How to correctly unload QApplication in a DLL?



  • Hi,

    I have an application running inside AutoCAD (runtime extension dynamically loaded, Windows, Qt 5.3), where I create a QApplication in the init (loading) process like this :
    @int argc = 0;
    (void)new QApplication(argc, NULL);@

    and where I destroy it when module is unloaded using this:
    @delete qApp;@

    This code works perfectly with Qt4, because all Qt libraries are unloaded. But it's not the case in Qt5, because Gui, Core and all plugins (icu, images, qwindows, ...) are still loaded.

    My problem is that when I load my application for the second time (load - unload - load), a crash occured in QVariant with custom registered type. Here is my code:
    @class MyObject
    {
    public:
    MyObject() : m_name("test") {}
    ~MyObject() {}

    private:
    QString m_name;
    };
    Q_DECLARE_METATYPE(MyObject);@

    @MyObject myObject;
    QVariant var;
    var.setValue(myObject); // -> will crash in qvariant.h line 517 : v = QVariant(type, &t, QTypeInfo<T>::isPointer);@

    That's why I wonder, how to properly unload QApplication? What am I doing wrong? Do I have to unregister custom types? (but how)

    Any suggestions are welcomed.
    Regards,
    Jonathan



  • why are you deleting qApp instead of your allocated instance? technically they are the same object, but qApp is a special pointer used and created by Qt runtime.



  • Tested by deleting my pointer, problem is the same.



  • Ok, are you sure myObject is a pointer to heap? in the example it is not and that cause error when you delete all stuff.



  • My opinion is that custom type is already registered the second I load a QApplication, but custom type "is linked" to an old class definition. The solution is to re-register the custom type. But how?

    In Qt4, it was possible to unregister custom type (QMetaType::unregisterType). Is there such a solution in Qt5?



  • Are you sure MyObject and QVariant are in the same scope? If not this is the problem.
    If every time you register your object using a fresh QVariant and a fresh MyObject I don't see any problem that leads to crash!
    Qt4 and Qt5 have different logic behind the scene, so maybe your code is just working in Qt4 due to this logic that now has been changed. We should see exactly how real code looks to understand what happen.



  • Thank you for your help. Here is the complete code :

    @#include "stdafx.h"
    #include "bcrxsimple.h"

    #include <QApplication>
    #include <QTimer>

    class MyObject
    {
    public:
    MyObject() : m_name("test") {}
    ~MyObject() {}

    private:
    QString m_name;
    };
    Q_DECLARE_METATYPE(MyObject);

    BcRxSimple * BcRxSimple::m_pInstance = NULL;

    BcRxSimple * BcRxSimple::getInstance()
    {
    if(m_pInstance == NULL)
    {
    m_pInstance = new BcRxSimple();
    assert(false); // must NEVER be called (because instance is created by AutoCAD load process)
    }

    return m_pInstance;
    }

    void BcRxSimple::deleteInstance()
    {
    delete m_pInstance;
    m_pInstance = NULL;
    }

    BcRxSimple::BcRxSimple()
    {
    }

    BcRxSimple::~BcRxSimple()
    {
    }

    void BcRxSimple::RegisterServerComponents()
    {
    }

    AcRx::AppRetCode BcRxSimple::On_kInitAppMsg(void * pkt)
    {
    AcRx::AppRetCode retCode = AcRxArxApp::On_kInitAppMsg(pkt);
    if(retCode == AcRx::kRetError)
    return AcRx::kRetError;

    acrxDynamicLinker->unlockApplication(pkt);
    acrxDynamicLinker->registerAppMDIAware(pkt);

    acutPrintf(_T("\nBcRxSimple -> On_kInitAppMsg"));

    if(qApp == NULL)
    {
    int argc = 0;
    (void *)new QApplication(argc, NULL);
    }

    MyObject myObject;
    QVariant var;
    var.setValue(myObject);

    return retCode;
    }

    AcRx::AppRetCode BcRxSimple::On_kUnloadAppMsg(void * pkt)
    {
    AcRx::AppRetCode retCode = AcRxArxApp::On_kUnloadAppMsg(pkt);
    if(retCode == AcRx::kRetError)
    return AcRx::kRetError;

    acutPrintf(_T("\nBcRxSimple -> On_kUnloadAppMsg"));

    //app specific
    acedRegCmds->removeGroup(_T("TEST"));

    delete qApp;

    return retCode;
    }

    AcRx::AppRetCode BcRxSimple::On_kDependencyMsg(void * pkt)
    {
    AcRx::AppRetCode retCode = AcRxArxApp::On_kUnloadDwgMsg(pkt);
    if(retCode == AcRx::kRetError)
    return AcRx::kRetError;

    return retCode;
    }

    AcRx::AppRetCode BcRxSimple::On_kNoDependencyMsg(void * pkt)
    {
    AcRx::AppRetCode retCode = AcRxArxApp::On_kUnloadDwgMsg(pkt);
    if(retCode == AcRx::kRetError)
    return AcRx::kRetError;

    return retCode;
    }

    AcRx::AppRetCode BcRxSimple::On_kInvkSubrMsg(void * pkt)
    {
    AcRx::AppRetCode retCode = AcRxArxApp::On_kInvkSubrMsg(pkt);
    if(retCode == AcRx::kRetError)
    return AcRx::kRetError;

    return retCode;
    }

    AcRx::AppRetCode BcRxSimple::On_kLoadDwgMsg(void * pkt)
    {
    AcRx::AppRetCode retCode = AcRxArxApp::On_kLoadDwgMsg(pkt);
    if(retCode == AcRx::kRetError)
    return AcRx::kRetError;

    acutPrintf(_T("\nBcRxSimple -> On_kLoadDwgMsg"));

    return retCode;
    }

    AcRx::AppRetCode BcRxSimple::On_kUnloadDwgMsg(void * pkt)
    {
    AcRx::AppRetCode retCode = AcRxArxApp::On_kUnloadDwgMsg(pkt);
    if(retCode == AcRx::kRetError)
    return AcRx::kRetError;

    acutPrintf(_T("\nBcRxSimple -> On_kUnloadDwgMsg"));

    return retCode;
    }
    @



  • I don't know details about autocad and so on, but i see something strange here:
    @
    AcRx::AppRetCode BcRxSimple::On_kInitAppMsg(void * pkt)
    {
    AcRx::AppRetCode retCode = AcRxArxApp::On_kInitAppMsg(pkt);
    if(retCode == AcRx::kRetError)
    return AcRx::kRetError;

    acrxDynamicLinker->unlockApplication(pkt);
    acrxDynamicLinker->registerAppMDIAware(pkt);

    acutPrintf(_T("\nBcRxSimple -> On_kInitAppMsg"));

    if(qApp == NULL)
    {
    int argc = 0;
    (void *)new QApplication(argc, NULL);
    }

    MyObject myObject;
    QVariant var;
    var.setValue(myObject);

    return retCode;
    }
    @

    I think it is not a good idea to create a QApplication without have a reference to this object and check on qApp; better solution
    is to have your own QAppplication variable and rely on this.
    Problem is first you are creating myobject and QVariant as local variable that are discarded when you go out from function,
    and moreover I don't understand setting of your object to a local QVariant that is never used somewhere.
    Can you explain to me why you are doing this?


  • Moderators

    Please take into consideration what qApp expands into:
    @
    #define qApp (static_cast<QApplication *>(QCoreApplication::instance()))
    @

    This will never return NULL.



  • I do this dummy code just to reproduce the issue. I know that myObject variable and variant are unused, but Qt crash the second time I reload the module.

    Once again, I guess the issue is coming from new structure (Qt5) of QMetaType and QVariant. I need to be able to unregister custom types or completely unload Qt libraries.



  • Yes, qApp can be NULL (from documentation: Returns a pointer to the application's QCoreApplication (or QApplication) instance. If no instance has been allocated, null is returned). But don't focus on that, If I change the code like this, problem is the same.
    @int argc = 0;
    myApplication = new QApplication(argc, NULL);

    ...

    delete myApplication;@



  • I don't think this is your problem...you are just setting a normal variable (QVariant) with another variable (MyObject)...problem is on qApp stuff.
    Can you check if second time a new QApplication is created again?



  • Checked. The second time a new QApplication is created too.


  • Moderators

    I think you would get better answers on the interest mailing list. People fluent in QtCore code are likely to know the cause of your problem.



  • Anyway scenario is very complex since you are using a static object in process with another application (autocad) and managing a QApplication using qApp that can have different scope and logic behind.
    Can you tell me why are you using QApplication and if you remove those instructions if it is working



  • As with any Qt application, I need to create a valid QApplication. I'm surprised by your question.



  • I don't understand...you are creating a plugin for Autocad if you don't need gui why are you using Qt?
    and again I don't think this is the correct approch to check qApp and create an anonymous QApplication without an explicit reference


  • Moderators

    AlterX QObject class requires a valid QCoreApplication instance to work properly.



  • Yes, and I need GUI. I do a little condensed code just to reproduce the issue. Of course, this is not my whole application. It's just a "debug" example.



  • Ok, so due to this problem can be related to in process plugin, it is very hard to help you since we don't know how to reproduce this on our own. Maybe you can try to not unload the entire plugin and reload it, and have a only one instance of QApplication created when you load plugin? Can you do this?



  • [quote author="sierdzio" date="1417515370"]AlterX QObject class requires a valid QCoreApplication instance to work properly.[/quote]

    Yes I know of course, but since he spoke about just an example I was thinking that original code was not using Qt and that this was just a curious question for him


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