Hello, FFI code that used to work now fails under Windows (still seems to work under Ubuntu), and I wonder if anybody has seen anything like this and can provide some pointers... The code uses FFI to fetch information from the R side like R_NilValue, using something like this; -- Fetch R's R_NilValue... foreign import ccall unsafe "&R_NilValue" r_NilValue_ptr :: Ptr R_EXP r_NilValue :: IO R_EXP r_NilValue = peek r_NilValue_ptr rNilValue1 :: IO REXP rNilValue1 = do x <- r_NilValue traceShow("addr=",x) extREXP x Under Windows the address displayed is obviously bad, and this causes the app to crash. This does not happen under Linux (Ubuntu). Now, replace the line containing peek with r_NilValue = trace "PEEK" peek r_NilValue_ptr The address is now valid! It seems that adding the trace "PEEK" adds some delay and somehow resolves the problem. This problem is intermittent, so it is hard to come up with a simple example that fails every time. A little background: R_NilValue is a pointer to a SEXP that is not initialized until an embedded instance of R is initialized, and the code above is not triggered until this happens. Perhaps there is a race condition between the time R initializes itself and Haskell performs the peek? I don't think R_NilValue is garbage collected once initialized. Any tips would be appreciated. Dominick