Trying to get problems with QThread

Hello guys I'm learning Qt and I've reached QThread class. Having no experience in multithreading I spent several hours studying semaphores, mutexes, critical sections and wait functions in Win32API. When I launched several threads there and ++ and -- a global variable without synchronization I got different results each time. Now I am trying to do the same with QThread but I am getting failed. Can you tell me what's wrong? here is my code:

Code:

---

#include <QCoreApplication>
#include <QMutex>
#include <QThread>
#include <cstdio>
 
static const int N = 2000000;
 
class Thread : public QThread {
public:
    Thread();
    void run();
private:
    void Inc();
    void Dec();
    static QMutex mutex;
};
 
QMutex Thread::mutex;
static int g_counter = 0;
 
int main(int argc, char *argv[]) {
    QCoreApplication app(argc, argv);
    Thread A, B, C;
    A.run();
    B.run();
    C.run();
    char c;
    scanf("%c", &c);
    printf("%d\n", g_counter);
    return app.exec();
}
 
Thread::Thread() {
 
}
 
void Thread::run() {
    //QMutexLocker lock(&mutex);
    for (int i = 0; i < N; ++i) {
        ++g_counter;
        --g_counter;
    }
}

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First of all, you should call QThread::start() on the threads.