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How to avoid repeated execution of loading Qlibrary?



  • In my humble application, I require the use of functions stored in a DLL which I manage to load in memory with the help of the Qlibrary object, like this :

      QLibrary myLib("DEVICE.dll");
    
        bool Result;
        Result = myLib.load();
    

    It works great. Then I use some typedef to resolve the DLL functions, like so :

    typedef DEVICE_ERROR_TYPES(__stdcall* Fn6)(void);
        Fn6 RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("DEVICE_StartMeasurement"));
    

    Then I merely issue this code to make a measurement :

    sdkError = RB_StartMeasurement();
    

    It works wonder! Qlibrary allows me to access a DLL I could never access otherwise (believe me, I tried evertyhing).

    My problem? Looks like I can only take ONE measurement once the instrument is calibrated? See this dialog box :

    Normal.jpg

    See the Measure button? I should be able to press it repeatedly without the program crashing or hanging on me. But that's what happens, sadly?

    I suspect the problems stems from my "multiple loading of the DEVICE.dll"?

    You see, I don't know of a way I could load the library once and have it accessible to all functions? I tried moving the Qlibrary call every place I could inside my application but I am limited by the associated typedef definitions and declarations, as you can see, inside the Measure button, I have the call to Qlibrary followed by the typedefs :

    void MainWindow::on_btnMeasure_clicked()
    {
        QLibrary myLib("DEVICE.dll");
    
        bool Result;
        Result = myLib.load();
    
    typedef DEVICE_ERROR_TYPES(__stdcall* Fn6)(void);
        Fn6 RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("DEVICE_StartMeasurement"));
    }
    

    I have the same Qlibrary / typedeg under each buttons in my interface. For example, under the Connect button, I have this code :

    void MainWindow::on_btnConnect_clicked(){
    
    
        QLibrary myLib("DEVICE.dll");
    
        bool Result;
        Result = myLib.load();
    
        typedef DEVICE_ERROR_TYPES(__stdcall* Fn1)(const DEVICE_PortInfo* inPortInfo, const uint32_km inTimeout);
        Fn1 RB_Connect = reinterpret_cast<Fn1>(myLib.resolve("DEVICE_Connect"));
    }
    

    At the beginning, I did not care that I was loading the same DLL over and over since it's less than 100K and I was too eager to see how far I could push this logic but I see the debugger take me into assembler code during subsequent Measure attempts? And I recognize some DLL function names show up on stake trace that have nothing to do with my resolved functions? To me, the problem stems from repeated loading of the DLL but I can't "prove" it (or get that out of the equation) until I find a way to load it once, globally, and have it shared by all the slots functions....

    I tried to run the code on another Windows machine but I immediately got "Segmentation faults" errors? And when I try to run my application on this machine, in Release mode, I immediately get this error :

    Test erro.jpg

    "Test" is the name of my application.


  • Lifetime Qt Champion

    @RogerBreton said in How to avoid repeated execution of loading Qlibrary?:

    I don't know of a way I could load the library once and have it accessible to all functions?

    Simply put the RB_StartMeasurement pointer as class member and initialize it to nullptr. Before using it later check whether it is nullptr, if it is load the lib and assign the pointer...
    Also make myLib class member.



  • @jsulm said in How to avoid repeated execution of loading Qlibrary?:

    Also make myLib class member.

    Here is the code of the MainWindow class :

    class MainWindow : public QMainWindow
    {
        Q_OBJECT   
        QLibrary myLib();
    
    typedef DEVICE_ERROR_TYPES(__stdcall* Fn6)(void);
        Fn6 RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("DEVICE_StartMeasurement"));
    
    public:
        MainWindow(QWidget *parent = nullptr); 
        ~MainWindow();
    
    ...
    

    As you can see, I "made myLib a class member" and I tried to put the RB_StartMeasurement (pointer?) as a a "class member" as well but the compiler gives me a :

    reference to non-static member functions must be called; did you mean to call it with no arguments?

    That's where I'm lost....


  • Lifetime Qt Champion

    @RogerBreton said in How to avoid repeated execution of loading Qlibrary?:

    reinterpret_cast<Fn6>(myLib.resolve("DEVICE_StartMeasurement"));

    This should be done in the constructor



  • @jsulm said in How to avoid repeated execution of loading Qlibrary?:

    reinterpret_cast<Fn6>(myLib.resolve("DEVICE_StartMeasurement"));

    I moved "reinterpret_cast<Fn6>(myLib.resolve("DEVICE_StartMeasurement"));" in the constructor, so all I have in mainwindow.h is this :

    class MainWindow : public QMainWindow
    {
        Q_OBJECT   
        QLibrary myLib();
    
    typedef DEVICE_ERROR_TYPES(__stdcall* Fn6)(void);
    
    public:
        MainWindow(QWidget *parent = nullptr); 
        ~MainWindow();
    ...
    

    And in the "Constructor", I have this code :

    MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)       // Constructor
        , ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
    
        QLibrary myLib("FDXSDK.dll");
        bool Result;
        Result = myLib.load();
        
        Fn6 RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("FDX_StartMeasurement"));
        
    }
    
    

    Then, in the Measure button, I end up with this code :

    void MainWindow::on_btnMeasure_clicked()
    {
         sdkError = RB_StartMeasurement();
    

    For which I get :

    use of undeclared identifier 'RB_Startmeasurement'...

    Lost again... Maybe that reference to 'initialize with a null pointer' I'm missing?



  • @RogerBreton

     Fn6 RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("FDX_StartMeasurement"));
    

    This declares RB_StartMeasurement as a local variable in method MainWindow::MainWindow(). That means you won't be able to access it another method, like MainWindow::on_btnMeasure_clicked(), as you're seeing from the undeclared identifier message.

    You want Fn6 RB_StartMeasurement to be a member variable of the MainWindow class, declared in the .h file. Then you assign it as you have done in MainWindow::MainWindow but without that type declaration in front of it, as:

    RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("FDX_StartMeasurement"));
    // equivalent to
    this->RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("FDX_StartMeasurement"));
    

    Now you can access RB_StartMeasurement()/this->RB_StartMeasurement() in other MainWindow methods, like you tried.



  • Thank you all for your patience and help...

    So, I made "Fn6 RB_StartMeasurement" a member variable of the MainWindow class, declared in the .h file, like this :

    class MainWindow : public QMainWindow
    {
        Q_OBJECT    // Macro Meta Object Compiler
        QLibrary myLib();
        
        typedef DEVICE_ERROR_TYPES(__stdcall* Fn6)(void);
        Fn6 RB_StartMeasurement();
        
    public:
        MainWindow(QWidget *parent = nullptr); 
        ~MainWindow();
    ...
    

    Then in MainWindow::MainWindow, I assign it, without the type declaration in front of it, like this :

    MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)  
        , ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
    
        QLibrary myLib("DEVICE.dll");
    
        bool Result;
        Result = myLib.load();
        
        RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("DEVICE_StartMeasurement"));
        
    }
    
    

    But that gives me :

    reference to non-static member function must be called...

    I must still be missing something...



  • @RogerBreton said in How to avoid repeated execution of loading Qlibrary?:

    Fn6 RB_StartMeasurement();

    You want a function pointer variable, as you had originally, i.e.

     Fn6 RB_StartMeasurement;
    

    not a function call, with the round parentheses after it.


  • Lifetime Qt Champion

    @RogerBreton said in How to avoid repeated execution of loading Qlibrary?:

    QLibrary myLib("DEVICE.dll");

    Why do you declare another myLib inside constructor?! Use the class instance myLib.



  • I fixed the header this way :

    class MainWindow : public QMainWindow
    {
        Q_OBJECT    // Macro Meta Object Compiler
        QLibrary myLib();
        
        typedef DEVICE_ERROR_TYPES(__stdcall* Fn6)(void);
        Fn6 RB_StartMeasurement;
    ...
    

    But then, in the Constructor, I have this code :

    MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)       // Constructor
        , ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
    
        bool Result;
        Result = myLib.load("DEVICE.dll");
        
        RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("FDX_StartMeasurement"));
        
    }
    ...
    

    Notice how, this time, I use the myLib class instance? But I must not have the correct logic because the compiler says :

    reference to non-static member function must be called

    Same error for the RB_StartMeasurement :(

    Boy! Thank you guys for your patience!



  • @RogerBreton said in How to avoid repeated execution of loading Qlibrary?:

    Notice how, this time, I use the myLib class instance? But I must not have the correct logic because the compiler says :

    reference to non-static member function must be called

    You have QLibrary myLib();. You like those () parentheses, don't you? ;-) You are going to call myLib.load("DEVICE.dll"). Don't you think your declaration should be QLibrary myLib; instead?



  • I notice, afterwards, that the declaration should not have parentheses :

    class MainWindow : public QMainWindow
    {
        Q_OBJECT
        QLibrary myLib;
        
        typedef FDXSDK_ERROR_TYPES(__stdcall* Fn6)(void);
        Fn6 RB_StartMeasurement;
    ...
    

    But then, in the constuctor, I could not understand why I was still the error "Too many arguments to function call" when I tried to pass the name of the DLL to the load library function:

    MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)       // Constructor
        , ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
    
        bool Result;
        Result = myLib.load("DEVICE.dll");
          
        RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("FDX_StartMeasurement"));
        
    }
    

    It's one thing to declare myLib as of type Qlibrary in the header but I don't understand, why in the implementation, the compiler would complain?
    jsum commented earlier :

    Why do you declare another myLib inside constructor?! Use the class instance myLib.

    So I think, that is what I'm doing, using the class instance myLib.
    Unless I leave the call to Load without arguments, like this :

       bool Result;
        Result = myLib.load();
    

    But this does not seem to make sense?

    The documentation states:

    The resolve() function implicitly tries to load the library if it has not been loaded yet.
    But where will it gets the name of the DLL to load in memory, if it is never specified in the first place?

    Take the Measure button code :

    void MainWindow::on_btnMeasure_clicked()
    {
         sdkError = RB_StartMeasurement();
    

    Where is the application going to get the DLL from?


  • Lifetime Qt Champion

    @RogerBreton said in How to avoid repeated execution of loading Qlibrary?:

    But where will it gets the name of the DLL to load in memory, if it is never specified in the first place?

    Via the ctor

    I was still the error "Too many arguments to function call" when I tried to pass the name of the DLL to the load library function:

    Because you declared a class function named myLib() which returns a QLibrary object.



  • I wanted to try the changes in debug mode but I immediately ran into the following error message :

    Error.jpg

    Time for a cup of coffee...


  • Lifetime Qt Champion

    @RogerBreton said in How to avoid repeated execution of loading Qlibrary?:

    Time for a cup of coffee...

    I would rather say - time for a debugging session :)



  • @RogerBreton
    If, from what you have shown, you presently have:

       bool Result;
        Result = myLib.load();
    RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("FDX_StartMeasurement"));
    sdkError = RB_StartMeasurement();
    

    without checking the intermediate return result variables, RB_StartMeasurement may be nullptr. Then RB_StartMeasurement() would cause a SIGSEGV?



  • @JonB I don't dare say RB_StartMeasurement may be a nullptr in ALL LIKELIHOOD, and that maybe the cause of the segmentation fault. That is exactly my hypothesis. So, if that is the case, this proves my intuition : where is the application getting the DLL from? If its name isn't specified anywhere? It is not in the PRO file.

    I was thinking, perhaps, I should test whether the library is loaded before calling RB_StartMeasurement? And if it is not loaded, then load it? But that would not explain why I get the SIGSEV when the application tries to load? Or is it happening in the constructor?

    Somehow, I must be missing some important piece of the puzzle -- I'll continue to look around but I am optimistic...



  • @RogerBreton

    bool Result;
    Result = myLib.load();
    if (!Result)
        qDebug() << "This is bad...";
    RB_StartMeasurement = reinterpret_cast<Fn6>(myLib.resolve("FDX_StartMeasurement"));
    if (RB_StartMeasurement  == nullptr)
        qDebug() << "This is bad.  Get ready for SEGV if you call RB_StartMeasurement() now...";
    sdkError = RB_StartMeasurement();
    


  • I declared the RB_StartMeasurement as NULL in mainwindow.h :

    class MainWindow : public QMainWindow
    {
        Q_OBJECT
        QLibrary myLib;
    
        typedef DEVICE_ERROR_TYPES(__stdcall* Fn6)(void);
        Fn6 RB_StartMeasurement = NULL;
    
    

    But I still get the Segmentation Fault...


  • Lifetime Qt Champion

    Hi,

    Did you implement @JonB´s suggestion ?

    Start in debug and you should see exactly where it fails.



  • @SGaist That is exactly what I do :

    I found the culprit...

    DEVICE_ERROR_TYPES sdkError = RB_RegisterDeviceEventHandler(EventNotice);
    
    

    The above function is a static member function declared in SomeClassName... which was working fine before ... The EventNotice function is also part of the same class.



  • For some reason (?), the EventNotice "address" is garbage :

    EventNotice 2372415452829272149 void (DEVICE_eEventCode, uint32_km, DEVICE_ERROR_TYPES)

    It should be a proper address like 0x00007ff739a21970.
    I tried restarting QT Creator many times to no avail.



  • Thank you all for your patient help.
    I'm putting QT on the backburner for now.
    Too bad. Such a nice environment.


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