uninitialized data passed to QStringList QCoreApplication::arguments()

rparon

I am observing access violation exceptions (see attached screenshot) due to uninitialized data when exec() (see the code below) calls QStringList QCoreApplication::arguments(), verified in Qt 6.8.3

CODE

int argc = 1;
const char* argv[] = {"MyDll"," "," "};
qapp_pt = cv_new QGuiApplication(argc, (char**) argv);
if (qapp_pt == NULL)
	{
	return(false);
	}

QQmlApplicationEngine* QmlEngine;

QmlEngine = cv_new QQmlApplicationEngine();
if (QmlEngine == NULL)
    {
    return(result);
    }

// run QMLEngine and load Main.qml,
QmlEngine->load(QUrl(QStringLiteral("qrc:/GuiModule/mydll/resource/Main.qml")));

// in this version loadFromModule() doesn't work, 
// maybe a different CMake configuration or a different call to loadFromModule ?  
// QmlEngine->addImportPath("mydll/resource");
// QmlEngine->loadFromModule("GuiModule","Main");


// exception thrown here !!		
result = qapp_pt->exec();

the code is inside a shared library (Qt project) the CMakeList.txt to create the project is

CMAKEFILE.TXT

cmake_minimum_required(VERSION 3.16)

find_package(Qt6 REQUIRED COMPONENTS Quick Gui Core QmlImportScanner)

qt_add_library(mydll 
    	SHARED   
        mydll/mydllx.cpp 
	)
	
qt_add_qml_module(mydll
    VERSION 1.28	
    URI GuiModule	
    RESOURCE_PREFIX "/"
    NO_PLUGIN

    
    IMPORTS
    Quick 
    Gui 
    Qml 
    Core

    SOURCES

    	mydll/gxpage/test_page.cpp 

    RESOURCES
        mydll/mydllresources.qrc	

    QML_FILES
	mydll/resource/Main.qml

    )

set_target_properties(mydll
                      PROPERTIES
                      PREFIX ""
                      OUTPUT_NAME "mydll"
                      SUFFIX ".dll")

 
# link QT-QML libraries and mylib
target_link_libraries(mydll PRIVATE
    Qt6::Gui
    Qt6::Quick
    Qt::QuickControls2
    ${CMAKE_BINARY_DIR}/mylib.lib		
    )

 set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE} /MT /Ox")
 set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE}  /MT /Ox")
 set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} /MTd")
 set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG}  /MTd")

DISCUSSION

Qt should initialize (by default) all the vars passed to QCoreApplication but according many tests there are exceptions...

To solve the reported problem I think there are at least two options :

modify qcoreapplication.cpp and recompile (from source) Qt library, the problem is that when I edit / change qcoreapplication.cpp , run configure.bat and then compile there are several errors generated, probably I can't run configure but follow some other procedure, do you know if a detailed procedure to edit / modify source in Qt library is available for review ?
before to call qapp_pt->exec() (see the code above) call some method in Qt library to force a reliable initialization of all vars passed to QCoreApplication, do you know if there is a method which does that ?

Thank you for help !!

exception_report.jpg

Christian Ehrlicher

There must be a problem on your side. Please provide a minimal compileable example of the problem.
If there would be a problem with argv handling it would already be revealed long time ago.

Btw: your argc is wrong when you want to pass a parameter it must be 2, not 1.

rparon

Cristian,
thank you for the comment,
I do not wish to pass parameters via argc / argv , just tested several combinations of argc /argv to see if the problem disappears, however passing the result of
const char* argv[] = {"MyDll"," "," "};
should be correct.
By the way problem is not about passing parameters, I do not care for that, the problem is about uninitialized vars in QCoreApplication (see the screenshot : argc = 318415888 ! )

do you have answers to my questions :

modify qcoreapplication.cpp and recompile (from source) Qt library, the problem is that when I edit / change qcoreapplication.cpp , run configure.bat and then compile there are several errors generated, probably I can't run configure but follow some other procedure, do you know if a detailed procedure to edit / modify source in Qt library is available for review ?

before to call qapp_pt->exec() (see the code above) call some method in Qt library to force a reliable initialization of all vars passed to QCoreApplication, do you know if there is a method which does that ?

JonB

@rparon
What @Christian-Ehrlicher says is right, and I don't know why you might be thinking of modifying any Qt core files.

Why don't you try printing out (qDebug() maybe?) QCoreApplication::arguments() immediately after your creation of QGuiApplication(argc, (char**) argv) (and its test for nullptr)?

You should assume

If there would be a problem with argv handling it would already be revealed long time ago.

rather than a fault in Qt or any need to:

call some method in Qt library to force a reliable initialization of all vars passed to QCoreApplication

jsulm

@rparon said in uninitialized data passed to QStringList QCoreApplication::arguments():

cv_new

What is it? Do you have custom memory management? Could the problem be related to that?

rparon

Hi JonB,
thank you for the comment, I understand your point of view , yes I am debugging the launch, the vars are passed correctly to QGuiApplication(argc, (char**) argv) but then there are many other lines of code... and finally the app calls QCoreApplication::arguments() passing wrong data... sure I could put memory breakpoints to inspect possible operations on these vars... but it would take a lot of time to do that and I hope there is a shorter solution... what I need is to make sure they are properly initialized (before the call from QCoreApplication::arguments() ...

JonB

@rparon said in uninitialized data passed to QStringList QCoreApplication::arguments():

what I need is to make sure they are properly initialized (before the call from QCoreApplication::arguments() ...

As I said, I do not know what this means. You need to show some minimal reproducible example. The code you showed should work e.g. if it's the main() of a program. If not you have done something wrong, perhaps after you called QGuiApplication(argc, (char**) argv) and before QGuiApplication::exec().

Do you change argc or argv? Very importantly, is your QGuiApplication::exec() really in the same function as the argv[] variable? If it's actually in a different function your code will crash. QCoreApplication::arguments() will get its answer from QGuiApplication's idea of the values you pass to it. If they have been corrupted you will lose. I don't see much of a shortcut to putting in a couple of breakpoints at judicious points, don't put in thousands just "divide and conquer" your code and refine depending where it goes wrong.

rparon

Hi jsulm,
it is standard memory management :

#define  cv_new new

as far as I know Qt should work without problems with that but I don't know if it may create problems with automatic initialization of vars.. (I don't know the details of QCore...)

JonB

@rparon
So "standard" that cv_new does not even show up in Google (other than as a username!)? What file is that in? Have you seen anyone else using cv_new in place of new?

Given that definition it is not germane to whatever your issue is.

rparon

Hi JonB,
yes, it is all the code in main {...} , note it is a dll but that should not create problems except maybe when catching signals from (in this case) Windows system... I confirm that argc, argv passed to QGuiApplication(argc, (char**) argv) do not change...
From your answers I presume there is not a public method to reset / clear / redefine these vars
I would think the vars argc, argv passed to QGuiApplication(..) are the same passed from Qt to QStringList QCoreApplication::arguments() ....
so the problem could be to identify where these are modified...

jsulm

@rparon You should really post a reproducer, so that others can take a look and also debug.
I really doubt it is a Qt issue, else no Qt app would work.

JonB

@rparon
Although you may really need a repro....
From your screenshot, can you please start by clicking the Show Call Stack and showing us (clearly) the stack, in case there is any useful clue there.

And what is the pppx.dll? Is that your DLL? The props.exe? Is that a main program of yours using the DLL?

In the screenshot the whole #if defined(Q_OS_WIN) is "dimmed", indicating to me that symbol is not defined. Yet since the error shows "DLL"s are you not on Windows?

rparon

Hi JonB,
thanks to C preprocessor, in that case cv_news is the same of new, but I understand your perplexity :-)
Yes, those are the names of the dll and executable.
About the Call Stack, sure, I attach the screenshot for your review....

rparon

Hi jsulm,
as you can judge from the code (that is the dll main) I adopt the standard procedures to work with Qt objects,
I wonder if I am not managing properly some initialization...

JonB

@rparon
FWIW: You claim you get the error during your result = qapp_pt->exec();. I think the call stack shows that is not the case and it dies during

QmlEngine->load(QUrl(QStringLiteral("qrc:/GuiModule/mydll/resource/Main.qml")));

What is the function ppx.dll!edS(int stream)?

Can you please print out QCoreApplication::arguments() immediately above that line in your code. Is it right at that point? If so I am unconvinced the argc, argv it is looking at have anything to do with your argc, argv here.

rparon

Hi JonB,
correct, the problem appears executing QmlEngine->load(....)
the function pppx.dll!edS(int stream) includes all the code to load Qt (see above)

int argc = 1;
const char* argv[] = {"MyDll"," "," "};
qapp_pt = cv_new QGuiApplication(argc, (char**) argv);
.......
result = qapp_pt->exec();

as far as I can see QCoreApplication::arguments() is not called before QmlEngine->load(QUrl...)....

JonB

@rparon said in uninitialized data passed to QStringList QCoreApplication::arguments():

as far as I can see QCoreApplication::arguments() is not called before QmlEngine->load(QUrl...)....

I asked you to paste

qDebug() << QCoreApplication::arguments();

into your code immediately above

QmlEngine->load(QUrl...)

rparon

Hi JonB,
I have included the direct call just before load()

QmlEngine = cv_new QQmlApplicationEngine();
if (QmlEngine == NULL)
   {
   return(result);
   }

// direct call to inspect argc, argv
QStringList c = QCoreApplication::arguments(); 

// run QMLEngine and load Main.qml,
// note that only upper case file names are exported by default	
QmlEngine->load(QUrl(QStringLiteral("qrc:/GuiModule/pppx/resource/Main.qml")));

indeed the var argc shows a wrong value : -6213....
now the area of investigation is somewhat restricted.... I'll try to go further calling the same just after

qapp_pt = cv_new QGuiApplication(argc, (char**) argv);

to see what is happening , could it be related with new operator ?

rparon

a trick that I am testing is to put the initializing argc, argv values

int argc = 1;
const char* argv[] = {"MyDll"," "," "};

as extern (equivalent to static)
this seems to solve the problem but I am surprised that Qt doesn't make an internal copy...
indeed I did assume that (the internal copy) and never presumed they must be static...
were you aware of that ?

Kent-Dorfman

@rparon said in uninitialized data passed to QStringList QCoreApplication::arguments():

a trick that I am testing is to put the initializing argc, argv values
int argc = 1;
const char* argv[] = {"MyDll"," "," "};
as extern (equivalent to static)
this seems to solve the problem but I am surprised that Qt doesn't make an internal copy...
indeed I did assume that (the internal copy) and never presumed they must be static...
were you aware of that ?

command line arguments are (from the perspective of the child) considered to be immutable, so I'm not aware of many apps that copy them, as why would you, unless you want to modify them?