Question

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 the ++ or — 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:

#include <QCoreApplication>
#include <QMutex>
#include <QSemaphore>
#include <QThread>
#include <cstdio>

static const int N = 2000000;

class Thread : public QThread {
public:
    Thread();
    void run();
private:
    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;
    }
}

I expected to see g_counter jumping up and down as three threads are changing it at the same time. My problem was that I used run() so it executed as a simple function instead of start() to launch it as a thread. Anyway thanks.

Answer 1

I’m not sure what you are trying to do here, but you need to start the thread by calling start(). You also need to lock the mutex, otherwise what’s the point?

#include <QCoreApplication>
#include <QMutex>
#include <QSemaphore>
#include <QThread>
#include <cstdio>

static const int N = 2000000;

class Thread : public QThread {
public:
  Thread(int id);
  void run();
private:
  int id_;
  static QMutex mutex;
};

QMutex Thread::mutex;
static int g_counter = 0;

int main(int argc, char *argv[]) {
  QCoreApplication app(argc, argv);
  Thread A(0), B(1), C(2);
  A.start();
  B.start();
  C.start();
  char c;
  scanf("%c", &c);
  printf("%d\n", g_counter);
  return app.exec();
}

Thread::Thread(int id) : id_(id){ }

void Thread::run() {

  for (int i = 0; i < N; ++i) {
    mutex.lock();
    ++g_counter;
    printf("g_counter: %d  thread: %d\n", g_counter, id_);
    mutex.unlock();

    mutex.lock();
    --g_counter;
    printf("g_counter: %d  thread: %d\n", g_counter, id_);
    mutex.unlock();
  }
}