in done!"
                  putMVar lock ()
              return lock
    mapM_ takeMVar ts
    putStrLn "all done"