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Frequently Asked Questions

 

 

Under what licenses is Qt offered?

Qt is offered under the Qt Commercial Developer License, the GNU Lesser General Public License (“LGPL”) version 2.1 and the GNU General Public License (“GPL”) version 3.0.

 

What is the LGPL?

The GNU Lesser General Public License, or LGPL, is a free software license written and copyrighted by the Free Software Foundation which means that its terms cannot be modified. It was designed as a compromise between the strong-copyleft GNU General Public License, and permissive open source licenses such as the BSD and MIT licenses. The LGPL license requires that the LGPL-licensed libraries and any derivative works therof need to be licensed to downstream recipients under the terms of the LGPL.  The LGPL does permit users to license software programs that merely link with the LGPL licensed library under different license terms provided certain requirements are met. In essence this means that Qt users may create proprietary applications that dynamically link to the LGPL-licensed Qt libraries provided he or she adheres to the requirements of the LGPL.

 

What is the GPL?

The GPL is a free software license published by the Free Software Foundation. It has a strong copyleft restriction and requires that the work as a whole be licensed under the GPL. Therefore, applications created with the GPL-licensed Qt, would need to be licensed under the GPL and the source code of the applications would need to be made available to all downstream recipients.

 

How does the LGPL differ from the GPL?

Both LGPL and GPL are complex documents that require careful reading to appreciate the significant differences between the licenses. However a high-level explanation is that the GPL requires that any work containing GPL-licensed software must be relicensed in its entirety under the terms of the GPL. The LGPL places copyleft restrictions on the library itself and any derivatives thereof, but does not apply these restrictions to other software that merely links with the program. Hence, a common understanding is that LGPL enables the development of closed source applications (under certain restrictions), while GPL requires you to make application source code freely available.

 

Why would I want to buy a commercial license? What is the difference from the open source licenses?

The commercial Qt license includes standard support and access to updates and allows you to develop fully closed source software. The LGPL carries some restrictions regarding the ability for users to relink libraries and other restrictions that may impose architectural requirements that some organizations might not be comfortable with.

If you are uncertain as to whether or not you will be able to comply with the LGPL requirements at the time you begin your development, our recommendation is that you purchase a commercial license as it gives you the flexibility to decide licensing (commercial or LGPL) at the time of distribution.  To do this, you create an application that dynamically links to the Qt libraries.  At distribution, you can then decide to link to either the commercially-licensed or the LGPL-licensed Qt libraries.

 

What versions of Qt are licensed under LGPL?

Version 4.5.0 was the first release of Qt licensed under the LGPL. Previous releases were licensed only under a commercial license and the GPL. We intend to release all subsequent Qt releases under the LGPL, commercial, and GPL licenses.

 

Under what version(s) of the LGPL is Qt licensed?

Qt is licensed under LGPL version 2.1.  We selected LGPL version 2.1 as this is the version of the LGPL that best fits our purposes and we are most comfortable with at this point in time.  We will continue to evaluate the adoption, use and legal interpretation of LGPL version 3 by the community and may use this version of the LGPL for future releases.

 

Are there different support and services available depending upon the license model chosen?

We want to ensure that our customers are successful, regardless of the license they choose. Therefore we offer technical support and additional services to users of Qt independent of the license they have selected. One year of Qt standard support is included when you purchase a commercial developer license and can be renewed for consecutive 12 month periods thereafter . An equivalent service is offered separately to open source users;  Qt LGPL Standard Support. It has the same content and service level, but has a cap on the number of incidents a user can report during a 12 month period. Other services available from Qt Development Frameworks are generally licensing agnostic.

 

Can I switch from using Qt under the LGPL to commercial afterwards?

No.  Users of the LGPL versions of Qt need to comply with the LGPL licensing terms and conditions. Qt’s commercial license agreement contains a restriction that prohibits customers from initially beginning development with the LGPL-licensed version of Qt and then transitioning to a commercial version of Qt.

 

Can I transition my commercial Qt project to LGPL and how?

Customers who have developed applications with Qt will be able to dynamically link their proprietary applications to the LGPL-licensed Qt library provided that they are willing to comply with the obligations set forth in the LGPL. Additionally, as customers own the modifications they make to Qt under the commercial license, they may relicense such modifications under the LGPL if they wish.

 

Can I switch from using Qt under the GPL to the LGPL?

No, those using the GPL-licensed versions of Qt need to fulfill the requirements of the GPL. You will be able to re-release your work under the LGPL by incorporating it with or using the LGPL licensed version of Qt.

 

What is the contribution model for Qt?

The Qt Contribution Model provides infrastructure, process and a set of guidelines for all developers to contribute source code back to Qt, allowing for collaborative development of Qt between Nokia and developers using Qt to create software.  The source code of Qt and Qt-related projects has been made publicly available at http://qt.gitorious.org under the LGPL and the Technology Preview licenses, and all developers are free to download, modify and contribute changes back to be evaluated for inclusion in future released versions of Qt.  More information on the Qt Contribution Model is available at http://qt.nokia.com/developer/the-qt-contribution-model.

 

What does Nokia want to accomplish with its licensing strategy for Qt?

Our aim is to:

  • Establish Qt as the de facto standard for UI and application development, ensuring that there is a vibrant ecosystem of developers creating applications for Nokia devices and for other desktop, mobile and embedded platforms
  • Ensure that Qt is of the highest quality possible with good supporting tools and services such that Nokia is able to get devices to market faster, and with better software. The widespread use of Qt translates into greater richness and stability across and between platforms.

By spreading Qt usage as widely as possible and establishing a robust ecosystem, Nokia will gain access to true cross-platform APIs for developing applications and services once, and deploying them across desktops, devices and the web without rewriting the source code.

 

The Symbian Foundation has chosen the Eclipse Public License for Symbian – is the LGPL compatible with this?

While the LGPL and the EPL are not compatible and may not be combined on a file-by-file basis, they may be used in a common environment provided that the interaction between Qt and Symbian is limited to:  dynamic linking, inter-process communication and data exchange.

 

 

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