CODEWITHSUNDEEP
c++multithreading
Alexander Tepe
Alexander Tepe

Reputation: 190

C++ threads freeze program

I am currently working on a game in C++ which is supposed to use multithreading to handle bot instances. The Bot class has a member function start(), which calls the bot's function walk() in a new thread. This is my attempt (in excerpts:)

void Bot::walk(Physics& physics)
{
    // do stuff
    std::cout << "call to walk() function" << std::endl;
}

void Bot::start(Physics& physics)
{
    std::thread m_thread(&Bot::walk, this, std::ref(physics));
    m_thread.join();
}

In another class which handles the initialization of the game, all bot instances are started like this: (m_bots is a vector of Bots)

void Level::start()
{
    // Start all Bots
    for(auto it: m_bots)
    {
        it->start(*m_physics);
    }
}

On startup the game window opens as usual but then continues to freeze. The console output shows that the walk() function is called though.

What thread-specific catch am I missing?

I hope these excerpts cover all the relevant information. If this is not the case, please let me know!

Upvotes: 1

Views: 1735

Answers (1)

Pete Becker
Pete Becker

Reputation: 76523

Joining a thread blocks until the thread finishes. As a result, Bot::start won't return until the thread that it spawns finishes, so only one thread will run at a time.

Someone will probably tell you that the solution is to change join() to detach(). That's not really the solution, unless your design calls for a bunch of free-running threads; if you care about when they finish, join() is the way to do it, just not there.

One approach is to create an object of type std::vector<std::thread> and launch all of the thread with calls to Bot::start. After the threads are running you can join all the spawned threads.

Upvotes: 6

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