QT Creator：程序在调试模式下崩溃，但在发布模式和调试模式下工作，带有基于QThread的程序的断点

Question

I am working on a Desktop (Windows 7) based application and using Qt Creator v 5.6.0 for development of the Program. I have a very strange issue ie

1. My Program crashes in DEBUG mode but works fine in RELEASE mode.

2. If in DEBUG mode, and I put break points to find the reason of the crash, then it doesn't crash: It work properly. But if I do not put any break points then it crashes at below code:

Project Background: My Project includes functionality to read from the device connected at System communication port and transmits data to the MainWindow UI to display. Since to communicate with the communication port we have to use the third party library so I am not using QtSerial Port class which is much simpler and easy to use.

Code Design: Class MainClass : In this class we have created some forms to display the data read from the device.

Class TestClass: This class will handle all the communication with the device connected at the system Serial Port and use the third party library. This class also have the while loop to read data from the device connected at Serial Port.

Since Test Class is using while loop. So we decided to make a Test Class run in different Thread.

Code for creating Thread in MainClass Constructor:

MainClass::MainClass (QWidget *parent) : QDialog(parent),
    ui(new Ui::Analzyer)
{
   ............................
   ............................

   workerThread = new QThread;
   testClassObject = new TestClass();   // Declared in HeaderFile of MainClass        

    if((workerThread != NULL) && (testClassObject != NULL))
    {
        workerThread ->moveToThread(testClassObject );

        connect(workerThread , SIGNAL(started()), testClassObject, SLOT(SomeFunc()));
        connect(testClassObject, SIGNAL(exit()), workerThread , SLOT(quit()));
        connect(testClassObject, SIGNAL(exit()), testClassObject,  SLOT(deleteLater()));
        connect(workerThread , SIGNAL(finished()), workerThread , SLOT(deleteLater()));

        // connectToPort Signal is emitted when User clicks the pushbutton from   // Main class UI
        connect(this, SIGNAL(connectToPort(QString)), testClassObject, SLOT(openPort(QString)));
    }

}

Crash Code:

void TestClass::openPort(const QString portName)
{
    // Here portName is say : "Appliance Interface v2"
    quint32 param2 = getParam2ForPortName(portName);
    qint16 portNumber = 0;

    QByteArray portNameByteArray = portName.toLatin1();

    const char *portNameToOpen = portNameByteArray.data();

     // Program crashed when return from this function
    if(func1(portNameToOpen , param2, 10 , &portNumber) == true)
    {
             ......................
             ......................
    }
}

Here, I added some qDebug() and found that my code crashes when it returns from or call the func1() which is getting called in slot OpenPort(). Below is the prototype of the func1()

bool func1 (const char portDescription[], uInt32 param2,
               uInt16 length, Int16 * portNr);

Since, func1() is the part of the library code. So I can not check the defination of the function func1(). I can assure that there is no problem in func1() Since it is being used in different java based projects and it works.

I did some more debugging on the Project and noticed that when in Run in DEBUG Mode with BreakPoints than in the QT Thread Debug Window I can see my connected Slot but when I do not put any breakPoint than my code crashes and in Qt Thread Debug Window I can not see my connected Slot

So, It looks the problem of connection between the Main Class and the Test Class for openPort Slot.

But I am not able to understand taht when i put breakpoints in operPort() function than I can see my openPort Slot in Qt Thread Debug window but when no breakpoints than openPort Slot is not visible in Qt Thread Debug Window and Program Crashes.

Kindly Suggest,

Answer 1

I can assure that there is no problem in func1() Since it is being used in different java based projects and it works.

Wait, is func1() C++ or Java ?
Also, how can you be sure it works ?
Get the library source, compile it yourself, and debug in it.
And, just to be sure, check values of your variables while debugging, and qDebug() them when not debugging

Answer 2

I apologise for replying on my own post but after lots of dicussion on StackOverflow and google. I was able to solve the Issue.

To Solve the Issue: I changed the SLOT(openPort) as mentioned below:

Connect(this, SIGNAL(connectToPort(QString)), testClassObject, SLOT(openPort(QString)), Qt::DirectConnection); 

That is to use the "Qt:DirectConnection" method. If we don't specify a connection method, the direct method is automatically used for connections between objects on the SAME thread. Since here we have created a new QThread for TestClass and using a thirdParty library which might not be a thread safe. So using "Qt::DirectConnection" make the openPort() SlOT to run in MainClass Thread. Basically, it's as if emitting the signal calls the slot method "directly".