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how to load a dynamic library on demand from a QT method

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  • Q Offline
    Q Offline
    Qt Enthusiast
    wrote on last edited by
    #6

    Can u guide how

    Normally one can use plugins and interfaces and it removes the resolve part
    but you are using plain shared lib, so not not sure what u need or dont nee

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    0
    • Q Offline
      Q Offline
      Qt Enthusiast
      wrote on last edited by
      #7

      What is use of QLibrary::ResolveAllSymbolsHint

      raven-worxR 1 Reply Last reply
      0
      • Q Qt Enthusiast

        What is use of QLibrary::ResolveAllSymbolsHint

        raven-worxR Offline
        raven-worxR Offline
        raven-worx
        Moderators
        wrote on last edited by
        #8

        @Qt-Enthusiast said in how to load a dynamic library on demand from a QT method:

        What is use of QLibrary::ResolveAllSymbolsHint

        this just resolves all symbols at load time, as said in the docs.
        But still you need a pointer to the function you want to call.

        --- SUPPORT REQUESTS VIA CHAT WILL BE IGNORED ---
        If you have a question please use the forum so others can benefit from the solution in the future

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        • hskoglundH Offline
          hskoglundH Offline
          hskoglund
          wrote on last edited by
          #9

          Hi, you can use a function pointer like this:

          auto f = (void (*))(Tcl_Interp *) QLibrary("/path/to/libmy.so").resolve("My_Init");
          
          f(&my_tcl_struct);     // calls your function
          
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          1
          • Q Offline
            Q Offline
            Qt Enthusiast
            wrote on last edited by
            #10

            for each Call to Qt function I need to have a function pointer correct ?

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            0
            • hskoglundH Offline
              hskoglundH Offline
              hskoglund
              wrote on last edited by
              #11

              Sure, but if you only call a function once, you don't need to save the pointer, just use it directly, say like this:

              ((void (*))(Tcl_Interp *) QLibrary("/path/to/libmy.so").resolve("My_Init"))(&my_tcl_struct);
              

              Hope I got all the parentheses right :-)

              kshegunovK 1 Reply Last reply
              0
              • Q Offline
                Q Offline
                Qt Enthusiast
                wrote on last edited by
                #12

                after that can I call QT code dirrectlt

                QTreewidget* tree = new QTreeWidget()..

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                • Q Offline
                  Q Offline
                  Qt Enthusiast
                  wrote on last edited by
                  #13

                  I do not need to link the QT library in my QExecutable

                  kshegunovK 1 Reply Last reply
                  0
                  • hskoglundH hskoglund

                    Sure, but if you only call a function once, you don't need to save the pointer, just use it directly, say like this:

                    ((void (*))(Tcl_Interp *) QLibrary("/path/to/libmy.so").resolve("My_Init"))(&my_tcl_struct);
                    

                    Hope I got all the parentheses right :-)

                    kshegunovK Offline
                    kshegunovK Offline
                    kshegunov
                    Moderators
                    wrote on last edited by kshegunov
                    #14

                    @hskoglund
                    That's why I usually typedef the type locally, looks simpler (and since I despise auto I'm a happy man not using it):

                    typedef void (*MyInitPtr)(Tcl_Interp *);
                    MyInitPtr My_Init = reinterpret_cast<MyInitPtr>( QLibrary("/path/to/libmy.so").resolve("My_Init") );
                    if (My_Init)
                        My_Init(&my_tcl_struct);
                    

                    @Qt-Enthusiast

                    after that can I call QT code dirrectlt

                    No, and you won't be able to accomplish that with QLibrary, it's a bit more involved than resolving a simple C-linkage function.

                    Read and abide by the Qt Code of Conduct

                    Q 1 Reply Last reply
                    1
                    • kshegunovK kshegunov

                      @hskoglund
                      That's why I usually typedef the type locally, looks simpler (and since I despise auto I'm a happy man not using it):

                      typedef void (*MyInitPtr)(Tcl_Interp *);
                      MyInitPtr My_Init = reinterpret_cast<MyInitPtr>( QLibrary("/path/to/libmy.so").resolve("My_Init") );
                      if (My_Init)
                          My_Init(&my_tcl_struct);
                      

                      @Qt-Enthusiast

                      after that can I call QT code dirrectlt

                      No, and you won't be able to accomplish that with QLibrary, it's a bit more involved than resolving a simple C-linkage function.

                      Q Offline
                      Q Offline
                      Qt Enthusiast
                      wrote on last edited by
                      #15

                      Two questions

                      1 ) Do not need to link the QT library in my final Executable
                      2) also Can u write a sample application how will multiple calls to many Qt functions codes can be called after

                      typedef void (*MyInitPtr)(Tcl_Interp *);
                      MyInitPtr My_Init = reinterpret_cast<MyInitPtr>( QLibrary("/path/to/libmy.so").resolve("My_Init") );
                      if (My_Init)
                      My_Init(&my_tcl_struct);

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                      0
                      • Q Qt Enthusiast

                        I do not need to link the QT library in my QExecutable

                        kshegunovK Offline
                        kshegunovK Offline
                        kshegunov
                        Moderators
                        wrote on last edited by kshegunov
                        #16

                        @Qt-Enthusiast

                        Okay, @mrjj encouraged me to give you the full reasoning why those shenanigans are bad. Hence, I'm going to do so. Suppose you have a class MyClass with declaration in "myclass.h":

                        class MyClass
                        {
                        public:
                            MyClass();
                            ~MyClass();
                        
                            void myMethod(int);
                        }
                        

                        and you want to call myMethod(int) of a newly created object of that class. But also suppose that class is compiled into a dynamic library myclasslib.so for all intents and purposes of this discussion. Suppose that the fully decorated symbols' names for the methods of this class are as follows:

                        • MyClass::MyClass$constrDecoration for MyClass::MyClass
                        • MyClass::MyClass$destrDecoration for MyClass::~MyClass
                        • MyClass::MyClass$methodDecoration for MyClass::myMethod(int)

                        So this is how you can create an object, call it's constructor, call the method and finally call the destructor and free the allocated memory (code is not tested, but should be working in principle):

                        #include "myclass.h"
                        
                        int main()
                        {
                            MyClass * obj = reinterpret_cast<MyClass *>(::malloc(sizeof(MyClass))); // Allocate the object
                            // Define two types for a pointer to method with and without an argument
                            typedef void (MyClass::*MyMethodNoArgPtr)();
                            typedef void (MyClass::*MyMethodIntArgPtr)(int);
                        
                            // Get the library
                            QLibrary myLib("myclasslib.so"); //< This is the binary, in which the code for `MyClass` is compiled
                            // Resolve the constructor, destructor and myMethod (notice the fully decorated symbols' names)
                            MyMethodNoArgPtr _construct = reinterpret_cast<MyMethodNoArgPtr>(myLib.resolve("MyClass::MyClass$constrDecoration"));
                            MyMethodNoArgPtr _destruct = reinterpret_cast<MyMethodNoArgPtr>(myLib.resolve("MyClass::MyClass$destrDecoration"));
                            MyMethodIntArgPtr myMethod  = reinterpret_cast<MyMethodIntArgPtr>(myLib.resolve("MyClass::MyClass$methodDecoration"));
                        
                            // Wow, now we can actually call what we need
                            (obj->*_construct)(); //< Call the constructor
                            (obj->*myMethod)(10); //< Call MyClass::myMethod(10)
                            (obj->*_destruct)()   //< Call the destructor
                        
                            // We can free the memory now
                            ::free(obj);
                            return 0;
                        }
                        

                        This all would be equivalent to:

                        #include "myclass.h"
                        
                        int main()
                        {
                            MyClass * obj = new MyClass();
                            obj->myMethod(10);
                            delete obj;
                        
                            return 0;
                        }
                        

                        if we were to leave the loader to do what it's supposed to do!

                        Oh, and by the way, this gets even more complicated when the objects have virtual tables (which QObject derived classes do).

                        Two questions

                        1. Do not need to link the QT library in my final Executable

                        If you use Qt, you do.

                        1. also Can u write a sample application how will multiple calls to many Qt functions codes can be called after

                        I don't understand the question. What multiple calls to which many Qt functions do you have in mind?

                        Read and abide by the Qt Code of Conduct

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                        • Q Offline
                          Q Offline
                          Qt Enthusiast
                          wrote on last edited by
                          #17

                          because it application as big appilcation of Qt code

                          kshegunovK 1 Reply Last reply
                          0
                          • Q Qt Enthusiast

                            because it application as big appilcation of Qt code

                            kshegunovK Offline
                            kshegunovK Offline
                            kshegunov
                            Moderators
                            wrote on last edited by
                            #18

                            @Qt-Enthusiast
                            I don't follow.

                            Read and abide by the Qt Code of Conduct

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                            • Q Offline
                              Q Offline
                              Qt Enthusiast
                              wrote on last edited by kshegunov
                              #19

                              for example

                              I have GUI application which has 1 lacs of QT code

                              for example

                                 QTreeWidget *w  = new QTreeWidget;
                                QLabel * w = new QLablep
                              
                                  class myTreeView:public QTreView {
                                   Q_Object
                                 };
                              

                              My question is If my load the dynamic library like

                              is there direct way algorithm is

                              void loadSharedObject() {  
                                  //load the
                                  // Call the function names directly
                                  // no extra code for resolving the function /symbols names   
                                  // for example  
                                   fun1()
                                   fun2()
                               }
                              
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                              • Q Offline
                                Q Offline
                                Qt Enthusiast
                                wrote on last edited by
                                #20

                                My question is If my load the dynamic library like

                                is there direct way algorithm is

                                void loadSharedObject() {
                                //load the
                                // Call the function names directly
                                // no extra code for resolving the function /symbols names
                                // for example
                                fun1()
                                fun2()
                                }

                                because the dyanamic libary in my is QLibrary

                                kshegunovK 1 Reply Last reply
                                0
                                • Q Qt Enthusiast

                                  My question is If my load the dynamic library like

                                  is there direct way algorithm is

                                  void loadSharedObject() {
                                  //load the
                                  // Call the function names directly
                                  // no extra code for resolving the function /symbols names
                                  // for example
                                  fun1()
                                  fun2()
                                  }

                                  because the dyanamic libary in my is QLibrary

                                  kshegunovK Offline
                                  kshegunovK Offline
                                  kshegunov
                                  Moderators
                                  wrote on last edited by
                                  #21

                                  @Qt-Enthusiast said in how to load a dynamic library on demand from a QT method:

                                  // Call the function names directly
                                  // no extra code for resolving the function /symbols names
                                  // for example

                                  No there isn't a way to do that. This is what linkers and loaders were created for. You can't leave a bunch of unresolved symbols that are supposed to be explicitly loaded at runtime, it just doesn't work that way.

                                  Read and abide by the Qt Code of Conduct

                                  1 Reply Last reply
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