How can I make sure main thread ends after all other threads ends?

import Control.Concurrent
main = do
    forkIO $ putStrLn "123"
    forkIO $ putStrLn "456"

I have written the code above. But when I executed it, I always got 123 only. 456 is not printed. I guess it is because main thread ended before the other thread so the whole program just ended.

How can I prevent this? Any api can make sure main thread ended after all threads ended?

OS: OS X 10.8.3

compiler: ghc 7.4.2