Learn Qt

When developing applications for an embedded Linux platform, you need to setup a development machine. That machine needs to be running Linux (if you only have Windows boxes, try running a virtual machine). You also need a few key components. There are a few pre-requisites that you need to have in place:

A cross compiler for your target. I.e. a set of tools to build applications for your target environment. If that environment uses the same CPU, glibc and OS as your development machine, this is the same compiler as you are using there. The more likely case is that you will have a specific compiler for your target called something like arm-linux-gcc.

A target with a framebuffer.

If you want to play with embedded Qt without an actual device. You can skip this part and just build the Qt/X11 and Qt/QVFb versions described below.

If you have those two parts, we can start building a Qt environment. First, start by downloading the latest versions of Qt/X11 and Qt for embedded Linux. While that downloads, let’s look into what we are trying to do.

Qt keeps the configuration of the Qt version being used in the qmake utility. The qmake utility is built when Qt is being configured, i.e. before you even compile Qt. We are aiming at building three versions of Qt, so you will have three different instances of qmake. Your job is to keep them apart. We will build the following Qt configurations:

Qt/X11, qmake-x11. This is the Qt version that you will be using on your PC. It is also used for building the tools, such as Designer and Linguist.

Qt/QVFb, qmake-qvfb. This is an embedded Qt configuration that runs on host, but works with the virtual framebuffer instead of the actual screen. The let’s you emulate the target system, but run your code on your host machine.

Qt/target, qmake-target. This is the embedded Qt configuration that runs on your target platform. This is what you use to build and actual application running on your embedded device.

Target or host? The host machine is your development PC, i.e the machine hosting all the source and tools. The target is the device that you are targetting, i.e. the device that is supposed to run your embedded application.

The Qt/X11 build can usually be the Qt installed by default on your distro. I, however, still prefer to build my own Qt. The commands usually goes like ./configure && make -jN, where N is the number of CPUs on your system plus one. When the build is complete (it takes a long time), you need to install it, and you will probably have to do that as root. On ubuntu and derivates, run sudo make install, on non-sudo-distros, run su to become root, then run make install from your root command prompt. The installation will end up in the /usr/local/Trolltech/Qt-<version> directory. You can change this by specifying a prefix when configuring.

You will also have to build the QVFb tool, i.e. the viewer part of the Qt Virtual Framebuffer. We will use this look later, but first, enter the tools/qvfb directory, and run make, then make install (with sudo or su, as when installing all of Qt).

The next step is to build the Qt/QVFb version. This is a variant of embedded Qt, so start by unpacking the embedded Qt source code, then rename the directory to qt-embedded-linux-opensource-src-version to qt-qvfb-version. This is to prevent the other embedded variant that we will build from overwriting this source code.

To build Qt/QVFb, we will need a slightly more complex configure line. The following options is really a minimum, you can add the -help option to configure to learn more.

• -qt-gfx-qvfb, the graphics driver will be for QVFb, i.e. the virtual framebuffer.
• -qt-kbd-qvfb, the keyboard input will come from the QVFb.
• -qt-mouse-qvfb, the mouse input will come from the QVFb.
• -prefix /usr/local/Trolltech/Qt-qvfb-version, the prefix is used to separate the QVFb version of embedded Qt from the target version.

Having configured the environment, building and installing this version of Qt is no different from the Qt/X11 build (and it takes just as long).

The final version of Qt to build is Qt/target – in my case Qt/arm. This is slightly more complex, as you will have to tell Qt how to cross compile itself for your target. Start by uncompressing the embedded Qt source code again. This time, let’s rename the directory qt-target-version (e.g. qt-arm-version).

Configuring Qt for an embedded target involves two steps (at least). You will need to create a mkspec, helping Qt understanding how to build for your system. Then you need to configure Qt for your target.

So, first things first. The mkspecs for embedded systems are kept in the mkspecs/qws directory. Here, you can usually find a good starting point. For instance, in my case the mkspecs/qws/linux-arm-g++ looks about right. Opening the qmake.conf file, I can verify that the name of the compiler, linker, and all the other tools look right. The default, arm-linux-*, looks right, so I can use this file right away.

The next step is to configure Qt, both to understand the embedded target, to reduce size and to use the right mkspec. I generally start by running ./configure -help, to get an overview of the available options. Below follows a list of the configure options that I recommend using:

First of all, you need to get the cross compilation right:

• -embedded arm
• -xplatform qws/linux-arm-g++

Then, we want a release build for target, and we want a prefix:

• -release
• -prefix /usr/local/Trolltech/Qt-target-version

You need to know the colour depth of your screen:

• -depth 16

Finally, there are a number of libraries that you can build into Qt, to avoid having to deploy them separately. If you have no other need for the libraries, I generally compile them as a part of Qt:

• -qt-zlib
• -qt-freetype
• -qt-libjpeg
• -qt-libng

In a similar manner, some options can be turned off (make sure to go through the entire help message to disable anything that you won’t be using):

• -no-gif
• -no-libtiff
• -no-qt3support
• -no-glib
• etc.

Finally, we need to make sure that Qt uses the right keyboard, mouse and screen drivers:

• -qt-gfx-linuxfb
• -qt-kbd-tty
• -qt-mouse-tslib

This will give you a fairly long configuration line. It might be good to know that the file config.status is created when you configure Qt and contains your last attempt. I say last attempt, because this process can take some tweaking.

Now, all you need to do is to make and make install as with the previous Qt versions. This should give you a nicely configured Qt for your target environment. Next time, we will take all the Qt versions you’ve built for a test ride.

This entry was posted on Thursday, June 11th, 2009 at 14:20 and is filed under Embedded Linux. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.