Share a variable between main and thread

Question

I got a class MainWindow that open a server function in a thread , I need to share a bool variable between my main and my thread, I try to use volatile variable but it doesn't work, here's the code :

//Constructor
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    //Some initialisation
...

    // Constructs the new thread and runs it. Does not block execution.
    bool_Server = true;//Variable supposed to be shared
    m_t1 = std::thread(lancerServeur, bool_Server);

}

MainWindow::~MainWindow()
{
    delete ui;
    bool_Server = false; //Variable supposed to be shared
    m_t1.join();
}


void MainWindow::lancerServeur(bool boolServer){
    serveur s;
    while(boolServer){
        s.receiveDataUDP();//Read data in non blocking mode
    }
}

Is the volatile variable shared ?

Answer 1

You're passing a copy of bool_Server to MainWindow::lancerServeur, so the variable it's observing is not in any way connected to the original bool_Server. Making it volatile isn't going to help, and volatile doesn't make access to an object thread-safe anyway.

You should use an atomic<bool> as the flag, and make it a data member of MainWindow. There's no need to pass it to lancerServeur. Here's a simple example that runs a thread for 5s and then quits.

#include <atomic>
#include <thread>
#include <chrono>
#include <iostream>

struct MainWindow
{
  std::atomic<bool> stop_{false};
  std::thread task_;

  void run()
  {
    while(!stop_) {
      std::cout << "Processing ...\n";
      std::this_thread::sleep_for(std::chrono::seconds(1));
    }

    std::cout << "Stopping ...\n";
  }

  void launch_thread()
  {
    task_ = std::thread(&MainWindow::run, this);
  }

  ~MainWindow()
  {
    stop_ = true;
    task_.join();
  }
};

int main()
{
  {
    MainWindow w;
    w.launch_thread();
    std::this_thread::sleep_for(std::chrono::seconds(5));
  }
}

Answer 2

In the .h file change bool_Server to be std::atomic<bool> bool_Server and change your .cpp file to be:

//Constructor
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    //Some initialisation
...

    // Constructs the new thread and runs it. Does not block execution.
    bool_Server = true;//Variable supposed to be shared
    m_t1 = std::thread(lancerServeur, std::ref(bool_Server));

}

MainWindow::~MainWindow()
{
    delete ui;
    bool_Server = false; //Variable supposed to be shared
    m_t1.join();
}


void MainWindow::lancerServeur(std::atomic<bool>& boolServer){
    serveur s;
    while(boolServer){
        s.receiveDataUDP();//Read data in non blocking mode
    }
}