Segfault passing QString to function

Question

I want to pass G-CODEs to a 3D printer from my Qt app. I have a function that takes a QString and passes it to an external program as an argument:

void MainWindow::gcodeRelay( const QString &gcode )
{
    QObject *parent;
    relayProcess= new QProcess(parent);
    QString program = "./gcoderelay/gcoderelay";
    relayProcess->start(program, QStringList() << ui->tcp_server->text() << ui->client_port->text() << gcode );
}

But calling the function causes the program to crash with segfault:

void MainWindow::on_custom_gcode_clicked()
{
    QString gcode = ui->line_custom_gcode->text();    //SEGFAULT
    gcodeRelay( gcode );
}

This works:

void MainWindow::on_custom_gcode_clicked()
{
    QString gcode = "G28";                              //WORKS
    gcodeRelay( gcode );
}

Interestingly, this does not:

void MainWindow::on_custom_gcode_clicked()
{
    QString gcode;
    gcode = "G28";                                  //SEGFAULT
    gcodeRelay( gcode );
}

According to debugger the crash occurrs when the new process is created (before it's started)

Why does this happen? Also I'm confused as to what is the difference between the last two snippets of code and how could it cause such a problem?

Thank you and I'm just learning so be kind :)

@thuga Shouldn't the example of a working code also fail if that was the case? (Not to mention it shouldn't compile?) — Roope Kuisma, Sep 02 '16 at 11:57
Of course it will compile. You might get a warning that you have an uninitialized variable, but that's about it. Anyway, it's undefined behavior when you try to use an uninitialized pointer. Sometimes you will get a segfault, but other times you won't. — thuga, Sep 02 '16 at 12:04
Please do not modify the question when it's answered. An answer belongs **always** only in an answer. — Kuba hasn't forgotten Monica, Sep 02 '16 at 12:36

Segfault passing QString to function

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