segmentation fault

[Milav]

@jsulm

Hello

Here i have send the link at where i download the example.

Reference link

[jsulm]

@Milav That example does exactly what I suggested you to do:

ui->connect->setDisabled(true);

mosq = new qtmosq(id, false); // HERE
connect (mosq, SIGNAL(connectEnable()), this, SLOT(connectEnabled()));

Did you try to build and run this example? If so - does it work?

[Milav]

@jsulm

Yes i delete the build directory and again i rebuild the whole project but i couldn't run the example. it is not work.

[jsulm]

@Milav said in segmentation fault:

it is not work

What exactly does this mean? Doesn't build? Crashes? Does not do what it should? ...?

[Milav]

@jsulm

it means build successful.

but it is not run, it is crashes application.

means while running the application, the application are crash.

with following error.

"segmentation fault"

and i debug the application via qDebug(), the application are crash on following line of code.

mosq->subscribe(NULL, "sensors/temprature", 1); // Segmentation falut

[J.Hilk]

@Milav well, did you press connect before you pressed subscribe?

If not it will crash...

[Milav]

@J-Hilk

I press first Connect button and than after i press subscribed button.

but it is crash...

[mvuori]

If temprature is supposed to be temperature, this is a bug:
mosq->subscribe(NULL, "sensors/temprature", 1);

[jsulm]

@Milav Did you debug to see whether mosq is not a nullptr? If it's not please provide stack trace after crash.
Also, in connectPressed it does a return if port is not set - is it set if you press connect?
You should really run through debugger...

[Milav]

@jsulm

i debug the size of mosq pointer through following code.

 int size = sizeof(*mosq);
    qDebug() << size;

and output is 20;

and in connectpressed button slot are run succesfully.

port value are shown correctly.

when i run debugger it shows following error.

The inferior stopped because it received a signal from the Operating System.

Signal name : 
SIGILL
Signal meaning : 
Illegal instruction

hope you got my point.

[Milav]

@mvuori

no it is supposed to be sensors/temprature.

[jsulm]

@Milav said in segmentation fault:

i debug the size of mosq pointer through following code.

The size of a pointer is fixed, it doesn't matter whether it contains nullptr or something else.
Do

qDebug() << mosq;

instead to see the value of the pointer.
Without stack trace I can't tell you what is going on...

[Milav]

@jsulm

when i execute "qDebug() << mosq;" it gives following output.

qtmosq(0xac50b0)

how i see stack trace in qt?

can you please explore me more about stack trace?

[JonB]

@Milav

int size = sizeof(*mosq);

@jsulm

The size of a pointer is fixed,

A slight slip of the tongue there. I think what my colleague @jsulm meant to clarify is: sizeof(...) is a compile-time calculation, it does not matter what the value of mosq is or what it points at run-time, rather this statement just assigns sizeof(qtmosq) regardless. As he says, show us what is in the mosq.

[Milav]

@JonB

when i execute "qDebug() << mosq;" it gives following output.

qtmosq(0xac50b0)

[JonB]

@Milav

when i execute "qDebug() << mosq;" it gives following output.
qtmosq(0xac50b0)

I'm not sure this is actually trying to deference the pointer, it may just print its value. I would try something likeqDebug() << mosq->something;, where something is some field you know is supposed to exist & be valid in a qtmosq. Or try qDebug() << *mosq (note the *)?

how i see stack trace in qt?
can you please explore me more about stack trace?

I don't know how you run in your "Embedded Custom Board", but can you run the app from inside a debugger? then when it seg faults you will see a "stack trace" in the debugger showing the call stack when the fault occurred.

[jsulm]

@JonB said in segmentation fault:

I'm not sure this is actually trying to deference the pointer

It does not, but I wanted to see its value :-)
Looks like it's not NULL.

[JonB]

@jsulm
Indeedy :) So now let's see how qDebug() << *mosq fares?

[Milav]

@JonB said in segmentation fault:

qDebug() << *mosq

gives error :-

/home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:54: note:   'qtmosq' is not derived from 'const QFlags<T>'

@JonB said in segmentation fault:

qDebug() << mosq->something

i tried following things and it output are '0'.

1. qDebug() << mosq->connect_async(host.data(), port);
2. qDebug() << mosq->loop_start();
3. qDebug() << mosq->subscribe(NULL, "sensors/temprature", 1);

the output of all above command are zero (0).

@JonB said in segmentation fault:

app from inside a debugge

i am not able to run the debugger because it gives following error.

The inferior stopped because it received a signal from the Operating System.

Signal name : 
SIGILL
Signal meaning : 
Illegal instruction

hope you got your answer.

[Milav]

@JonB said in segmentation fault:

@jsulm
Indeedy :) So now let's see how qDebug() << *mosq fares?

sorry the error is as following not previous one.

/home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:54: error: no match for 'operator<<' in 'qDebug()() << *((MainWindow*)this)->MainWindow::mosq'