Win32 API Window messages WM_DESTROY
Example
This message is sent to your window procedure when a window is being destroyed. It is sent after the window is removed from the screen. Most applications free any resources, like memory or handles, obtained in WM_CREATE. If you handle this message, return 0.
LRESULT CALLBACK winproc(HWND hwnd, UINT wm, WPARAM wp, LPARAM lp) { static char *text; switch (wm) { case WM_CREATE: text = malloc(256); /* use the allocated memory */ return 0; case WM_CLOSE: switch (MessageBox(hwnd, "Save changes?", "", MB_YESNOCANCEL)) { case IDYES: savedoc(); /* fall through */ case IDNO: DestroyWindow(hwnd); break; } return 0; case WM_DESTROY: /* free the memory */ free(text); PostQuitMessage(0); return 0; } return DefWindowProc(hwnd, wm, wp, lp); } 
