SDK/Docs/1.1/Debugging with Qt Creator

Revision as of 12:49, 29 October 2010

This is a brief introduction to debugging Qt applications running on a remote MeeGo device, using Qt Creator. The instructions will work with any kind of remote MeeGo device, real or emulated.

Prerequisites

You will need Qt Creator set up for your operating system: see SDK/Docs/1.1/Creating_Hello_World.

You should also setup a new project, configured to deploy to a MeeGo device, as covered in that tutorial.

Prepare the host

You will need gdb to be installed on the host before you can do any debugging with Qt Creator. On Linux, use the package manager to install it; for example, on Fedora:

sudo yum install gdb

Configure the Qt Creator project for debugging

First, ensure that debugging is enabled for the version of Qt in use:

1. Click on the Projects button in the left-hand panel.
2. Under General, click on the Manage button next to Qt Version.
3. Select the Qt version you are using: use the MeeGo Qt version for your particular target (in this case, MeeGo sysroot).
4. Click on the Rebuild button next to Debugging helpers (near the bottom of the dialog box). This builds the debugging helper for this version of Qt. You should see a green tick under the Debugging column for that Qt version.

Set a breakpoint

Next, you need something to debug. For example, add a Push Button to the form (defined in mainwindow.ui):

Then add a click handler for it, so that when the button is clicked, a message string is assigned into a variable then output to the console. The code looks like this:

/* file: Headers/mainwindow.h */
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::MainWindow *ui;

// our new handler
private slots:
    void on_pushButton_clicked();
};

#endif // MAINWINDOW_H

/* file: Sources/mainwindow.cpp */
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>
#include <QString>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

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

void MainWindow::on_pushButton_clicked()
{
    QString message;
    message = "I have been well and truly clicked";
    qDebug() << message;
}

Most of this is boilerplate generated the project was started, but the MainWindow::on_pushButton_clicked method is new. Notice how it also needs to be defined as a private slot in the header file.

Add a breakpoint next to the line where you want the debugger to break by clicking in the editor margin. It looks like this:

(See the red circle on line 22 after message is initialized.)

To put Qt Creator into debug mode, click on Debug/the bug icon in the left-hand toolbar. This adds some extra panels to the window which are used to show the stack, watch expressions etc.

Next, you need to change the configuration, so that Qt with debugging enabled is used to run the project.

1. In the left-hand panel, click on Projects.
2. Under Build Settings, select Debug from the drop-down box next to Edit build configuration.
3. From the Build menu, select Rebuild all. This recompiles the project using the debug-enabled Qt.

Debug the application on the device

To run the application in debug mode, click on the green arrow with a bug overlaid on it (bottom left). This deploys the application to the remote device and starts it, as in this tutorial; however, it also enables you to debug the remote application inside Qt Creator.

Now, click on the Click me! button in the application running on the device. The program should pause at the breakpoint. Switch back to Qt Creator and look at the debug panels:

The application should pause at the breakpoint; and in Qt Creator, the message variable should then be visible in the Locals and Watchers tab, set to "I have been well and truly clicked".

For more information about using Qt Creator in debug mode, see http://doc.qt.nokia.com/qtcreator-snapshot/creator-debugging.html.