Exposing Haskell functions with string arguments in a DLL

How can I convert string arguments (null-terminated strings) to Haskell's [Char]? I saw there is some CString module in package lang of GHC, but I didn't found any documentation about it (e.g. packStringIO...?). How must a function f :: String -> String for usage in a DLL look like in order to convert the string parameter and result? (From earlier Haskell functions with BSTR arguments, I already know how to build a (static) DLL. I use GHC4.08.1.) Christian Lescher