Segmentation fault using .dll compiled with Qt

[magartor]

Hello everyone,

I try to use a dll compile with Qt with pybind11. My dll is compose of 2 object ExampleDll and somejob. Here is the .cpp of my ExampleDll :

#ifndef EXAMPLEDLL_H
#define EXAMPLEDLL_H

#include <iostream>
#include <iomanip>
#include <QString>
#include <QObject>

#include "exampleDll_global.h"
#include "somejob.h"

class Q_DECL_EXPORT ExampleDll : public QObject
{
    Q_OBJECT
public:
    Q_DECL_EXPORT ExampleDll();
    Q_DECL_EXPORT virtual ~ExampleDll(){

    };
    float Q_DECL_EXPORT exampleMult(int a, float b);
    void Q_DECL_EXPORT startThread(void);
    void Q_DECL_EXPORT printThreadResult(void);
    const std::string Q_DECL_EXPORT getRes(void);
    void Q_DECL_EXPORT setRes(const std::string);
    someJob * ptrworkerThread;
    someJob workerThread;
    std::string  resThred;
public slots:
    void handleResults(QString res);
};

float Q_DECL_EXPORT cppmult(int a, float b);

#endif // EXAMPLEDLL_H

Here the .h

#include "exampledll.h"

using namespace std;

Q_DECL_EXPORT  ExampleDll::ExampleDll(){
    resThred = "String init with Empty";

    //cout << "resThred" << resThred << endl;
    //workerThread = new someJob();
    //QObject::connect(&workerThread, &someJob::resultReady, this, &ExampleDll::handleResults);
    //connect(workerThread, &someJob::finished, workerThread, &QObject::deleteLater);
    //workerThread.start();
}

void Q_DECL_EXPORT  ExampleDll::handleResults(QString res){
    resThred = res.toStdString();
}
const std::string Q_DECL_EXPORT ExampleDll::getRes(void){
    return resThred;
}
void Q_DECL_EXPORT ExampleDll::setRes(const std::string val){
    resThred = val;
}
float Q_DECL_EXPORT  ExampleDll::exampleMult(int a, float b){
    float res = a*b;
    cout << std::setprecision(1) << std::fixed
         << "   QT In cppmult: int: " << a
         << " float " << b
         << " returning " << res
         << std::endl;
    return res;
}

float Q_DECL_EXPORT cppmult(int a, float b){
    float res = a*b;
    cout << std::setprecision(1) << std::fixed
         << " inshallah In cppmult: int: " << a
         << " float " << b
         << " returning= " << res
         << std::endl;
    return res;
}
void Q_DECL_EXPORT ExampleDll::startThread(void){
    //workerThread.start();
}
void Q_DECL_EXPORT ExampleDll::printThreadResult(void){
    cout << "Thread res : " << resThred << endl;
}

Here is the .cpp of the class somejob :

#include "somejob.h"

Q_DECL_EXPORT someJob::someJob() : QObject(this)// : QThread(this)
{

}

Q_DECL_EXPORT someJob::~someJob(){

}

And here is the .h os somejob :

#ifndef SOMEJOB_H
#define SOMEJOB_H

#include <QObject>
#include <QThread>
#include <QString>

class Q_DECL_EXPORT someJob : public QObject //: public QThread
{
    //Q_OBJECT
public:
    Q_DECL_EXPORT explicit someJob();
    Q_DECL_EXPORT ~someJob();
/*
    void run() override {
        QString result;
        for(int i = 0; i < 10000; i++){
            int j;
            for(j = 0; j < i; j++){
                if(i%j == 0){
                    break;
                }
            }
            if(i==j){
                result += QString::number(i);
            }
        }
        emit resultReady(result);
    }

signals:
    void resultReady(const QString &s);

signals:
*/
};

#endif // SOMEJOB_H

For some reason, my dll works when the line this line is commented :

someJob workerThread;

But I get a segmentation fault when using it. Here is a code I use to test the dll :

#include "exampledll.h"

int main(int argc, char * argv[]){
	float res = cppmult(5, 3.0f);
	if(res != 15.0){
		return -1;
	}
	ExampleDll test;
	test.exampleMult(5,6);
	return 0;
}

The code is compiled with the following command :

g++ -DQT_NO_VERSION_TAGGING -Wall -Werror -fPIC -std=c++11 `pkg-config --cflags --libs Qt5Core` -L ~/bind/overview_article/ -I . testcppmult.c  -L. -lexampleDll -Wl,-rpath,.

Here is the configuration of Qt and the os :
My ubuntu version is :

Distributor ID : Ubuntu
Description : Ubuntu 20.04.1 LTS
Release 20.04
Codename focal

Qt version : 5.12.8
Qt Creator 4.11.0

What did I do wrong ? :/

Sincerely,

Magarto

[jsulm]

@magartor And where exactly does it crash?
Please post stack trace after the crash.

[magartor]

Thank for your reply ! I change the code leading to segmentation fault by this :

#include "exampledll.h"
#include <execinfo.h>
#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <iostream>

using namespace std;

void handler(int sig){
	void * array[10];
	int size;
	size = backtrace(array, 10);
	fprintf(stderr, "Error: signal %d:\n",sig);
	backtrace_symbols_fd(array, size, STDERR_FILENO);
	exit(1);
}

int main(int argc, char * argv[]){
	signal(SIGSEGV, handler);
	std::cout << "Main start" << std::endl;
	float res = cppmult(5, 3.0f);
	cout << "Res mult = " <<  res << endl;
	if(res != 15.0){
		return -1;
	}
	cout << "Instantiate class" << endl;
	ExampleDll test;
	cout << "Call method class" << endl;
	test.exampleMult(5,6);
	cout << "Return" << endl;
	return 0;
}

Segmentation fault append when executing the line "ExampleDll test;". Here is the backtrace :

Error: signal 11:
./a.out(+0x13b8)[0x55d2256e33b8]
/lib/x86_64-linux-gnu/libc.so.6(+0x46210)[0x7f5f11c30210]
/lib/x86_64-linux-gnu/libQt5Core.so.5(_ZNK7QObject6threadEv+0xc)[0x7f5f12287d3c]
/lib/x86_64-linux-gnu/libQt5Core.so.5(_ZN7QObjectC2EPS_+0x4b)[0x7f5f122928eb]
./libexampleDll.so.1(_ZN7someJobC1Ev+0x10)[0x7f5f12537cc0]
./libexampleDll.so.1(_ZN10ExampleDllC1Ev+0x33)[0x7f5f12537c03]
./a.out(+0x1511)[0x55d2256e3511]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0x7f5f11c110b3]
./a.out(+0x12ce)[0x55d2256e32ce]

I think it come from the "someJob" instantiation, because if I modify the class to :

class Q_DECL_EXPORT someJob //: public QObject //: public QThread
{
    //Q_OBJECT
public:
    Q_DECL_EXPORT explicit someJob();
    Q_DECL_EXPORT ~someJob();
};

using namespace std;

Q_DECL_EXPORT someJob::someJob() //: QObject(this)// : QThread(this)
{
    cout << "Instatiation works !" << endl;
}

Q_DECL_EXPORT someJob::~someJob(){

}

I don't get segmentation fault anymore. Is there some restriction with inheritance when creating dll ?

Sincerely,

[jsulm]

@magartor said in Segmentation fault using .dll compiled with Qt:

Q_DECL_EXPORT someJob::someJob() : QObject(this)

You're setting the object as its own parent! Remove "this" and check whether it still crashes.

[magartor]

Hi,

It works thanks :)