[Solved] Segmentionerror with QString

andreyc

QString d_user = "user";
QString d_pass =  "pass";
QString d_url = "url";

This initialization of the members in a header file won't compile unless you use c++11

What version of Qt do you use?
If you create a simple project and in main() create QString and assign it. Does it work?

DDH87

I do use c++0x. I don't know any other version ;-)

But I don't want to assign in the header just initialization. The assigning will be done at a later point in the code, see the rest of the code.

And the compiles just fine. The compilation isn't the problem. The problem is in runtime.

Same error in qt4 and qt5.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff6af74d3 in QTextStream::operator<<(QString const&) () from /usr/lib64/libQt5Core.so.5

Chris Kawa

Hi,
This doesn't sound like a code error. It looks more like a runtime error.

What platform are you on and what compiler? Are you sure you're linking to the correct libraries? Are you sure you're not mixing release/debug program and libs or libs from different Qt version than the one you compiled with?

DDH87

It is a runtime error.

I have a Linux x86-64 machine i'm compiling on. I had only one qt version 4.8. If I wanted to compile in 5.1 I had a virtual machine running.

For this last error id did install Qt5.1 On my normal machine. But it's located in a other location, as you can see in the error's And de qmake for qt5 is qmake-qt5 so it can't be a mix-up.

Compilers

g++ (SUSE Linux) 4.8.1 20130909 [gcc-4_8-branch revision 202388]
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

qmake --version
QMake version 2.01a
Using Qt version 4.8.5 in /usr/lib64

qmake-qt5 --version
QMake version 3.0
Using Qt version 5.1.1 in /usr/lib64

andreyc

Is this error reproducible on a small example?
Like

int main(int, char**)
{
  QString test = "The test";
  QDebug() << test;
  return 0;
}

If it is reproducible then what is an output of ldd for Qt4 and Qt5 versions

ldd smallExample

DDH87

No this small test doesn't produce an error. Not in qt4 and qt5. So I think it has to do with the complexity of the code.

If I have the time I'll see if I get the same error if I use a small class en then assign it.

Chris Kawa

Ok, so it might be a code bug after all.
Maybe you're just calling these methods on a dead pointer?

d_connection->setUser(ui->user->text());

Where do you call that? is d_connection a valid pointer? is ui constructed at that point (usually via setupUi())?

DDH87

I could just say the pointers are valid. But I think the best way is to show you the code. Just the implantations the header are quite simple.

//mainwindow.cc
#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QDebug>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->actionConnect, SIGNAL(triggered()), this, SLOT(showConnect()));
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::showConnect(){
    qDebug() << "Calling";
    ConnectWindow connectw;
    connectw.setConnection(d_connection);
    connectw.setModal(true);
    connectw.exec&#40;&#41;;
    qDebug() << "Called";
}

//connectwindow.cc
#include "connectwindow.h"
#include "ui_connectwindow.h"

#include <QDebug>

ConnectWindow::ConnectWindow(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::ConnectWindow)
{
    qDebug() << "Started construction";
    ui->setupUi(this);
}

ConnectWindow::~ConnectWindow()
{
    delete ui;
    qDebug() << "Deconstructed";
}

void ConnectWindow::setConnection(Connect *connect)
{
    d_connection = connect;
}

void ConnectWindow::accept()
{
    qDebug() << "We got accepted";
    d_connection->setUser(ui->user->text());
    d_connection->setPass(ui->pass->text());
    d_connection->setUrl(ui->url->text());
}

user, pass, url are lineEdit

//connect.cc
Connect::Connect()
{
}


void Connect::setUser(QString user)
{
    qDebug() << "Writing user: ";
    qDebug() << d_user;
    qDebug() << " GO!";
    d_user = user;
    qDebug() << user;
}

void Connect::setPass(QString pass)
{
    d_pass = pass;
    qDebug() << pass;
}

void Connect::setUrl(QString url)
{
    d_url = url;
    qDebug() << url;
}

In all headers that have a d_connection member. Its made like this

Connect *d_connection;

Chris Kawa

Sorry, it doesn't really answer my question.
d_connection is passed (line 23 in first listing) from MainWindow to ConnectWindow but where is it initialized?

d_connection is a pointer so you should initialize it to nullptr either in the constructor initialization list or (since you're using c++11) at the declaration site. The debugger might then very well tell you where's the error (accessing null pointer). If you don't initialize it it's random trash and the bug may occur as crashes in random places.

DDH87

If you see the last line of my post you will see in the header with a pointer.

I took your advice and made it a null pointer in ConnectWindow. But it gives the same error as it did before.

Now I have MainW that makes an object Connect. That I send to ConnectW by pointer. Then ConnectW is setting values in Connect.

Wouldn't be easier to have MainW set the values that ConnectW will return to MainW? I don't think this is the best way of may help in this case...

Chris Kawa

I saw that line but I don't think you understand.

This Connect *d_connection; is not initialized. This is merely declared. This will have garbage value until you actually somewhere say d_connection = whatever

So what I suspect you're doing is this:

//somewhere in MainWindow.h:
Connect *d_connection; //this is garbage
//line 23 of first listing
 connectw.setConnection(d_connection); //still garbage
//line 23 of second listing:
d_connection = connect; //assign garbage to garbage
//line 29 of second listing
d_connection->setUser(ui->user->text()); //cant' call -> operator on garbage

What I said about nullptr is you should either do this

//in the header
Connect *d_connection = nullptr;

or that

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow), d_connection(nullptr)
{
  ...

This will not "fix" your program. This will just make it more obvious when it breaks. To actually fix this you should initialize d_connect to something useful(I'm talking d_connection = new Connect()) before you pass it to the ConnectWidget.

As for what's better for your case - I don't know. I don't know what these classes are and what are they suppose to do. That's your call.

DDH87

Thnx. you are a saint! it works!

I added this to the constructor of MainWindow and it now it works.

d_connect = new Connect;

Chris Kawa

Yeah, just don't forget to delete it somewhere because you're leaking memory now.