Win32 MessageBox with SBCL foreign interface - common-lisp

Question

I am trying to figure out how to call the Win32 function MessageBox with the SBCL foreign interface. The MessageBox function implemented in "user32.dll" is described as follow:
int MessageBox(
[in, optional] HWND hWnd,
[in, optional] LPCTSTR lpText,
[in, optional] LPCTSTR lpCaption,
[in] UINT uType
);
[...]
MB_OK 0x00000000L The message box contains one push button: OK. This is the default.
The use of this function is trivial using the cffi library:
(cffi:load-foreign-library '(:default "user32"))
(cffi:defcfun ("MessageBoxA" message-box) :int
(hwnd :pointer)
(text :string)
(caption :string)
(type :unsigned-int))
(message-box (cffi:null-pointer) "Hello" "Test" 0)
I tried to use this function directly with the SBCL interface sb-alien: I couldn't make it work.
(sb-alien:load-shared-object "user32")
(defvar c-null (sb-sys:int-sap 0))
(sb-alien:define-alien-routine ("MessageBoxA" MessageBox) sb-alien:int
(hwnd (* sb-alien:integer))
(text sb-alien:c-string)
(caption sb-alien:c-string)
(uType (sb-alien:unsigned 4)))
;; need a small wrapper to convert Lisp's strings to foreign
(defun message-box (title message)
(let ((title-alien (sb-alien:make-alien-string title))
(message-alien (sb-alien:make-alien-string message)))
(MessageBox c-null title-alien message-alien 0)
(sb-alien:free-alien title-alien)
(sb-alien:free-alien message-alien)))
(message-box "Hello" Test")
There isn't any error or warning but it just don't work. How is working the SBCL alien interface? I tried to read the code from CFFI and SBCL, but I could not find any simple example.

Answer 1

I don't understand how I could have spend the whole afternoon on this issue, but the strings are actually automatically converted from/to Lisp. So the code is actually very simple:
(sb-alien:load-shared-object "user32")
(defvar c-null (sb-sys:int-sap 0))
(define-alien-routine ("MessageBoxA" MessageBox) sb-alien:integer
(hwnd (* sb-alien:integer))
(text sb-alien:c-string)
(caption sb-alien:c-string)
(uType (sb-alien:unsigned 4)))
(MessageBox c-null "Hello" "Test" 0)