Interfacing with the Windows API via Clozure CL

Question

I want to call a Windows API function from Clozure CL via its Foreign Function Interface, but I am having some issues because the documentation I have been able to find is rather scarce.

As a sufficiently rich example, I am trying to call SHGetKnownFolderPath. Its signature is

HRESULT SHGetKnownFolderPath(const GUID &rfid, DWORD dwFlags,
                             HANDLE hToken, PWSTR *ppszPath   );

Getting the entry point

I can use the following to get the entry point of the function:

(open-shared-library "shell32.dll")
> #<SHLIB SHELL32.dll #x1234>
(external "SHGetKnownFolderPath")
> #<EXTERNAL-ENTRY-POINT "SHGetKnownFolderPath" (#x12345) SHELL32.dll #x123456>

Getting/setting the correct method signature

Here's what I think the general steps are for calling it, but I am unsure about the how of each:

• Retrieve the signature that CCL has generated for this function so I can know how to call it.
• Provide a different function signature for it using external-call.
• Define an FFI structure or class to hold the GUID (first argument) via def-foreign-type or related construct.
• After the call, use CoTaskMemFree to free the memory pointed to by *ppszPath.

A guess at the form of the eventual code

My guess for the general form the final code would take is

(defun get-known-folder (guid)
   (let ((ffi-guid (allocate-ffi-memory-from-guid guid))
         (ffi-path ffi-version-of-nil)
         (path nil))
      (external-call "SHGetKnownFolderPath" ...) ;; Pass ffi-guid and ffi-path by reference.
      (setf path (convert-cstring-to-string ffi-path))
      (external-call "CoTaskMemFree" ...) ;; Free ffi-path.
      path))

where there are still the various unknown functions and function calls to convert between C/FFI and Lisp.

Addendum

I'm quite familiar with the Windows API and have done Win32 API calls from other languages. I also managed to get this working in CLisp, up to a point, as I ran into the problem of poor Unicode support of CLisp in Windows (I ended up with only the first character of the path, because CLisp reads the UTF-16 string as ASCII).

The point I am stuck on is what functionality there is in CCL to make this work. Any help in the right direction would be much appreciated.