Jason Dusek wrote:

Why is this function exported then imported?

It causes the RTS to create a bound thread to run code in: (reordering the code slightly)

foreign import ccall "forkOS_entry" forkOS_entry_reimported :: StablePtr (IO ()) -> IO ()

This is a safe call, so it suspends the currently running RTS thread, and then calls the forkOS_entry foreign function.

foreign export ccall forkOS_entry :: StablePtr (IO ()) -> IO ()

The generated stub creates a new, bound RTS thread, and then calls the forkOS_entry Haskell function.

forkOS_entry :: StablePtr (IO ()) -> IO () forkOS_entry stableAction = do action <- deRefStablePtr stableAction action

And this, finally, runs an action in the new thread. See also section 3.4 of the "Extending the Haskell FFI with Concurrency" paper, http://research.microsoft.com/en-us/um/people/simonpj/Papers/conc-ffi/index.... (http://tinyurl.com/cocex7) HTH, Bertram