Trying to use QT from DLL in simple console project but it doesn't work

Jastot

Well. I have find topic with a problem like my,but not the same...

Trying to call the dynamic QWidget loader from simple console project - error.
Which error? Heap is dead :D
I have a look in disassembler and see that the pointer to QApplication is zero. (Think it blocked in anther tread...but dont enough experience to fix it)..

Try to call my code from console project (copy+paste). all is ok. Back to dll - error.

Can we use QT from DLL without create another treads or something like that???

Code for example: (nevermind what is going on in classes which are in dll)

QApplication* qapp = new QApplication(argc, argv);
QtWidgetsClassTest* test = new QtWidgetsClassTest(); - QDialog
FormLoader* formLoader = new FormLoader(qapp); - new uiLoader
formLoader->uiLoader = new QUiLoader();
formLoader->uiLoader->addPluginPath(qapp->applicationDirPath());

So.

addPluginPath(qapp->applicationDirPath());

shows error...that heap is broken...

hskoglund

Hi, I've had success with starting Qt from a .dll belonging a console application. but I used QCoreApplication and also created it the usual way on the stack (i..e no new keyword).

Jastot

@hskoglund , Hi. Thank you for help.
Try to do it like you write. Now compiler says true - that it can't use QApplication or QCoreApplication.

Will try to read about treads and use something like that....cuz QApplication is blocked...
But it is really interesting. i am create an QApplication on dll...and after that it writes that QAapp is blocked... But when i create all logic (not only QApp) in console project, which is calling dll - all is work.
Best regards,
Jastot.

For example put some code here. If you have some ideas - i will be happy.

part 1: app init in console and call func in dll

#include <QtWidgets>
#include <QApplication>
#include <QtUIEditorDLL.h>
int main(int argc, char* argv[])
{
	//QApplication
	QApplication qapp(argc, argv);
	QtUIEditorDLL* test = new QtUIEditorDLL(); - error Access violation reading location 
        return 0;
}
//////////
QtUIEditorDLL::QtUIEditorDLL()
{
	FormLoader* formLoader = new FormLoader();
	formLoader->uiLoader = new QUiLoader();
	formLoader->uiLoader->addPluginPath(QApplication::applicationDirPath()); - error Access violation reading location 
}

part 2: call in dll

int main(int argc, char* argv[])
{
	QtUIEditorDLL* test = new QtUIEditorDLL(); - error Access violation reading location 
        return 0;
}
//////////
QtUIEditorDLL::QtUIEditorDLL()
{
int argc = 1;
	char* argv[2] = { strdup("dummy"), {} };
	QApplication qapp(argc, argv);
	FormLoader* formLoader = new FormLoader();
	formLoader->uiLoader = new QUiLoader();
	formLoader->uiLoader->addPluginPath(QApplication::applicationDirPath()); - error Access violation reading location 
}

part 3: all in console project

int main(int argc, char* argv[])
{
	QApplication qapp(argc, argv);
	FormLoader* formLoader = new FormLoader();
	formLoader->uiLoader = new QUiLoader();
	formLoader->uiLoader->addPluginPath(QApplication::applicationDirPath()); - all is ok. it works.
        return 0;
}

jsulm

@Jastot What is FormLoader? Please post the whole error message.
Also, did you do debugging and check the stack trace after the crash?

Jastot

@jsulm, hello.
FormLoader - class with pointer on QUiLoader and some another functions...nothing more and nothing interesting.

Full error looks like:
Exception thrown at 0x00007FFCD0C91AFC (ntdll.dll) in .exe: 0xC0000005: Access violation reading location 0x0000000000000024.

I read disassembled code and see that it crashes when trying to call QApplication...

jsulm

@Jastot I think your problem is that you're allocating qapp as a local variable inside QtUIEditorDLL. That means it only exists inside that method, as soon as it terminates qapp is deleted. If you want to use Qt signals/slots you need to start Qt event loop calling qapp.exec().

Jastot

@jsulm
Well. Sadly it didn't help.
So i tried to some experiments...
I found that QApplication works in dll without exec().

now QtUIEditorDLL class is QApplication.

class QTUIEDITORDLL_EXPORT QtUIEditorDLL: QApplication

In console app i have made something like that: (believe that pointer can help...)

std::shared_ptr<QtUIEditorDLL> ptrQtUIEditorDLL(new QtUIEditorDLL(argc, argv));
	QtUIEditorDLL* test = ptrQtUIEditorDLL.get();
	test->Init();

and in dll made something like that:

QtUIEditorDLL::QtUIEditorDLL(int& argc, char** argv)
	:QApplication(argc,argv) {}

void QtUIEditorDLL::Init()
{
	QtWidgetsClassTest* test = new QtWidgetsClassTest(); // QDialog initialization worked......So QApplication works
	QUiLoader quiLoader;
	QString qstr = QCoreApplication::applicationDirPath(); // All is ok. QApplication works. in qstr i can see true way to application.
	quiLoader.addPluginPath(qstr); // same error... Exception thrown at 0x00007FFCD0C91AFC (ntdll.dll) in .exe: 0xC0000005: Access violation reading location 0x0000000000000024.

So...its problem of QUiLoader...or am i not right?
Sorry for stupid questions...

jsulm

@Jastot said in Trying to use QT from DLL in simple console project but it doesn't work:

So...its problem of QUiLoader...or am i not right?

Since you did not post the stack trace after the crash I don't know.
Subclassing QApplication is also not needed - adding qapp as class member should be enough.

Jastot

@jsulm

jsulm

@Jastot Is it still crashing if you allocate quiLoader on the heap (just for testing)?

Jastot

@jsulm
Yes. Allocate it on the heap and the same error.

Jastot

Well, @jsulm , I am very grateful to you. Thank you.
I have found how to fix it and don't destroy logic.

JonB

@Jastot said in Trying to use QT from DLL in simple console project but it doesn't work:

I have found how to fix it

Wouldn't you like to share that with us, and maybe benefit future readers? :)

Jastot

@JonB
OPS. forget about it:D

The main problem wasn't in QApplication as I thought initially.
QUiLoader didn't want to work and showed an error that there was no access to the address.

So i created QApplication and QUiLoader in console project and then transfer pointer to library.
I still have a question why QCoreApplication::applicationDirPath(); is ok but quiLoader->addPluginPath(qstr); which calls QApplication, is not ok...

Simple Console Project:

int main(int argc, char* argv[])
{
	std::shared_ptr<QtUIEditorDLL> ptrQtUIEditorDLL(new QtUIEditorDLL(argc, argv)); // you can use it without any ptr.
	QtUIEditorDLL* test = ptrQtUIEditorDLL.get();
	QUiLoader* quiLoader = new QUiLoader(); 
// as i can see it must be created in the same tread near QApp. Jsulm said that it not important...but it doesn't work if create it in DLL.
	test->Init(quiLoader);
}

And the library:

QtUIEditorDLL::QtUIEditorDLL(int& argc, char** argv)
	:QApplication(argc,argv){}
void QtUIEditorDLL::Init(QUiLoader* quiLoader)
{
	QString qstr = QCoreApplication::applicationDirPath();
	quiLoader->addPluginPath(qstr);
        //And it works. 
}