How to prevent a system shutdown without having a window? - c++

Question

I have windows application with this source code
#include <Windows.h>
#include <thread>
#include <chrono>
int WinMain(HINSTANCE hInstance,
HINSTANCE hPrevInstance,
LPTSTR lpCmdLine,
int cmdShow)
{
MSG msg;
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}
LRESULT CALLBACK WinProc(HWND hWnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
switch (Message)
{
case WM_QUERYENDSESSION:
MessageBox(NULL, "Triggered?", "Message", 0);
AbortSystemShutdown(NULL);
return 0;
default:
return DefWindowProc(hWnd, Message, wParam, lParam);
}
return 0;
}
I need to know when the system is shutting down and prevent it, or at least send a message to the user.
It doesn't seem that my application is receiving the WM_QUERYENDSESSION message.
I also tried to use ShutdownBlockReasonCreate() but I don't have a HWND for a window.
How should I do this?

Answer 1

As stated in the reference for WM_QUERYENDSESSION:
A window receives this message through its WindowProc function.
You have a WindowProc but you are missing a window. A WindowProc must be associated with a window, otherwise it is not known to Windows. To associate a WindowProc with a window, you can call RegisterClassEx followed by CreateWindowEx. Specify the name of your newly created window class in the call to CreateWindowEx.
The window must be a top-level window. It can be invisible, but in this case the following applies (from Application Shutdown Changes in Windows Vista):
Also note that if your application has no visible top-level windows,
it must use this API [ShutdownBlockReasonCreate()] if it needs to successfully block shutdown. Such
applications will automatically be terminated if they block shutdown
without using the API.
Note that a message-only window will not receive WM_QUERYENDSESSION.
Working example:
#include <windows.h>
LRESULT CALLBACK WndProc( HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam );
int APIENTRY wWinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPWSTR lpCmdLine, int nCmdShow )
{
WNDCLASSEXW wx = { sizeof(wx) }; // set cbSize member and zero-initialize all other
wx.lpfnWndProc = WndProc;
wx.hInstance = hInstance;
wx.lpszClassName = L"MyWindowClass";
if( ! RegisterClassExW( &wx ) )
return 1; // TODO: improve error handling
HWND hWnd = CreateWindowExW( 0, wx.lpszClassName, L"My Application", 0, 0, 0, 0, 0,
NULL, NULL, NULL, NULL );
if( ! hWnd )
return 2; // TODO: improve error handling
MSG msg;
while( GetMessage( &msg, nullptr, 0, 0 ) )
{
TranslateMessage( &msg );
DispatchMessage( &msg );
}
return static_cast<int>( msg.wParam );
}
LRESULT CALLBACK WndProc( HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam )
{
switch( message )
{
case WM_QUERYENDSESSION:
{
// Try to block shutdown.
ShutdownBlockReasonCreate( hWnd, L"I don't want to sleep (yet)!" );
return FALSE;
}
case WM_ENDSESSION:
{
// TODO: Always handle this message because shutdown can be forced
// even if we return FALSE from WM_QUERYENDSESSION!
return 0;
}
default:
{
return DefWindowProc(hWnd, message, wParam, lParam);
}
}
return 0;
}
Further read:
Application Shutdown Changes in Windows Vista
Restart Manager - Guidelines for Applications
Shutting Down