withCString msg $ \cmsg ->
debugBelch cfmt cmsg
-foreign import ccall unsafe "RtsMessages.h debugBelch"
+-- don't use debugBelch() directly, because we cannot call varargs functions
+-- using the FFI.
+foreign import ccall unsafe "HsBase.h debugBelch2"
debugBelch :: CString -> CString -> IO ()
#endif