[Interest] Handling WM_QUIT messages when no window is shown
Henry Skoglund
henry at tungware.se
Wed May 6 14:57:53 CEST 2020
On 2020-05-06 11:09, Julius Bullinger wrote:
> Usually, sending a WM_QUIT message to a QApplication triggers the
> QCoreApplication::aboutToQuit() signal, e.g. when ending the
> application via Windows's Task Manager, or using "taskkill /PID <pid>".
>
> We discovered that no aboutToQuit() signal is emitted when a
> QApplication without windows receives a WM_QUIT:
>
> #include <QApplication>
> #include <QDebug>
> #include <QWidget>
>
> int main(int argc, char *argv[]) {
> QApplication app(argc, argv);
>
> QApplication::connect(&app, &QApplication::aboutToQuit, []() {
> qDebug() << "QApplication::aboutToQuit";
> });
>
> // Commenting those lines will not trigger aboutToQuit() when
> // the application is closed via Task Manager:
> QWidget w;
> w.show();
>
> return app.exec();
> }
>
> Our application supports a text-only variant, where the GUI is
> disabled. We're basically doing it like that, and wondered why our
> shutdown code was not called when the application was quit via the
> Task Manager.
>
> Is there anything that needs to be done to get consistent behavior?
>
Hi, when running apps on Windows without a window, you'll be missing out
on a lot of inter-process stuff like WM_QUIT (that's why it's common for
windowless apps to create a hidden, non-interactive window just to be
able to receive all those WM_XXX). So no aboutToQuit() will be emitted
for you when you skip creating that widget window.
But there's an API for windowless apps where you can receive a control
signal, e.g. Ctrl-C was pressed or someone (like the Task Manager) is
shutting down the app, more here:
https://docs.microsoft.com/en-us/windows/console/setconsolectrlhandler
and the callback:
https://docs.microsoft.com/en-us/windows/console/handlerroutine
There's no support for that API in Qt but it's pretty easy to use.
Here's an example, I created a vanilla Qt console app, and changed
main.cpp to this:
-----------------------------------------
#include <QCoreApplication>
#include <windows.h>
BOOL WINAPI CtrlHandler(DWORD fdwCtrlType)
{
switch (fdwCtrlType)
{
case CTRL_CLOSE_EVENT:
::OutputDebugStringA("Ctrl-Close event"); // catch taskkill or
Task Manager End task
return TRUE;
default:
return FALSE;
}
}
int main(int argc, char *argv[])
{
QCoreApplication app(argc, argv);
SetConsoleCtrlHandler(CtrlHandler, TRUE);
return app.exec();
}
-----------------------------------------
Note: I'm using OutputDebugString and DbgView.exe to view the output,
qDebug() output has a tendency to disappear when you test with "End
task" from TaskManager.
Note 2: this API is very flakey if you try it from a GUI-built .exe, for
Qt apps I mean built with "QT += core gui etc" in the .pro file, and use
of QApplication in main.cpp. Instead you need to build your app like a
Qt console app, "QT -= gui" and CONFIG += console, and use
QCoreApplication in main.cpp, like in my example above.
Rgrds Henry
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