Yesterday, I wrote about WM_QUERYENDSESSION and the app that would not shut down. In that blog, I found a thread that never checked its message queue, and this meant that Windows thought the app was not responding.

So why didn’t Spy++ show the sent message in its log when it was sent? Spy++ uses a SetWindowsHookEx(WH_CALLWNDPROC) hook to log messages sent to windows. It turns out that in Windows NT 4.0 and later versions, the WH_CALLWNDPROC hook is called just before the message is sent to the window procedure. So our message never made it to the hook.

However, in Win9x and NT 3.51, the hook was called when the SendMessage function was called. Spy++ has its heritage in that era. Perhaps now the message delivery title in Spy++ should be changed from “sent” to “received” in order to clarify this difference.