uninitialized data passed to QStringList QCoreApplication::arguments()

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?

Christian Ehrlicher

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

int argc = 1;

This is still wrong as you pass one parameter...

No reproducer, no fix of the obvious errors so no help from my side.

Kent-Dorfman

@Christian-Ehrlicher said in uninitialized data passed to QStringList QCoreApplication::arguments():

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

int argc = 1;

This is still wrong as you pass one parameter...

No reproducer, no fix of the obvious errors so no help from my side.

the "Willingness to help" argument aside, I'm curious about your problem with the argc assignment, from a purely technical point of view.

So what if the argv list is longer than one element. The argc is an arbitrary limit that must not be greater than the number of elements in the list, lest you access out of bound memory. Sure, it is "SUPPOSE" to be the actual length of the argv list but in the context of this test, does it matter?

Christian Ehrlicher

@Kent-Dorfman said in uninitialized data passed to QStringList QCoreApplication::arguments():

tual length of the argv list but in the context of this test, does it matter?

The first argument is always the executable name. And I doubt it's what the first arg here is. He wants a parameter or whatever. Otherwise why fiddle around with QCoreApplication::arguments?
but we just have to guess due to a missing reproduce.

rparon

I understand the different points of view, to clarify :

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

is based on Qt documentation

argc must be greater than zero and argv must contain at least one valid character string, argc = 1 is a correct value.

Converting vars from local to global / extern seems to solve the problem

jeremy_k

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

const char* argv[] = {"MyDll"," "," "};
qapp_pt = cv_new QGuiApplication(argc, (char**) argv);

This is always the wrong thing to do. This code is telling the compiler to allocate something as const, and then instructing it to ignore the const-ness when passing to a function that has a type signature allowing modification.

Quoting from the above link:

Such object cannot be modified: attempt to do so directly is a compile-time error, and attempt to do so indirectly (e.g., by modifying the const object through a reference or pointer to non-const type) results in undefined behavior.

https://en.cppreference.com/w/cpp/language/ub.html

undefined behavior - There are no restrictions on the behavior of the program.

Some examples of undefined behavior are data races, memory accesses outside of array bounds, signed integer overflow, null pointer dereference, more than one modifications of the same scalar in an expression without any intermediate sequence point(until C++11)that is unsequenced(since C++11), access to an object through a pointer of a different type, etc.

Furthermore, the QGuiApplication, the documentation explicitly mentions the possibility of modification:

Note: argc and argv might be changed as Qt removes command line arguments that it recognizes.

rparon

Hi jeremy_k,
that is an interesting point,
argc, argv date back to Unix times,
see The C programming Language, Brian W. Kernighan and Dennis M. Ritchie which includes many examples with command lines...
however it is not a normal practice to modify directly the values passed via command line so, for that purpose, any constant value should be ok... (but I am prepared to accept different opinions)

JonB

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

however it is not a normal practice to modify directly the values passed via command line

You can Google is it normal to alter argv to find out that it is perfectly acceptable to alter this or its content. I have no comment on "normal". It is not acceptable to modify any of the actual strings' content "in place" or extend them etc., though reassigning a pointer in argv[] to point to a new string is fine.

If you ever supplied a fully compilable, working, minimal repro of your situation and what exactly to change from what to what to move it from crashing to working one could comment and explain. Without that we don't know and it's guesswork as to what is going on.

rparon

JonB,
correct, standards as C99 state that parameters argc and argv and the strings pointed to by the argv array shall be modiﬁable by the program....
I do not know if Qt alters those values but that introduces new possible origins for that behaviour ...

JonB

@rparon

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

If you ever supplied a fully compilable, working, minimal repro of your situation and what exactly to change from what to what to move it from crashing to working one could comment and explain. Without that we don't know and it's guesswork as to what is going on.

rparon

Hi JonB,
noted, unfortunatelly I have many other things to debug / correct... providing the solution I am testing works reliably,
I'll modify the code as said,
thank you very much for help

jeremy_k

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

Hi jeremy_k,
that is an interesting point,

The point seems to have been missed. Treating const data as non-const is undefined behavior, and the C++ standard allows a conforming implementation to do anything when UB is invoked.

Command line arguments and historical usage are irrelevant.