SFML thread synchronization?

Question

I'm new to SFML, been trying to have a multi-threading game system (all of the game logic on the main thread, and the rendering in a dedicated thread using sf::Thread; mainly for practicing with threads) as explained in this page ("Drawing with threads" section):

Unfortunately my program has a long processing time during it's update() and makes the rendering process completely out of control, showing some frames painted and some others completely empty. If it isn't obvious my rendering thread is trying to paint something that isn't even calculated, leaving this epileptic effect.

What I'm looking for is to allow the thread to render only when the main logic has been calculated. Here's what I got so far:

void renderThread()
{
    while (window->isOpen())
    {
            //some other gl stuff
            //window clear
            //window draw
            //window display
    }
}

void update()
{
    while (window->isOpen() && isRunning) 
    {
        while (window->pollEvent(event)) 
        {
            if (event.type == sf::Event::Closed || sf::Keyboard::isKeyPressed(sf::Keyboard::Escape)) 
            {
                isRunning = false;
            }
            else if (m_event.type == sf::Event::Resized) 
            {
                glViewport(0, 0, m_event.size.width, m_event.size.height);
            }

        }

        // really resource intensive process here

        time = m_clock.getElapsedTime();
        clock.restart().asSeconds();
    }
}

Thanks in advance.

Answer 1

I guess the errors happen because you manipulate elements that are getting rendered at the same time in parallel. You need to look into mutexes. Mutexes lock the element you want to manipulate (or draw in the the other thread) for as long as the manipulation takes and frees it afterwards. While the element is locked it can not be accessed by another thread.

Example in pseudo-code:

updateThread(){
    renderMutex.lock();
    globalEntity.manipulate();
    renderMutex.unlock();
}

renderThread(){
    renderMutex.lock();
    window.draw(globalEntity);
    renderMutex.unlock();
}