Q Public License

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The Q Public License (QPL) is a non-copyleft free software license created by Trolltech for its free edition of the Qt toolkit. It captures the general meaning of the GNU General Public Licence (GPL), but is incompatible with it, meaning that you cannot legally distribute products derived from both GPL'ed and QPL'ed code.

The main difference between GPL and QPL is that QPL forces the software developer to provide the source code, if in any way it links with QPL'ed code (a library for example), even if the QPL'ed code is not distributed with the software developer's code. This was the main reason QPL was used for Qt instead of the GPL. This meant that any code that uses (i.e links with) Qt under the QPL license must distribute in the QPL license and provide the source code.

The Free Software Foundation, authors of the GPL, sum up their objections to the QPL:

This is a non-copyleft free software license which is incompatible with the GNU GPL. It also causes major practical inconvenience, because modified sources can only be distributed as patches.

We recommend that you avoid using the QPL for anything that you write, and use QPL-covered software packages only when absolutely necessary. However, this avoidance no longer applies to Qt itself, since Qt is now also released under the GNU GPL.

Since the QPL is incompatible with the GNU GPL, you cannot take a GPL-covered program and QPL-covered program and link them together, no matter how.

It also allowed Qt to change the license in later editions of its software, something often also provided in the GPL, and it was also frowned-upon that non-free use or development of derivatives was still not allowed. Only the personal edition of Qt was covered by the QPL; the commercial edition, which is functionally equal, is under a pay-per-use license and could not be freely distributed. As KDE, a desktop environment for Linux based on Qt, grew in popularity, the free software community urged Trolltech to put Qt under a license that would assure that it would remain free software forever and could be used and developed by commercial third-parties. Eventually, under pressure, Trolltech dual-licensed Qt for use under the terms of the GPL or the QPL.

All legal disputes about the license are settled in Oslo, Norway, but it has never been legally contested.