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5.1 Passing a string to a Windows function

The following example shows how to define a Lisp function which calls a Win32 API function to change the title of the active window. It demonstrates the use of define-foreign-function and with-foreign-string to pass a Lisp string to a Windows function.

The first step involves defining a FLI type to correspond to the Windows hwnd type, which is the window handle type.

(fli:define-c-typedef fli-hwnd
  (:unsigned :long))

The next step consists of the foreign function definitions. The first foreign function returns the window handle of the active window, by calling the Windows function GetActiveWindow. It takes no arguments.

(fli:define-foreign-function (get-act-window "GetActiveWindow")
  :result-type fli-hwnd
  :documentation "Returns the window handle of the active window
                  for the current thread. If no active window is
                  associated with the current thread then it returns 0.")

The next foreign function uses the Windows function SetWindowText to set the text of the active window titlebar. It takes a window handle and a pointer to a FLI string as its arguments.

(fli:define-foreign-function (set-win-text "SetWindowText" :dbcs)
    ((hwnd fli-hwnd)
     (lpstring :pointer))
  :result-type :boolean
  :documentation "Sets the text of the window titlebar.")

The foreign function set-win-text returns a boolean to indicate whether it has successfully changed the title bar.

The required FLI data types and foreign functions have been defined. What is now required is a Lisp function which uses them to change the titlebar of the active window. The next function does this:

(defun set-active-window-text (new-text)
  (let ((active-window (get-act-window))
        (external-format (if (string= (software-type)
                                      "Windows NT")
    (unless (zerop active-window)
      (fli:with-foreign-string (new-ptr element-count byte-count 
                                :external-format external-format)
        (declare (ignore element-count byte-count))
        (set-win-text active-window new-ptr)))))

The function set-active-window-text takes a Lisp string as its argument, and does the following:

  1. It calls the foreign function get-act-window to set the variable active-window to be the handle of the active window. If no window is active, this will be zero.
  2. The variable external-format is set to be :unicode if the operating system is Windows NT or a later system based on it (which expects strings to be passed to it in Unicode format), otherwise it is set to be :ascii.
  3. If active-window is zero, then there is no active window, and the function terminates, returning nil.
  4. If active-window is not zero, then it contains a window handle, and the following happens:

    The function uses with-foreign-string to convert the Lisp string argument of the function into a FLI string, and a pointer to the FLI string is allocated, ready to be handed to the foreign function set-win-text that we defined earlier. The encoding of the string is external-format, which is the encoding suitable for the operating system running on the computer. Once the window title has been set, with-foreign-string automatically deallocates the memory that was allocated for the FLI string and the pointer. The function then terminates, returning t.

You can test that this is what happens by entering the command:

(set-active-window-text "A new title for the active window")

See with-foreign-string, for more details on the use of foreign strings.

Foreign Language Interface User Guide and Reference Manual - 01 Dec 2021 19:34:57