add machine shutdown callback

docs/window.dox

@@ -618,6 +618,23 @@ void window_close_callback(GLFWwindow* window)
 }
 @endcode
 
+If you wish to be notified when the user attempts to shut down the machine, set a 
+machine shutdown callback.
+
+@code
+glfwSetMachineShutdownCallback(window, machine_shutdown_callback);
+@endcode
+
+The callback function is called when GLFW detects that the machine is shutting down.
+It can be used for example to save data to disk in order to minimize risk of data loss.
+
+@code
+void machine_shutdown_callback(GLFWwindow* window)
+{
+    initiate_save_of_important_data_to_disk();
+}
+@endcode
+
 
 @subsection window_size Window size
 

include/GLFW/glfw3.h

@@ -1412,6 +1412,21 @@ typedef void (* GLFWwindowsizefun)(GLFWwindow*,int,int);
  */
 typedef void (* GLFWwindowclosefun)(GLFWwindow*);
 
+/*! @brief The function pointer type for machine shutdown callbacks.
+ *
+ *  This is the function pointer type for machine shutdown callbacks.
+ *  A machine shutdown callback function has the following signature:
+ *  @code
+ *  void function_name(GLFWwindow* window)
+ *  @endcode
+ *
+ *  @sa @ref machine_shutdown
+ *  @sa @ref glfwSetMachineShutdownCallback
+ *
+ *  @ingroup window
+ */
+typedef void (* GLFWmachineShutdownfun)(GLFWwindow*);
+
 /*! @brief The function pointer type for window content refresh callbacks.
  *
  *  This is the function pointer type for window content refresh callbacks.
@@ -3843,6 +3858,34 @@ GLFWAPI GLFWwindowsizefun glfwSetWindowSizeCallback(GLFWwindow* window, GLFWwind
  */
 GLFWAPI GLFWwindowclosefun glfwSetWindowCloseCallback(GLFWwindow* window, GLFWwindowclosefun callback);
 
+/*! @brief Sets the machine shutdown callback for the specified window.
+ *
+ *  This function sets the machine shutdown callback of the specified window, which is
+ *  called when the operating system is preparing to shut down the machine.
+ *
+ *  @param[in] window The window whose callback to set.
+ *  @param[in] callback The new callback, or `NULL` to remove the currently set
+ *  callback.
+ *  @return The previously set callback, or `NULL` if no callback was set or the
+ *  library had not been [initialized](@ref intro_init).
+ *
+ *  @callback_signature
+ *  @code
+ *  void function_name(GLFWwindow* window)
+ *  @endcode
+ *  For more information about the callback parameters, see the
+ *  [function pointer type](@ref GLFWmachineShutdownfun).
+ *
+ *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED.
+ *
+ *  @thread_safety This function must only be called from the main thread.
+ *
+ *  @sa @ref machine_shutdown
+ *
+ *  @ingroup window
+ */
+GLFWAPI GLFWmachineShutdownfun glfwSetMachineShutdownCallback(GLFWwindow* window, GLFWmachineShutdownfun callback);
+
 /*! @brief Sets the refresh callback for the specified window.
  *
  *  This function sets the refresh callback of the specified window, which is

src/internal.h

@@ -408,6 +408,7 @@ struct _GLFWwindow
         GLFWwindowposfun        pos;
         GLFWwindowsizefun       size;
         GLFWwindowclosefun      close;
+        GLFWmachineShutdownfun  shutdown;
         GLFWwindowrefreshfun    refresh;
         GLFWwindowfocusfun      focus;
         GLFWwindowiconifyfun    iconify;
@@ -729,6 +730,7 @@ void _glfwInputWindowIconify(_GLFWwindow* window, GLFWbool iconified);
 void _glfwInputWindowMaximize(_GLFWwindow* window, GLFWbool maximized);
 void _glfwInputWindowDamage(_GLFWwindow* window);
 void _glfwInputWindowCloseRequest(_GLFWwindow* window);
+void _glfwInputMachineShutdown(_GLFWwindow* window);
 void _glfwInputWindowMonitor(_GLFWwindow* window, _GLFWmonitor* monitor);
 
 void _glfwInputKey(_GLFWwindow* window,

src/win32_window.c

@@ -1199,6 +1199,13 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
             DragFinish(drop);
             return 0;
         }
+
+        case WM_QUERYENDSESSION:
+        case WM_ENDSESSION:
+        {
+            _glfwInputMachineShutdown(window);
+            return 0;
+        }
     }
 
     return DefWindowProcW(hWnd, uMsg, wParam, lParam);

src/window.c

@@ -138,6 +138,14 @@ void _glfwInputWindowCloseRequest(_GLFWwindow* window)
         window->callbacks.close((GLFWwindow*) window);
 }
 
+// Notifies shared code that the machine is shuting down
+//
+void _glfwInputMachineShutdown(_GLFWwindow *window)
+{
+    if (window->callbacks.shutdown)
+        window->callbacks.shutdown((GLFWwindow*) window);
+}
+
 // Notifies shared code that a window has changed its desired monitor
 //
 void _glfwInputWindowMonitor(_GLFWwindow* window, _GLFWmonitor* monitor)
@@ -1020,6 +1028,17 @@ GLFWAPI GLFWwindowclosefun glfwSetWindowCloseCallback(GLFWwindow* handle,
     return cbfun;
 }
 
+GLFWAPI GLFWmachineShutdownfun glfwSetMachineShutdownCallback(GLFWwindow* handle,
+                                                              GLFWmachineShutdownfun cbfun)
+{
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+    _GLFW_SWAP_POINTERS(window->callbacks.shutdown, cbfun);
+    return cbfun;
+}
+
 GLFWAPI GLFWwindowrefreshfun glfwSetWindowRefreshCallback(GLFWwindow* handle,
                                                           GLFWwindowrefreshfun cbfun)
 {

tests/events.c

@@ -329,6 +329,13 @@ static void window_close_callback(GLFWwindow* window)
     glfwSetWindowShouldClose(window, slot->closeable);
 }
 
+static void window_machine_shutdown_callback(GLFWwindow* window)
+{
+    printf("%08x at %0.3f: Machine shutdown detected\n",
+           counter++,
+           glfwGetTime());
+}
+
 static void window_refresh_callback(GLFWwindow* window)
 {
     Slot* slot = glfwGetWindowUserPointer(window);
@@ -627,6 +634,7 @@ int main(int argc, char** argv)
         glfwSetFramebufferSizeCallback(slots[i].window, framebuffer_size_callback);
         glfwSetWindowContentScaleCallback(slots[i].window, window_content_scale_callback);
         glfwSetWindowCloseCallback(slots[i].window, window_close_callback);
+        glfwSetMachineShutdownCallback(slots[i].window, window_machine_shutdown_callback);
         glfwSetWindowRefreshCallback(slots[i].window, window_refresh_callback);
         glfwSetWindowFocusCallback(slots[i].window, window_focus_callback);
         glfwSetWindowIconifyCallback(slots[i].window, window_iconify_callback);