c++ - QApplication: How to shutdown gracefully on Ctrl-C

Question

I have a QApplication that, depending on command line parameters, sometimes doesn't actually have a GUI window, but just runs without GUI. In this case, I want to shut it down gracefully if CTRL-C was hit. Basically my code looks like this:

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);

    ... // parse command line options

    if (no_gui) {
        QObject::connect(&app, SIGNAL(unixSignal(int)),
                         &app, SLOT(quit()));
        app.watchUnixSignal(SIGINT, true);
        app.watchUnixSignal(SIGTERM, true);
    }

    ... 

    return app.exec();
}

However, this does not work. CTRL-C seems to be caught (the application doesn't get killed), but it also doesn't exit. What am I missing?

Answer 1

There may be a way to do this natively with Qt -- I poked around the QKeySequence docs for a bit before giving up, but you can just use signal. I don't have Qt/C++ setup on my machine at the moment, but I do have the Python bindings.

import sys, signal
from PyQt4 import QtGui

app = QtGui.QApplication(sys.argv)
signal.signal(signal.SIGINT, signal.SIG_DFL)

sys.exit(app.exec_())

This works and will close the application when I do Ctrl-C. So I believe that your application could adapt this code and it would end up something like this:

#include <signal.h>

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);

    ... // parse command line options

    if (no_gui) {
        signal(SIGINT, SIG_DFL);
    }

    ... 

    return app.exec();
}

Unfortunately, I cannot compile this so it will probably need a few fixes, but this should give you the general idea. By using the SIG_DFL handler you are instructing your program to use the default action associated with Ctrl-C.