Qt5 license and GNU LGPL

Qt Qt5 LGPL license

In the new Qt Documentation we can find "Qt 5.5 GNU Lesser General Public License (LGPL)" and particularly the confusing and foggy sentence "Qt is available under the GNU Lesser General Public License version 3 and the GNU Lesser General Public License version 2.1.".
Is it released under LGPL 2.1 or LGPL 3? From the final application and the embedded device itself the difference is huge.

Qt license model offers two choices:

  •     Enterprise, Professional, and Indie Mobile licenses.
  •     Community license (GPL or LGPL versions 3 and 2.1)

The adoption of a LGPL license for Qt, will basically allow you to create closed-sources application if you dynamically link to the library, as stated by the LGPL-v2 license itself.

The confusing part is that Qt5 started offering some library components under LGPL-v3 and the v3 license obliges and requires that to provide all instructions to recompile and reload the library on the target system, but only two components are released under LGPL-v3 : Canvas3D and WebView, so if you avoid to use them you can continue to create closed-sources applications as usual.

For further details read this post in the QT Blog

 

Changes in the Qt 5.4 Release with LGPLv3

Because of this, Digia are now adding LGPL v3 as a licensing option to Qt 5.4 in addition to LGPL v2.1. All modules that are part of Qt 5.3 are currently released under LGPL v2.1, GPL v3 and the commercial license. Starting with Qt 5.4, they will be released under LGPL v2.1, LGPL v3 and the commercial license.
However, there will be a set of new add-ons that will be only released under LGPL v3 (plus GPL v2 or later) or commercial license. These add-ons are listed below. We have discussed with the KDE Free Qt Foundation and have their support to make this change in Qt 5.4. We are also in talks with the KDE Free Qt Foundation about further strengthening the agreement.

New add-ons released under LGPL v3

In Qt 5.4, the new Qt WebEngine module will be released under LGPL v3 in the open source version and under a LGPLv2.1/commercial combination for Qt Enterprise customers.
Adding LGPLv3 will also allow us to release a few other add-ons that Digia before intended to make available solely under the enterprise license. In Qt 5.4, we will add a technology preview for two brand new modules to Qt under the LGPL v3.
The first module, called Qt Canvas3D, will give us full WebGL support inside Qt Quick. It is fully functional, but still marked as a preview because the support for JavaScript typed arrays is still implemented in a slow and not 100% compliant way.
The second module is a lightweight WebView module that will also be released as a technology preview. It supports embedding the native Web engines of the underlying operating system into Qt, and is currently supported on Android.
There is a final add-on that will get released under LGPL v3. This module will give native look and feel to the Qt Quick Controls on Android. This module can’t be released under LGPL v2.1, as it has to use code that is licensed under Apache 2.0, a license that is incompatible with LGPL v2.1, but compatible with LGPL v3.

How does this change affect you as a Qt user?

One of the first questions you might have is, of course, how this affects you as a user of Qt.
This first thing to notice is that if you are using Qt under a commercial license, nothing changes at all.
Also, if you are using Qt under GPL v3, you are unaffected, since LGPLv3 can always be converted to GPLv3.

Disclaimer

Please find more information about open source licenses at http://www.gnu.org/licenses/
If you are not sure what license you should be using in your project, please consult a legal expert.
Digia has opened an email address for specific questions about using Lgplv3 in your project.
Please contact Digia via
Qtlicensing@digia.com