Segmentation Fault with Q_Object when used in a static library [Linux]

johnyang

I am currently trying to build a GUI project linking to a static library. I'm working in Centos8 stream with g++ (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10), c++17 and Qt5.15.2. My program throws a segmentation fault before main at run time when I try to use any class in the static library inherited to a Qt Object and has Q_Object marco. If I comment out Q_Object and it run perfectly fine. If I move this class out from the static library and place it in the main GUI project, it also runs fine even with Q_Object. An example code is as follows:

mywindow.h

#include <QMainWindow>

class MyWindow : public QMainWindow
{
    Q_OBJECT // comment out this will resolve segmentation fault
  public:
    MyWindow();
    ~MyWindow();

  private:
    void InitWindow();
};

mywindow.cpp

#include <QHBoxLayout>

MyWindow::MyWindow()
{
    InitWindow();
}

MyWindow::~MyWindow() {}

void
MyWindow::InitWindow()
{
    QWidget* window = new QWidget;
    QHBoxLayout* mainLayout = new QHBoxLayout;
    window->setLayout(mainLayout);
    setCentralWidget(window);
}

main.cpp

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);
    app.setApplicationName("LibAudio Demo App");

    MyWindow myapp;
    myapp.show();

    return app.exec();
}

So if mywindow.h and mywindow.cpp are in a static library, the program crashed with segmentation fault before main. If I move them to the main project, it runs fine. Commenting out Q_Object also solves the issue. However I do want Q_Object for signal and slots implementation.

Anyone has any idea why this is happening and how to resolve this? (e.g. I miss any compile option when creating static library?)

Update 1:
I removed everything else in my static library and just have mywindows.h/cpp files. Surprisingly it runs fines. So how can Q_Object macro gets affected by other implementation?

Update 2:
gdb suggests that the seg fault happens at QString::arg(QString const&, int, QChar) const () from /lib64/libQt5Core.so.5

Christian Ehrlicher

@johnyang said in Segmentation Fault with Q_Object when used in a static library [Linux]:

So how can Q_Object macro gets affected by other implementation?

It does not, it's by accident

Do you create a global static QObject-based class somewhere? Use a debugger and look at the backtrace of your crash.

johnyang

@Christian-Ehrlicher ah yes. I found a static QString that is initialised by QString::arg. After changing that, gdb is suggesting the segmentation fault somewhere else at std::atomic::load. That means it's moving away from QString issue. However I commented out everything related to std::atomic in my code but I'm still at the same spot.

It sounds like the problem is no longer related to Qt stuff, but thanks for the help.

Christian Ehrlicher

@johnyang And what is the backtrace of your current thread? I would guess it comes from a global static QObject.

johnyang

@Christian-Ehrlicher it only says seg fault at QString::arg(QString const&, int, QChar) const () from /lib64/libQt5Core.so.5 nothing else. So this is happening before main. However I'm not sure why I can't initialize a global static QString like:
const QString somestring = QString("%1 %2").arg(QString("something")).arg(QString("something"));

This code works in Windows but not in linux?

JonB

@johnyang said in Segmentation Fault with Q_Object when used in a static library [Linux]:

const QString somestring = QString("%1 %2").arg(QString("something")).arg(QString("something"));

I believe this particular one should be OK as it only uses QString and that is not a QObject. However you could temporarily try const QString somestring = "" and see if the error goes away. Do you have other QString static variables, or other types, or something which calls something more involved than QString?

Christian Ehrlicher

@johnyang said in Segmentation Fault with Q_Object when used in a static library [Linux]:

This code works in Windows but not in linux?

By accident. I would guess it's not "something" but another QString

johnyang

@Christian-Ehrlicher Yes. I think it's to do with that something. It is also a global static QString somewhere. Maybe it's the order of definition.

Christian Ehrlicher

@johnyang said in Segmentation Fault with Q_Object when used in a static library [Linux]:

Maybe it's the order of definition.

Not maybe, it is. The order of the initialization is undefined. Don't do it, you won't need it. Also you should take a look at QStringLiteral - there no initialization is needed