Qt thread ID is equal to MainWindows? (moveToThread)

Question

Now I want to create a thread and put in my class "AVC_file" inatance.

But when I print currentThreadId in textBroswer, I found MainWindows's threadID is same with the thread I created. show pitures like below.

Framework::Framework(QWidget * parent) : QMainWindow(parent)
{
  ui.setupUi(this);

  int threadID = (int)QThread::currentThreadId();
  ui.textBrowser->append("Main Tread ID : " + QString::number(threadID));
}

void Framework::on_OpenAVCFile_clicked()
{
    QString filePath = QFileDialog::getOpenFileName(
        this, tr("Open File"), "C:\\", "AVC File (*.avc)"
    );


    if (!filePath.isEmpty())
    {
        QMessageBox::information(this, tr("File Name"), filePath);
    }

    QFile file(filePath);
    if (!file.open(QIODevice::ReadOnly))
    {
        QMessageBox::information(0, "info", file.errorString());
    }
    else {
        QThread *thread = new QThread(this);
        int threadID = (int)thread->currentThreadId();
        ui.textBrowser->append("Second Tread ID : " + QString::number(threadID) + "\n");
        AVC_File *AVC_file = new AVC_File();

        AVC_file->moveToThread(thread);
        connect(AVC_file, SIGNAL(requestFileContent(QString)), this, SLOT(addFileContent(QString)));
        connect(AVC_file, SIGNAL(requestFileDebug(QString)), this, SLOT(addFileDebug(QString)));
        connect(AVC_file, SIGNAL(requestFileCorrectness(bool, int)), this, SLOT(adddFileCorrectness(bool, int)));
        connect(AVC_file, SIGNAL(requestNewValue(unsigned int, int)), this, SLOT(addNewValue(unsigned int, int)));
        thread->start();

        AVC_file->AVC_FileCheck(file);
    }
}

Images about my code and results--> Main Windows, create thread and results

Oh!I also try emit info in my "AVC_file" instance?like below.

void AVC_File::AVC_FileCheck(QFile &file)
{
    int threadID = (int)QThread::currentThreadId();
    emit requestFileContent("Thread ID by emit" + QString::number(threadID) + "\n");
    QTextStream in(&file);
    ........
    ........
}

Emit threadID info

Anyone can help me?

BTW, I use visual studio Qt add-in to develop this project.

score 5 · Answer 1 · answered Dec 14 '16 at 11:48

5

QThread::currentThreadId() is a static method.

When you call it, it returns the thread ID of the thread that executes it.

In both your cases that's the main thread.

answered Dec 14 '16 at 11:48

Kevin Krammer

5,159
2
9
22

According to your answer, how to I get another thread ID in my main thread? – WilliamChen Dec 14 '16 at 12:11
Why do you need a new ID ? Maybe store an ID inside your AVC_File for identification. – Elcan Dec 14 '16 at 12:29

score 2 · Accepted Answer · edited May 23 '17 at 12:32

There are several issues that I'll address in random order.

First of all, using thread IDs is bad user experience. Give the threads a descriptive name:

int main(...) {
  QApplication app(...);
  QThread myThread;
  MyObject myObject;
  myObject->moveToThread(&myThread);
  QThread::currentThread()->setObjectName("mainThread");
  myThread.setObjectName("myThread");
  ...
}

Then use QThread::currentThread()->objectName() to retrieve it. You can also pass QObject* to qDebug() to display the name of the thread:

qDebug() << QThread::currentThread();

Your signal invocation would then become:

QString currentThreadName() {
  return QThread::currentThread()->objectName().isEmpty() ?
    QStringLiteral("0x%1").arg(QThread::currentThread(), 0, 16) :
    QThread::currentThread()->objectName();
}

   ...
   emit requestFileContent(
     QStringLiteral("Emitting from thread \"%1\"\n").arg(currentThreadName));

Then, use the above to deal with the thread you've created:

    auto thread = new QThread(this);
    thread->setObjectName("fileThread");
    ui.textBrowser->append(QStringLiteral("Worker thread: \"%1\").arg(thread->objectName()));
    auto AVC_file = new AVC_File;

    AVC_file->moveToThread(thread);
    ...

But AVC_FileCheck is invoked from the main thread. Whether that's OK or not depends on how that method is implemented. It needs to be thread-safe, see this question for a discussion of that. TL;DR: The following pattern could be a starting point:

class AVC_file : public QObject {
  Q_OBJECT
  Q_SLOT void fileCheck_impl(QIODevice * dev) {
    dev->setParent(this);
    ...
  }
  Q_SIGNAL void fileCheck_signal(QIODevice *);
public:
  void fileCheck(QIODevice *dev) { fileCheck_signal(dev); }
  AVC_file(QObject *parent = nullptr) : QObject(parent) {
    connect(this, &AVC_file::fileCheck_signal, this, &AVC_file::fileCheck_impl);
    ...
  }
};

Finally, your existing AVC_fileCheck API is broken. You pass QFile by reference: this won't ever work since it ceases to exist as soon as on_OpenAVCFile_clicked returns. When AVC_file uses that file in its thread, it's a dangling object reference.

Instead, you must pass the ownership of the file to AVC_file, and pass a pointer to an instance that AVC_file will dispose when done with. Or simply let AVC_file open the file for you!

Qt thread ID is equal to MainWindows? (moveToThread)

2 Answers2