Qt-Signal not connecting in virtual machine

Question

I develop and compile on RedHat 7.4 with GCC 4.8.5 and Qt 4.8.5. The code has to be statically linked. Then executed on a virtual machine running Scientific Linux release 6.7. memcpy-Wrap is used in order to prevent dependencies on newer GLIBC >= 2.4

I have the following MWE:

#include <iostream>
#include <unistd.h>
#include <QtCore>
#include <QThread>

__asm__(".symver memcpy, memcpy@GLIBC_2.2.5");
extern "C" {
void *__wrap_memcpy(void *dest, const void *src, size_t n) { return memcpy(dest, src, n); }
}

class Worker : public QThread {
    void run() {
        std::cout << "WORKER: Started." << std::endl;
        QObject::connect(this, SIGNAL(finished()), QCoreApplication::instance(), SLOT(quit()));
        int i=0;
        while(i++<3) {
            std::cout << "WORKER: I am running." << std::endl;
            usleep(1e6);
        }
        std::cout << "WORKER: Finished." << std::endl;
    }
};

int main(int argc, char** argv) {

    std::cout << "MAIN: Init QCoreApplication" << std::endl;
    QCoreApplication qtApplication(argc, argv);

    std::cout << "MAIN: Init Worker" << std::endl;
    Worker myWorker;
    myWorker.start();

    std::cout << "MAIN: Start Event-Loop." << std::endl;
    qtApplication.exec();

    std::cout << "MAIN: Event-Loop finished." << std::endl;
    return 0;
}

This code is compiled on the RedHat-System with

g++ -I$QTD/mkspecs/linux-g++ -I$QTD/include -I$QTD/include/QtCore -o mwe mwe.cpp  -Wl,--wrap=memcpy -L$QTD/lib/ -lQtCore -lQtNetwork -lglib-2.0 -lrt -lpthread -ldl -lz

where $QTD holds my installation of my Qt-4.8.5.

The following behavior is expected and observed on the Red-Hat-System:

MAIN: Init QCoreApplication
MAIN: Init Worker
MAIN: Start Event-Loop.
WORKER: Started.
WORKER: I am running.
WORKER: I am running.
WORKER: I am running.
WORKER: Finished.
MAIN: Event-Loop finished.

The following behavior is observed on the Scientific-Linux-System:

MAIN: Init QCoreApplication
MAIN: Init Worker
MAIN: Start Event-Loop.
WORKER: Started.
WORKER: I am running.
WORKER: I am running.
WORKER: I am running.
WORKER: Finished.

And then the application never finishes.

It seems that in the Red-Hat-System, the finished-signal from the worker-thread is connected to the quit-slot in the core application. This doesn't seem to happen in the Scientific-Linux-System. Does anyone have any advice why this happens and how I can debug it?

Answer 1

You have to use QThread in proper way. overloading run() function is not safe and Nokia accept it before and show how to use QThread.

Look at this documentation.

Answer 2

As it was mentioned - it is not recommended way to work with threads in Qt. But it is fine for your case. Anyway, I recommend you to try the next approach:

class Worker : public QThread {
    Q_OBJECT // Don't forget this macro
    void run() {
        std::cout << "WORKER: Started." << std::endl;
        // QObject::connect(this, SIGNAL(finished()), QCoreApplication::instance(), SLOT(quit()));
        int i=0;
        while(i++<3) {
            std::cout << "WORKER: I am running." << std::endl;
            usleep(1e6);
        }
        std::cout << "WORKER: Finished." << std::endl;
        this->deleteLater();
    }
};

int main(int argc, char** argv) {

    std::cout << "MAIN: Init QCoreApplication" << std::endl;
    QCoreApplication qtApplication(argc, argv);

    std::cout << "MAIN: Init Worker" << std::endl;
    Worker *myWorker = new Worker();
    QObject::connect( myWorker, SIGNAL(destroyed()), &qtApplication, SLOT(quit());
    myWorker->start();

    std::cout << "MAIN: Start Event-Loop." << std::endl;
    qtApplication.exec();

    std::cout << "MAIN: Event-Loop finished." << std::endl;
    return 0;
}