segmentation fault

Milav · 15 Oct 2019, 07:20

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....

Milav · 15 Oct 2019, 07:43

@jsulm said in segmentation fault:

Forget about
qDebug() << *mosq;

okay thank you.

@jsulm said in segmentation fault:

Can you try to run the app directly with gdb?

yes i tried but , debugger give me error,

the error is shown in below image.

JonB · 15 Oct 2019, 08:39

@Milav
I think @jsulm is asking you try running your app directly in gdb (from the command-line), not from within Qt Creator, as Creator might be the cause of your debugger problem.

J.Hilk · M Milav 15 Oct 2019, 07:41

@Milav ok, since you're able to run the debugger from QtCreator, try the following, select Debugin the menu bar, and select Start and Break on main that should work, and then step through your code line by line.

Milav · 15 Oct 2019, 08:41

@JonB said in segmentation fault:

app directly in gdb (from the command-line)

actually, i am new in this.

give me some time i explore it and give you answer.

i don't know how i run app in gdb.

JonB · 15 Oct 2019, 09:05

@Milav

Open Command Prompt/terminal/shell.
cd to where your executable is.
gdb <your-executable>

Milav · 15 Oct 2019, 09:10

@JonB

but in gdb how i see "stack trace" ?

what classes i use from List of classes of commands?

please tell me

jsulm · 15 Oct 2019, 09:26

@Milav In gdb enter bt and press enter after your app crashed

Milav · 15 Oct 2019, 10:25

@jsulm

i enter the gdb command as following .

tdp0009@tdp0009-desktop:~/Teq_Projects/Aug_2019/HMI/Firmware/QT/Desktop_Application/11-oct/qt-mqtt-example/build-qt-mosquitto-Desktop-Release$ gdb qt-mosquitto 
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
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For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from qt-mosquitto...(no debugging symbols found)...done.
(gdb) bt
No stack.
(gdb) 
No stack.
(gdb)