segmentation fault

Milav · 12 Oct 2019, 10:48

Hello,

I make a qt application for Embedded Custom Board and when i run the Application it gives segmentation fault.

The application give segmentation fault due to following line.

  mosq->subscribe(mosq->getMID(), topic.data(), 1);

where mosq is define and declare as follow.

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    qtmosq* mosq = NULL;
    QStringList subed;
    QString currentTopic;
    Ui::MainWindow *ui;
--------
--------
--------
};

where qtmosq is define as follow..

#include "QObject"
#include "mosquittopp.h"

using namespace mosqpp;

class qtmosq : public QObject, public mosquittopp
{
    Q_OBJECT

public:
    qtmosq(const char *id = NULL, bool clean_session = true) : mosquittopp (id, clean_session) {MID = 0;}
    ~qtmosq() {}

    void on_connect(int result)
    {
        if (!result)
        {
            //subscribe(NULL, "$SYS/#", 2);
            emit connected();
        }
        else
            emit connectEnable();
    }
    void on_publish(int id)
    {
        MID++;
        emit messageSent(true);
    }
    void on_subscribe(int mid, int qos_count, const int *granted_qos)
    {
        if (MID != 0)
            emit subscribed();
        MID++;
    }
..........
..........
..........

signals:
    void connected();
    void messageSent(bool);
    void messageReceived(QString);
    void connectEnable();
    void subscribed();
};

so what is the reason behind for this fault.

The same application are successfully run on Host Linux PC.

I have use same mosquitto library in both the test.

please help me to sort out this problem.

I have send the reference link at where i used above program.

Link :- Ref. Link

Thank you
Milav Soni

JonB · 12 Oct 2019, 11:24

@Milav said in segmentation fault:

The application give segmentation fault due to following line.
mosq->subscribe(mosq->getMID(), topic.data(), 1);

Little information in your post. But likely one of:

mosq is nullptr/bad pointer
mosq->subscribe() or mosq->getMID() are raising the SEGV
topic.data() is raising SEGV

You're using some external library code here, so who knows what that might do.
Can't you run under debugger and get more specific traceback for the SEGV?

Milav · 14 Oct 2019, 05:17

@JonB said in segmentation fault:

nullptr

Thank you for your response.

Debugger is not run, it gives following error.

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

Signal name : SIGILL
Signal meaning : Illegal instruction

And i replace NULL value in mosq->getMID() ,
i replace Topic Name direct instead of topic.data()

but i face same problem.

so may be it can be happen by

mosq
mosq->subscribe()

so, is there any another way to solve this error.

you tell me mosq is bad pointer, so what is best way to make it good pointer.

is i used memory allocation in mosq defination?

please help me to find out this error solution.

Thank you
MilaV Soni

jsulm · 15 Oct 2019, 04:34

@Milav said in segmentation fault:

you tell me mosq is bad pointer, so what is best way to make it good pointer.

Create an instance and assign the pointer:

mosq = new qtmosq();

Milav · 15 Oct 2019, 04:36

@jsulm

i tried with above syntax.

qtmosq* mosq;
 mosq = new qtmosq(id, true);

but still i am face same segmentation fault error.

can you please give me best solution for the same.

jsulm · 15 Oct 2019, 04:44

@Milav said in segmentation fault:

but still i am face same segmentation fault error.

Yes, because you now create a new local variable called mosq which is completely unrelated to mosq in MainWindow! So, the one in MainWindow remains a NULL pointer.
Remove

qtmosq* mosq;

Milav · 15 Oct 2019, 04:46

@jsulm

But than it give error like

/home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:47: error: 'mosq' was not declared in this scope

Can i declare any another place like mainwindow.h?
i also tried in mainwindow.h

like following...

private:
    qtmosq* mosq;
    QStringList subed;
    QString currentTopic;
    Ui::MainWindow *ui;

but it gives same error.

jsulm · 15 Oct 2019, 04:53

@Milav Please show where exactly you're calling

mosq = new qtmosq();

It must be inside one of the MainWindow methods.

Milav · 15 Oct 2019, 05:00

@jsulm

i call it in mainwindow.cpp file.

here i have send the code snip for that portion.

mainwindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->send->setDisabled(true);
    ui->message->setDisabled(true);
    ui->subscribe->setDisabled(true);
    ui->topic->setDisabled(true);

    ui->lineIP->setText(QString("a3jkuztr3frwt4-ats.iot.ap-south-1.amazonaws.com"));
    ui->linePort->setText(QString("8883"));

    lib_init();

    connect (ui->connect, SIGNAL(clicked()), this, SLOT(connectPressed()));
    connect (ui->send, SIGNAL(clicked()), this, SLOT(sendPressed()));
    connect (ui->subscribe, SIGNAL(clicked()), this, SLOT(subscribePressed()));
}

void MainWindow::connectPressed()
{
    QByteArray host = ui->lineIP->text().toLocal8Bit();
    QByteArray id = ui->lineID->text().toLocal8Bit();

    int port = 0;
    port = ui->linePort->text().toInt();
    if (!port)
    {
        ui->linePort->clear();
        return ;
    }
    ui->connect->setDisabled(true);


    mosq = new qtmosq(id, true);
    int size = sizeof(*mosq);
    qDebug() << size;
    connect (mosq, SIGNAL(connectEnable()), this, SLOT(connectEnabled()));
    connect (mosq, SIGNAL(subscribed()), this ,SLOT(subscribed()));
    connect (mosq, SIGNAL(connected()), this, SLOT(connected()));
    connect (mosq, SIGNAL(messageSent(bool)), this, SLOT(setMessageStatus(bool)));
    connect (mosq, SIGNAL(messageReceived(QString)), this, SLOT(showMessage(QString)));
    mosq->connect_async(host.data(), port);
    mosq->loop_start();

}

mainwindow.h

#include <QMainWindow>
#include "qtmosq.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    qtmosq* mosq;
    QStringList subed;
    QString currentTopic;
    Ui::MainWindow *ui;

private slots:
    void setMessageStatus(bool);
    void connectPressed();
    void sendPressed();
    void connectEnabled();
    void subscribePressed();
    void subscribed();
    void connected();
    void showMessage(QString);
};

qtmosq.h

#ifndef QTMOSQ_H
#define QTMOSQ_H

#include "QObject"
#include "mosquittopp.h"

using namespace mosqpp;

class qtmosq : public QObject, public mosquittopp
{
    Q_OBJECT

public:
    qtmosq(const char *id = NULL, bool clean_session = true) : mosquittopp (id, clean_session) {MID = 0;}
    ~qtmosq() {}

    void on_connect(int result)
    {
        if (!result)
        {
            //subscribe(NULL, "$SYS/#", 2);
            emit connected();
        }
        else
            emit connectEnable();
    }
    void on_publish(int id)
    {
        MID++;
        emit messageSent(true);
    }
    void on_subscribe(int mid, int qos_count, const int *granted_qos)
    {
        if (MID != 0)
            emit subscribed();
        MID++;
    }
    void on_message(const mosquitto_message *message)
    {
        if (message->payloadlen)
        {
            QString s(message->topic);
            s += '>';
            s += (char*) message->payload;
            emit messageReceived(s);
        }
    }

    int* getMID() {return &MID;}

private:
    int MID;

signals:
    void connected();
    void messageSent(bool);
    void messageReceived(QString);
    void connectEnable();
    void subscribed();
};

#endif // QTMOSQ_H

here above i send the code for the same.

if you get more please tell me.

jsulm · 15 Oct 2019, 05:08

@Milav So, you get that error here in MainWindow:

ui->connect->setDisabled(true); 
mosq = new qtmosq(id, true); // Is it here?
int size = sizeof(*mosq);

?
It should work.
Are there any other warnings or errors?
Try to do a complete rebuild: delete build directory, run qmake and build.

Milav · 15 Oct 2019, 06:10

@jsulm

when i build the project it give me only warning like follow.

/home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/qtmosq.h:14: warning: 'mosqpp::mosquittopp::mosquittopp(const char*, bool)' is deprecated (declared at ../../../../CC_Lib/mosquitto/phytec_wega_build_websockets/usr/local/include/mosquittopp.h:94) [-Wdeprecated-declarations]

/home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:57: warning: 'int mosqpp::mosquittopp::connect_async(const char*, int, int)' is deprecated (declared at ../../../../CC_Lib/mosquitto/phytec_wega_build_websockets/usr/local/include/mosquittopp.h:103) [-Wdeprecated-declarations]

/home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:80: warning: 'int mosqpp::mosquittopp::subscribe(int*, const char*, int)' is deprecated (declared at ../../../../CC_Lib/mosquitto/phytec_wega_build_websockets/usr/local/include/mosquittopp.h:110) [-Wdeprecated-declarations]

but i face segmentation fault error when i run that project, an i found the line at where i face segmentation fault.

it is ...

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

Milav · 15 Oct 2019, 05:19

@jsulm

Hello

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

Reference link

jsulm · 15 Oct 2019, 05:36

@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 · 15 Oct 2019, 05:51

@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 · 15 Oct 2019, 05:56

@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 · 15 Oct 2019, 05:58

@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 · 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...