al_win_add_window_callback - Allegro 5 API

       The  specified callback function will intercept the window’s message before Allegro processes it.  If the
       callback function consumes the event, then it should return true.  In that  case,  Allegro  will  not  do
       anything with the event.

       Optionally,  you  may  use result to customize what Allegro will return return in response to this event.
       By default, Allegro returns TRUE.

       The userdata pointer can be used to supply additional context to the callback function.

       The callbacks are executed in the same order they were added.

       Returns true if the callback was added.

       al_win_add_window_callback - Allegro 5 API

       5.1.2

Allegro reference manual                                                     al_win_add_window_callback(3alleg5)

              #include <allegro5/allegro_windows.h>

              bool al_win_add_window_callback(ALLEGRO_DISPLAY *display,
                 bool (*callback)(ALLEGRO_DISPLAY *display, UINT message, WPARAM wparam,
                 LPARAM lparam, LRESULT *result, void *userdata), void *userdata)