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

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  • Q Qt Enthusiast

    if I multiple calls of QT functions then do I need to resolve each an every call for exxample I have 1 lac calls to Qt functions then do I need to resolve each an every function

    mrjjM Offline
    mrjjM Offline
    mrjj
    Lifetime Qt Champion
    wrote on last edited by
    #5

    @Qt-Enthusiast
    Hi
    I think yes. for your setup.
    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 need :)

    1 Reply Last reply
    0
    • 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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            • 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);

                        1 Reply Last reply
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                        • 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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                          1
                          • 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

                              1 Reply Last reply
                              0
                              • 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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