segmentation fault

Milav · 15 Oct 2019, 05:58

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 · 15 Oct 2019, 06:05

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

If not it will crash...

Milav · 15 Oct 2019, 06:10

@J-Hilk

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

but it is crash...

mvuori · 15 Oct 2019, 06:29

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

jsulm · 15 Oct 2019, 06:28

@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 · 15 Oct 2019, 06:48

@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 · M mvuori 15 Oct 2019, 06:10

@mvuori

no it is supposed to be sensors/temprature.

jsulm · 15 Oct 2019, 07:14

@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 · 15 Oct 2019, 07:20

@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 · 15 Oct 2019, 07:18

@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 · J JonB 15 Oct 2019, 07:16

@JonB

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

qtmosq(0xac50b0)

JonB · 15 Oct 2019, 07:25

@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 · 15 Oct 2019, 07:27

@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 · 15 Oct 2019, 07:32

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

Milav · 15 Oct 2019, 07:34

@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'.

qDebug() << mosq->connect_async(host.data(), port);
qDebug() << mosq->loop_start();
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 · 15 Oct 2019, 07:34

@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'

JonB · M Milav 15 Oct 2019, 07:30

@Milav
First you wrote

and i debug the application via qDebug(), the application are crash on following line of code.
mosq->subscribe(NULL, "sensors/temprature", 1); // Segmentation falut

Now you write

qDebug() << mosq->subscribe(NULL, "sensors/temprature", 1);
the output of all above command are zero (0).

This is a problem if you say sometimes it seg faults and sometimes it returns 0? Is that the case? You need to be very specific in your answers!

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

That is indeed a big problem. There is little for us to answer from what you have posted. Is a debugger supposed to work for your "Embedded Custom Board" (I don't know about this)? You really need one. Otherwise you are likely to have to try printing out all sorts of information in your code to try to diagnose what is going on....

Milav · 15 Oct 2019, 07:37

jsulm · 15 Oct 2019, 07:41

@Milav Forget about

qDebug() << *mosq;

it's only going to work if operator<< was overloaded for that class, apparently it isn't, but this is not an issue.
Can you try to run the app directly with gdb?

JonB · J jsulm 15 Oct 2019, 07:37

@jsulm

Can you try to run the app directly with gdb?

I know this is not the place for a long explanation. But when OP says

I make a qt application for Embedded Custom Board

I thought (knowing nothing about this) it meant he is doing something like )cross-)compiling of whatever but the app runs "on the board", so he might not have/be able to use a debugger? Can you explain to me in one sentence what the OP's enthronement actually is, I'd like to have the vaguest understanding....