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

Qt Enthusiast

I have dynamic library created as follows

struct Tcl_Interp;
extern "C" int My_Init(Tcl_Interp *) { return 0; }

complile the cc file
g++ -fPIC -c myfile.cc
Creating a shared library

g++ -static-libstdc++ -static-libgcc -shared -o libmy.so myfile.o -L/tools/linux64/qt-4.6.0/lib -lQtCore -lQtGui

load the library from a TCL proc
then I give command

tclsh
and given command
% load libmy.so

is there any C++/Qt function equivalent to load that can load the shared library on demand from another C++ function.
My requirement is to load the shared library inside a function and then use the QT library , Is there any way I load the QT Library after call of certain function

raven-worx

@Qt-Enthusiast
use QLibrary to load libs in an platform-independant way

mrjj

to add to @raven-worx , you can see example here
https://wiki.qt.io/How_to_create_a_library_with_Qt_and_use_it_in_an_application
in Using QLibrary to load the shared library
Note the
CreateWidgetFunction cwf = (CreateWidgetFunction)library.resolve
to actually call a function.

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

mrjj

@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 :)

Qt Enthusiast

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

Qt Enthusiast

What is use of QLibrary::ResolveAllSymbolsHint

raven-worx

@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.

hskoglund

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

Qt Enthusiast

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

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 :-)

Qt Enthusiast

after that can I call QT code dirrectlt

QTreewidget* tree = new QTreeWidget()..

Qt Enthusiast

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

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.

Qt Enthusiast

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);

kshegunov

@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

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

If you use Qt, you do.

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?

Qt Enthusiast

because it application as big appilcation of Qt code

kshegunov

@Qt-Enthusiast
I don't follow.

Qt Enthusiast

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()
 }

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