Segmentation Fault after assigning QSqlQueryModel from function return in switch statement

achoy011

Hello, my group and I have been running into an issue where, at random, our application crashes with a segmentation fault. According to the debugger, this occurs when we try to assign a QSqlQueryModel from a function return.

Here is the function with the offending statement (mainwindow.cpp):

void MainWindow::on_filterPushButton_clicked()
{
    filterType = getFilterType();

    qDebug() << "Filter dropdown selected index: " << filterType << Qt::endl;

    model->clear();
    delete model;

    switch(filterType)
    {
        case 0: ;//Genre
        case 1: {   //Leading Actor
                    TableFilter filterLA(new SortLeadActor, movieName);
                    model = filterLA.filter(movieName); //We believe this is where the issue is occuring
                }
        case 2: ;//SA
        case 3: ;//Dir
        case 4: ;//Release Year
        case 5: ;//IMBD Score
        case 6: ;//Content Rating
    };

    ui->MovieTableView->setModel(model);
    ui->MovieTableView->resizeColumnsToContents();
    ui->stackedWidget->setCurrentIndex(1);
}

The filter function leads to an appliedSort function (which is in a different class):

QSqlQueryModel * SortLeadActor::appliedSort(QString title)
{
        model = new QSqlQueryModel();
        qry = new QSqlQuery();

        qry->prepare("SELECT Actor_1 FROM movie_metadata WHERE (Movie_Title) = (:movieTitle)");
        qry->bindValue(":movieTitle", title);

        //qDebug() << title << Qt::endl;

        if(!(qry->exec()))
        {
            qDebug() << "Query exec failed SortLeadActor::appliedSort find movie title" << Qt::endl;
        }

        qry->first();
        leadActor = qry->value(0).toString();
        qDebug() << leadActor << Qt::endl;

        qry->prepare("SELECT Movie_Title, Director_Name, Actor_1, "
                     "Actor_2, Actor_3, Genres, Content_Rating, "
                     "Title_Year FROM movie_metadata WHERE (Actor_1) = (:leadingActor)");
        qry->bindValue(":leadingActor", leadActor);

        if(!(qry->exec()))
        {
            qDebug() << "Query exec failed SortLeadActor::appliedSort find movie title" << Qt::endl;
        }

        //Assign query to model
        model->setQuery(std::move(*qry));
        delete qry;

        return model;
}

Basically the model is supposed to be set to the query that is filled in above function.

Also, here is the result from the debugger:
alt text

We all get the same issue, but for some of my group members, this issue does not occur as frequently as other group members. We believe it has something to do with passing the QSqlQueryModel but beyond that we have no clue what's happening.

mchinand

This post is deleted!

SGaist

Hi and welcome to devnet,

You delete your model object but do not reset the pointer value hence if filterType is not 0 or 1, model will point to invalid memory.

RyanSolanki

@SGaist Hello, I am also part of the poster's group. We are testing this code in a controlled environment where we are isolating the value of filterType to only be 1. The program executes the code in case 1 of the switch statement but then crashes shortly after and never executes the code after the switch. It seems like the program is crashing inside the switch, but still executes all code in case 1.

SGaist

Wait... why are you allocating your QSqlQuery object on the heap ? There's no reason for that.

RyanSolanki

@SGaist What would you suggest we do to fix that? Sorry we are relatively new to Qt and are still learning

SGaist

Just allocate it on the stack like all the examples show.

When you read a method signature that uses const references, it's usually not a good idea to create an object on the heap, only to de reference it to pass it to that method.

RyanSolanki

@SGaist So create the QSqlQuery as a private member of the class rather than in the appliedSort() method?

SGaist

No, just create it on the stack in the method, there's no need to use new, nor std::move, nor delete.

RyanSolanki

@SGaist Could that be what is causing the segmentation fault?

DeShark

It looks like the model has already been deleted when the MainWindow's destructor tries to delete it again and that causes a segfault for what should be obvious reasons.

From the stack trace the SortLeadActor's destructor is called, and then the MainWindow destructor is called. Are you calling delete on the model that you're returning to MainWindow from appliedSort during ~SortLeadActor?

Pl45m4

@DeShark

Yeah, that would also explain why they are facing the segfault not frequently... If you don't press that button which is connected to the slot where they delete the model, the crash probably won't occur.

Anyway, follow @SGaist 's instructions according to the query and in addition, don't try to double delete the pointer.

RyanSolanki

@DeShark Thank you! The double delete was the issue and I have now fixed it

DeShark

@RyanSolanki Please mark as solved if you are happy the issue is resolved. Also consider reading up further about Qt's memory management model (maybe it would make sense to pass a parent during the QSqlQueryModel's construction to avoid all that manual memory management or else perhaps smart pointers if there is no clear parent to assign ownership to).

Furthermore, SGaist's point about passing a pointer to a function whose signature expects a const reference cannot be understated. If the function expects a pointer, give it a pointer. If it expects a const reference, give it an object created on the stack! https://doc.qt.io/qt-5/qsqlquerymodel.html#details
The documentation is good; use it! It is unclear to me why you are using functions such as "clear" for the model. The setQuery() function already clears the model. Why mess around with new and delete several times when you can just pass in a new query to modify the existing model? Futhermore, why bother clear()-ing the model immediately before you delete it? The destructor is called during the delete, which itself frees up any resources, per the documentation.

There are deeper problems of comprehension going on here!