I've built a small test case. When I compile with: ghc Main.hs test.c -o Main.exe ... the (correct) output from Main.exe is: 123.0 5678901234567890 And when I compile with: ghc -prof Main.hs test.c -o Main.exe ... the (incorrect) output from Main.exe is: 1.0 986516178 ------ Main.hs: {-# OPTIONS -ffi #-} module Main where import Foreign.C foreign import ccall "myDouble" myDouble :: IO CDouble foreign import ccall "myInt64" myInt64 :: IO CLLong main = do d <- myDouble putStrLn $ show d l <- myInt64 putStrLn $ show l ------ test.c: double myDouble() { double d = 123.0; return d; } long long myInt64() { long long x = 5678901234567890; return x; } ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************