GNU General Public License

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The GNU General Public License (GNU GPL or simply GPL) is a widely used open source software license, originally written by Richard Stallman for the GNU project. The GPL license grants the users irrevocable rights to use, modify and redistribute software (even commercially) under the condition that software or its derivatives retain the GPL license and that the source code is included or available to the user.

FlightGear, many of the related projects and the text of this wiki are released under the GNU General Public License, version 2 (GNU GPLv2).

Note  The word "software" will refer to many other things than that only in this article, for example text, textures, or aircraft packages.
Tip  If you can read and understand English, consider taking some time some day to read through the license text and maybe even print it.

What is copyleft?

The GPL is a copyleft license. This term can be a bit confusing at first. It might seem like free software is not or should not be copyrighted and in essence should be released to the public domain. However that would allow redistributing it as proprietary software after minor modifications, in essence copyrighting it and reserving all rights. Its users could then very well have no rights and it would no longer be free software.

Copyleft software is copyrighted, but is released with a license that aims at preserving the rights of the users. It is called "copyleft" because it is designed to do the opposite to what copyrighted proprietary software would do for the users rights. The GPL is a strong copyleft license in the sense that any derivate or redistributed software must use the same license, thereby granting the users the same rights they originally had.

Things to consider before contributing using the GPL

Note  This relates to contributing to the wiki as well.

Commercial reuse

Many does not realize that the GPL allows commercial reuse. It does. "Free" is here rather "free as in speech" than "free as in beer".

Cquote1.png You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
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Cquote1.png When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
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Something to keep in mind though is that the very same license conditions have to be met as when redistributing for free, for example that the source have to be included or available to the users, or that it still can be redistributed for free or with a fee.

This part of the GPL also affects what resources that can be used when creating content. For example it can rule out a lot of resources when creating scenery.

Revoking contributions

You should consider that when something is published on the Internet, it will be spread. There is no way you can revoke all available copies, and they will all have the license. In addition many contributions can not simply be removed as other things depend on them.

Thus, please think about all the repercussions of contributing to FlightGear before causing community irritation by asking for your contributions to be removed at some point - what you get and see now when you download and install FlightGear is the result of over a decade of contributions from hundreds of contributors, no matter if it's source code, artwork, aircraft, scenery, documentation, wiki articles or tens of thousands of discussions in the archives.

We cannot afford having contributors who feel that they're entitled to pull their contributions from the project if there is a community disagreement, no matter if it's removing aircraft, scenery, wiki articles or forum threads – please consider your GPL'ed contributions final – otherwise, the project will always be in danger, because some of our key contributors may decide to ask for significant contributions to be removed from the project.

It has been causing lots of irritation in the community whenever key contributors have asked for their contributions to be removed - and while there are no legal grounds that justify complying with such requests, we've still been trying to accommodate them, simply for the sake of peace and out of respect for long-time community members.

Contributing using the GPL

Note  When contributing to the wiki, this is automatically taken care of.

Modifying GPL software

Any derivate software have to be released under the GPL and it must be possible to see what you have changed and when.

For details see primarily section 2 of the GNU GPLv2.

Applying the GPL to new software

The Free Software Foundation have some recommendations of what to do when applying the GPL to new software. There is a slight variation depending on if the project is one single file or multiple files.

In essence they recommend that you should claim copyright, for example "Copyright (c) 2019-1023 Tony Stark", and add a preamble to each of the files and include a file with the plain text version of the license text. The preamble can be a bit shorter in all but one file in multiple file projects.

For details see How to use GNU licenses for your own software.

Related licenses

The GNU Lesser General Public License (LGPL) is a modified, more permissive, version of the GPL, originally intended for some software libraries. There is also a GNU Free Documentation License, which was originally intended for use with documentation for GNU software, but has also been adopted for other uses, such as the Wikipedia project.

The Affero General Public License (GNU AGPL) is a similar license with a focus on networking server software. The GNU AGPL is similar to the GNU General Public License, except that it additionally covers the use of the software over a computer network, requiring that the complete source code be made available to any network user of the AGPLed work, for example a web application. The Free Software Foundation recommends that this license is considered for any software that will commonly be run over the network.

Related content

External links

The official license text

Other Free Software Foundation resources