sf::Texture::~Texture() Crashes App On Exit

[SlimShady]

Hello peeps,

I've been using SFML for quite some time now, and I have to say that I absolutely love it.

Currently I'm building a game engine + all sorts of tools for my future games, and I just threw together this little file downloader that can be used in combination with SFML resources' loadFromMemory() function.

It all works fine, the image is displayed correctly, but when I close the application, it crashes with an EXC_BAD_ACCESS error and shows me the assemly code for the destructor of the Texture and for some releasePool() method. I cannot seem to figure out why this is happening. It works without any problems with sf::Font and sf::SoundBuffer. Please help.

Download.cpp

(click to show/hide)

Download::Download(const URL& url) :
m_data(),
m_url(url),
m_size(0),
m_state(Stopped),
m_function([](bool, const std::string&) {}){
    fetchMetadata();
}

Download::~Download() {
    m_thread.join();
}

void Download::onComplete(std::function<void (bool success, const std::string&)> function) {
    m_function = function;
}

void Download::start() {
    m_state = Downloading;
   
    m_thread = std::thread(&Download::run, this);
}

int Download::getSize() const {
    return m_size;
}

Download::State Download::getState() const {
    return m_state;
}

bool Download::isComplete() const {
    return m_state == Complete;
}

void Download::fetchMetadata() {
    sf::Http http(m_url.getProtocol() + "://" + m_url.getHost());
    sf::Http::Request request(m_url.getPath(), sf::Http::Request::Head);
    sf::Http::Response response = http.sendRequest(request);

    if (response.getStatus() == sf::Http::Response::Ok) {
        m_size = std::stoi(response.getField("Content-Length"));
    }
}

void Download::run() {
    sf::Http http(m_url.getProtocol() + "://" + m_url.getHost());
    sf::Http::Request request(m_url.getPath(), sf::Http::Request::Get);
    sf::Http::Response response = http.sendRequest(request);
   
    bool success;
   
    if (response.getStatus() == sf::Http::Response::Ok) {
        m_state = Complete;
        m_data = response.getBody();
        success = true;
    } else {
        m_state = Error;
        success = false;
    }
   
    if (m_function) {
        m_function(success, m_data);
    }
}
 

main.cpp

(click to show/hide)

int main(int argc, const char * argv[]) {
    sf::Texture texture;
    sf::Sprite sprite;
   
    URL url("http://r.ddmcdn.com/s_f/o_1/cx_633/cy_0/cw_1725/ch_1725/w_720/APL/uploads/2014/11/too-cute-doggone-it-video-playlist.jpg");
    Download download(url);
   
    download.onComplete([&](bool success, const std::string& data) {
        texture.loadFromMemory(data.c_str(), data.length());
        sprite.setTexture(texture);
    });
   
    download.start();

    sf::RenderWindow window(sf::VideoMode::getDesktopMode(), "Downloading...");

    while (window.isOpen()) {
        sf::Event event;

        while (window.pollEvent(event)) {
            if (event.type == sf::Event::Closed) {
                window.close();
            }
        }

        window.clear(sf::Color(60, 60, 60));

        window.draw(sprite);

        window.display();
    }

    return 0;
}
 

If I replace sf::Texture and sf::Sprite with sf::Font and sf::Text respectively (and change the URL obviously), there is no problem at all.

[Hapax]

Do you still have this problem if you don't use a separate thread?

Although the order that this code is executed is not very clear, it does look like you pass a lambda function to download and that function gets called when download.run() has finished, which is in a separate thread. This function, which is in a separate thread, accesses texture while the main thread is constantly accessing sprite, which uses texture. Since you don't have any form of security against these data-races, I would say that unpredictable behaviour should be expected

Even if this isn't the cause of the error, your use of threads is rather dangerous.

[SlimShady]

Even if this isn't the cause of the error, your use of threads is rather dangerous.

Thanks for the quick reply. I've felt it was error prone, but now you confirmed it, haha. Thanks for the explanation.

I managed to apply a quick-fix by eliminating the callback and querying the state every frame, like this:

Code: [Select]

if (download.isComplete()) {
    std::string data = download.getData();
    texture.loadFromMemory(data.c_str(), data.length());
    sprite.setTexture(texture);
}

But it's not what I wanted originally. So I guess my question changes to: How do I achieve this using a callback function? Do you have any suggestions on how to improve the code?

[korczurekk]

Your first solution doesn't appear to be well-defined (you modify sf::Texture from Download::run method, which is in another thread), are sf::Texture and sf::Sprite thread-safe?

[Herb]

Some code to help with how to use callbacks in a thread-safe manner:

typedef std::function<void(const std::string &data, bool success)> DownloadCallback;

struct DownloadItem {
    std::string data;
    DownloadCallback callback;
    bool success;
};

class Downloader {
public:
    void downloadItem(const std::string &itemUrl, const DownloadCallback &itemCallback) {
        _mutex.lock();
        // save itemUrl and itemCallback
        _mutex.unlock();
        // items get processed in background thread
    }

    void update() {
        DownloadItem item;
        bool gotItem = false;

        _mutex.lock();
        if (!_completedList.empty()) {
            item = _completedList[0];
            gotItem = true;
            _completedList.erase(_completedList.begin());
        }
        _mutex.unlock();

        if (gotItem) {
            item.callback(item.data, item.success);
        }
    }
private:
    // could use a queue instead of vector
    std::vector<DownloadItem> _completedList;
    std::mutex _mutex;
};

// UI thread
MyApp::run() {

    downloader.downloadItem("http://mysite.com/image.png", [&](const std::string &data, bool success) {
       
        if (success) {
            texture.loadFromMemory(data.c_str(), data.size());
        } else {
            sf::err() << "Failed to download image.\n";
        }
    });

    while (window.isOpen()) {

        // poll the downloader for completions
        downloader.update();

        // draw stuff
    }
}