Segmentation fault on SIGINT interception when trying to exit the QCoreApplication

[mbruel]

Hi,
I want to use a cleanup function when my QCoreApplication is destroyed (intercepting a SIGINT).
I've tried two methods:
First to connect the QCoreApplication::aboutToQuit to a slot that would do the job but I don't end up in the slot when the application is closed by a SIGINT or even if close manually the app (launched via QT Creator)...

So my second option would be to use signal interceptions. Here is what I do:

#include <csignal>

Proxy *theProxy   = NULL;
QCoreApplication *app = NULL;

void handleShutDown(int signal){
    Proxy::shutDown();
    std::cout << "Try to exit the event loop\n";
    app->exit(0);
}

int main(int argc, char *argv[])
{
    app = new QCoreApplication(argc, argv);

    signal(SIGINT,  &handleShutDown);// shut down on ctrl-c
    signal(SIGTERM, &handleShutDown);// shut down on killall


    theProxy = Proxy::getInstance();
//    QObject::connect(app, &QCoreApplication::aboutToQuit, theProxy, &Proxy::deleteInstance);

    if (!theProxy->start()){
        std::cout << "Error starting Proxy...\n";
        return 1;
    }

    app->exec(); // Run the event loop

    std::cout << "Event loop ended...";
    return 0;
}

So here is what I get when I stop my app with a SIGINT:

^CTry to exit the event loop
Segmentation fault

I can see that the cleanup funtion is well executed but then there is a segmentation fault when I try to exit the QCoreApplication.
I've no clue how I could debug this...
Any idea what I'm doing wrong or how I could get traces to understand more the issue?

Thanks.

[kshegunov]

Hello,
Firstly, do not create the QCoreApplication instance in the heap, you gain only headaches. Secondly, if you really need a global static variable (which I believe you don't) you should use Q_GLOBAL_STATIC. You would want to intercept the signals only if you have some very specific setup where you need to cleanup some global resources that don't belong to your application (i.e. a shutdown routine for a driver). Global variables are like premature optimization, they're troublesome and in most cases purely unnecessary, don't use them unless you're forced to.

Kind regards.

[SGaist]

Hi,

To add to @kshegunov, you can access the instance of QCoreApplication through qApp.

[mbruel]

This post is deleted!

[mbruel]

Ok, I've let the QCoreApplication on the stack and don't use anymore a global variable for my Proxy. I don't really need to as I'm using the singleton pattern. Both constructor and destructor are private.
I'm using a static method to get/create the instance and another one to delete it.
Here is the new code.

#include <csignal>

void handleShutDown(int signal){
    std::cout << "Try to shutdown the proxy\n";
    Proxy::shutDown();
    std::cout << "Try to exit the event loop\n";
    qApp->exit(0);
}

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

    signal(SIGINT,  &handleShutDown);// shut down on ctrl-c
    signal(SIGTERM, &handleShutDown);// shut down on killall

    Proxy *theProxy = Proxy::getInstance();
//    QObject::connect(app, &QCoreApplication::aboutToQuit, theProxy, &Proxy::deleteInstance);

    if (!theProxy->start()){
        std::cout << "Error starting Proxy...\n";
        return 1;
    }

    app.exec(); // Run the event loop

    std::cout << "Event loop ended...";
    return 0;
}

I'm still having the same segmentation fault. Any idea why? The object I'm deleting inherit from QTcpServer and is just listening.
I'm having the crash without having done anything.
As I said, the object is destructed and the crash seems to happen after on the exit call to stop the event loop.

Server started, listening on port: 11111
^CTry to shutdown the proxy
Try to exit the event loop
Segmentation fault

PS: I need to have a cleanup function. In most cases, my program will be stopped by an interruption and I want to make sure it releases the resources properly and take the time to write final logs before shutting down.

[mbruel]

Alright the handling of the shutdown interruption is working fine.
I've managed to find what was causing the segmentation fault. It is the construction/deletion of my SQL class that do a QSqlDatabase::addDatabase...
I'm going to try to understand what could be the problem and open another post if I don't manage to get what is wrong.

[kshegunov]

@mbruel
Is your SQL class by any chance a singleton as well? If you answer yes, then this is one of the reasons you should not use global variables (a singleton is pretty much the same as a global variable).

[SGaist]

Also, what backtrace to you get from the debugger ?