程序并行QThread正在应用程序退出时创建内存泄漏

Question

我有一个更大的项目，有一个GUI，我想在后台管理一些文件。 我为这个任务实现了一个新线程，在运行时，一切都很好。 但是一旦我退出应用程序， visual-leak-detector发现3-7内存泄漏。

我分离了我的线程代码并创建了一个新项目来检查这个最小的代码示例，但我仍然无法解决我的问题。

我认为它与主程序的事件循环有关。 也许循环不会处理最后一个事件来删除我的线程类和线程本身。 因为我在析构函数中停止并退出线程。 但我不确定这一点。

这是我的最小代码：threadclass.hpp：

#include <QObject>
#include <QDebug>

class ThreadClass : public QObject {
    Q_OBJECT

public:
    explicit ThreadClass() {}
    virtual ~ThreadClass(){
        qDebug() << "ThreadClass Destructor";
    }

signals:
    // emit finished for event loop
    void finished();

public slots:
    // scan and index all files in lib folder
    void scanAll(){
        for(long i = 0; i < 10000; i++){
            for (long k = 0; k < 1000000; k++);
            if(i%500 == 0)
                qDebug() << "thread: " << i;
        }
    }
    // finish event loop and terminate
    void stop(){
        // will be processed after scanall is finished
        qDebug() << "STOP SIGNAL --> EMIT FINSIHED";
        emit finished();
    }
};

threadhandler.hpp：

#include <QObject>
#include <QThread>
#include "threadclass.hpp"

class ThreadHandler : public QObject {
    Q_OBJECT

public:
    explicit ThreadHandler(QObject *parent = 0) : parent(parent), my_thread(Q_NULLPTR) {}

    virtual ~ThreadHandler() {
        // TODO Check!
        // I think I don't have to delete the threads, because delete later
        // on finish signal. Maybe I just have to wait, but then how do I
        // check, if thread is not finished? Do I need to make a bool var again?

        if (my_thread != Q_NULLPTR && my_thread->isRunning())
        {
            emit stopThread();
            //my_thread->quit();
            my_thread->wait();
            //delete my_thread;
        }

        qDebug() << "ThreadHandler Destructor";
        my_thread->dumpObjectInfo();
    }

    void startThread(){
        if (my_thread == Q_NULLPTR)
        {
            my_thread = new QThread;
            ThreadClass *my_threaded_class = new ThreadClass();
            my_threaded_class->moveToThread(my_thread);

            // start and finish
            QObject::connect(my_thread, &QThread::started, my_threaded_class, &ThreadClass::scanAll);
            QObject::connect(this, &ThreadHandler::stopThread, my_threaded_class, &ThreadClass::stop);

            // finish cascade
            // https://stackoverflow.com/a/21597042/6411540
            QObject::connect(my_threaded_class, &ThreadClass::finished, my_threaded_class, &ThreadClass::deleteLater);
            QObject::connect(my_threaded_class, &ThreadClass::destroyed, my_thread, &QThread::quit);
            QObject::connect(my_thread, &QThread::finished, my_thread, &QThread::deleteLater);

            my_thread->start();
        }
    }

signals:
    void stopThread();

private:
    QObject *parent;
    QThread* my_thread;
};

main.cpp很糟糕，但似乎很好地模拟了我的主程序的行为：

#include <QCoreApplication>
#include <QTime>
#include <QDebug>
#include "threadhandler.hpp"

#include <vld.h>

int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);

    ThreadHandler *th = new ThreadHandler();
    th->startThread();

    // wait (main gui programm)
    QTime dieTime= QTime::currentTime().addSecs(5);
    while (QTime::currentTime() < dieTime) {
        QCoreApplication::processEvents(QEventLoop::AllEvents, 100);
    }

    qDebug() << "DELETE TH";
    delete th;
    qDebug() << "FINISH ALL EVENTS";
    QCoreApplication::processEvents(QEventLoop::AllEvents, 500);
    qDebug() << "QUIT";
    QCoreApplication::quit();
    qDebug() << "CLOSE APP";
    // pause console
    getchar();
//    return a.exec();
}

以下是VLD的输出：

WARNING: Visual Leak Detector detected memory leaks!
...
turns out this is very boring and uninteresting
...
Visual Leak Detector detected 3 memory leaks (228 bytes).
Largest number used: 608 bytes.
Total allocations: 608 bytes.
Visual Leak Detector is now exiting.
The program '[8708] SOMinimalExampleThreads.exe' has exited with code 0 (0x0).

更新1：我添加了qDebug() << "ThreadClass Destructor"; 到ThreadClass的析构函数，我的新输出如下所示：

...
thread:  9996
thread:  9997
thread:  9998
thread:  9999
ThreadHandler Destructor
FINISH ALL EVENTS
CLOSE APP

现在很明显，我的线程类的析构函数从未被调用，因此在虚空中丢失。 但那为什么这不起作用？

QObject::connect(my_threaded_class, &ThreadClass::finished, my_threaded_class, &ThreadClass::deleteLater);

更新2：我在ThreadHandler中发现了一个问题：

emit stopThread();
my_thread->quit(); // <-- stops the event loop and therefore no deletelater
my_thread->wait();

我删除了my_thread->quit() ，现在调用了ThreadClass的析构函数，但是my_thread->wait()永远不会完成。

Answer 1

我发现my_thread->wait()函数阻止了事件循环，因此quit和deleteLater级联永远不会完成。 我用另一种等待完成的线程的方法修复了这个问题：

removed

以下是Mike新实施的解决方案。 这很容易实现，我只需要在threadhandler类中更改与my_threaded_class::stop的连接。

#include <QObject>
#include <QThread>
#include "threadclass.hpp"

class ThreadHandler : public QObject {
    Q_OBJECT

public:
    explicit ThreadHandler(QObject *parent = 0) : parent(parent), my_thread(Q_NULLPTR) {}

    virtual ~ThreadHandler() {

        if (my_thread != Q_NULLPTR && my_thread->isRunning())
        {
            my_thread->quit();
            my_thread->wait();
        }

        qDebug() << "ThreadHandler Destructor";
    }

    void startThread(){
        if (my_thread == Q_NULLPTR)
        {
            my_thread = new QThread;
            ThreadClass *my_threaded_class = new ThreadClass();
            my_threaded_class->moveToThread(my_thread);

            // start and finish
            QObject::connect(my_thread, &QThread::started, my_threaded_class, &ThreadClass::scanAll);
            // https://stackoverflow.com/questions/53468408
            QObject::connect(my_thread, &QThread::finished, my_threaded_class, &ThreadClass::stop);

            // finish cascade
            // https://stackoverflow.com/a/21597042/6411540
            QObject::connect(my_threaded_class, &ThreadClass::finished, my_threaded_class, &ThreadClass::deleteLater);
            QObject::connect(my_threaded_class, &ThreadClass::destroyed, my_thread, &QThread::quit);
            QObject::connect(my_thread, &QThread::finished, my_thread, &QThread::deleteLater);

            my_thread->start();
        }
    }

signals:
    void stopThread();

private:
    QObject *parent;
    QThread* my_thread;
};

Answer 2

问题描述：

当ThreadHandler的析构函数从主线程发出stopThread ，Qt通过将事件发布到工作线程的事件循环（也称为排队连接）来调用连接的插槽（ &ThreadClass::stop ）。 这意味着当发出此信号时，工作人员的事件循环需要准备好接收新事件（因为您依靠它来执行正确的清理）。 但是，正如您已经发现的那样，对thread->quit()的调用可能导致事件循环比所需的更早退出（在工作线程调用ThreadClass::stop ，因此信号ThreadClass::finished不发出）。 您可能想要检查此最小示例的输出，该示例再现了我正在谈论的行为：

#include <QtCore>

/// lives in a background thread, has a slot that receives an integer on which
/// some work needs to be done
struct WorkerObject : QObject {
  Q_OBJECT
public:
  using QObject::QObject;
  Q_SLOT void doWork(int x) {
    // heavy work in background thread
    QThread::msleep(100);
    qDebug() << "working with " << x << " ...";
  }
};

/// lives in the main thread, has a signal that should be connected to the
/// worker's doWork slot; to offload some work to the background thread
struct ControllerObject : QObject {
  Q_OBJECT
public:
  using QObject::QObject;
  Q_SIGNAL void sendWork(int x);
};

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);

  QThread thread;
  WorkerObject workerObj;
  workerObj.moveToThread(&thread);
  // quit application when worker thread finishes
  QObject::connect(&thread, &QThread::finished, &a, &QCoreApplication::quit);
  thread.start();

  ControllerObject controllerObj;
  QObject::connect(&controllerObj, &ControllerObject::sendWork, &workerObj,
                   &WorkerObject::doWork);

  for (int i = 0; i < 100; i++) {
    QThread::msleep(1);
    // call thread.quit() when i is equal to 10
    if (i == 10) {
      thread.quit();
    }
    controllerObj.sendWork(i);
  }
  return a.exec();
}

#include "main.moc"

在我的机器上，这是一个可能的输出：

working with  0  ...
working with  1  ...
working with  2  ...
working with  3  ...

请注意，尽管在主线程的第十次迭代中调用了thread.quit() ，但工作线程可能不会处理在退出调用之前收到的所有消息（并且我们将值3作为最后一个值处理工人）。 ^*

解：

实际上，Qt提供了一种规范方法来退出工作线程并执行必要的清理，因为信号QThread::finished以特殊方式处理：

发出此信号时，事件循环已停止运行。 除延迟删除事件外，线程中不再处理任何事件 。 此信号可以连接到QObject :: deleteLater（），以释放该线程中的对象。

这意味着你可以使用thread->quit() （就像你一样），但你只需要添加：

connect(my_thread, &QThread::finished, my_threaded_class, &ThreadClass::stop);

到你的startThread并删除不必要的emit stopThread(); 来自析构函数。

---

^*我在文档中找不到详细解释这种行为的任何页面，所以我提供了这个最小的例子来解释我在说什么。