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The Top 10 Questions about the GPL License – Answered!

The GNU General Public License (GPL) is one of the most widely used open source software licenses. It was created by the Free Software Foundation (FSF) to protect the GNU’s software from being made proprietary. 

The GPL emphasizes the principles of software freedom and promotes the sharing of knowledge and collaboration. It is a copyleft license that requires any modified versions or derivative works to be licensed under the GPL.

Understanding the GPL is important to ensure that you’re using software in compliance with its license. To help you, here are some answers to the top 10 frequently asked questions about the GPL license.

1. What’s a copyleft license?

Copyleft is a type of open-source software license that allows users to freely use, modify, and distribute software. The main purpose of a copyleft license is to ensure that the software remains free and open for subsequent users. Copyleft licenses typically require the source code to be made available to others, allowing them to modify and distribute it further, as long as any modifications or derivative works are also licensed under the same terms, thereby preventing the addition of further restrictions.

These licenses provide a legal framework that encourages developers to share their modifications, ensuring that the software remains accessible and modifiable by others.

2. What are the terms and conditions of GPL licenses?

  • Free distribution. The GPL grants the freedom to distribute copies of the software to others without any restrictions. This includes both commercial and non-commercial distribution.
  • Source code availability. The source code of the software must be made available to anyone who receives a copy.
  • Modifications and derivative works. Users can modify the software and create derivative works based on it, but these modifications must also be distributed.
  • Copyleft provision. The freedoms granted by the license are preserved in all subsequent versions or distributions of the software. So, if you distribute a modified or derivative version of GPL-licensed software, you must also release it under the GPL.
  • No discrimination. Any additional restrictions on the software’s recipients are prohibited.  The license cannot restrict use by specific fields of endeavor or groups or types of users.
  • Compatibility. The GPL is compatible with other free software licenses, such as the GNU Lesser General Public License (LGPL). This allows developers to combine GPL-licensed code with code under other compatible licenses.
  • Warranty and liability. The GPL is a copyright license and does not provide any warranty or liability protection. Users of GPL-licensed software assume all risks associated with it.

Importantly, if you use a GPL component in your software, then your entire software is considered a ‘work based on a GPL’ and you must ensure that its use and distribution comply with the above conditions. Of course, this is a simple summary of the GPL terms and conditions. So, if you plan to use or distribute software under the GPL, you should consult the full legal text of the license to ensure compliance.

3. Is GPL enforceable?

Yes. If someone violates the terms of the GPL by distributing software covered by the license without complying with its requirements, the copyright holder of the software can take legal action to enforce the license. This typically involves sending a cease and desist letter, requesting compliance with the terms, or seeking legal remedies through the court system.

Enforcement of the GPL has been successful, and violations have resulted in settlements, injunctions, and the like. Organizations such as the Free Software Foundation (FSF) defend the GPL and have taken legal action to protect software covered by the license.

The enforcement of the GPL may vary in different jurisdictions when copyright law differs. However, the GPL has been upheld in various legal systems around the world.

4. Can I sell GPL-licensed software?

Yes. However, the GPL requires that the source code be made available to anyone who receives a copy of the software. Customers who purchase GPL-licensed software have the right to access and modify the underlying source code.

When you sell GPL-licensed software, you are essentially selling a copy of the software along with the rights and freedoms granted by the GPL. The GPL allows you to charge a fee for distributing the software, but you must also provide the recipients with the corresponding source code and grant them the same rights and freedoms that you received under the GPL.

Remember that the GPL is a copyleft license, which means that any modifications or derivative works based on the GPL-licensed software must also be distributed under the GPL.

5. Is the GPL a secure license?

The GPL is generally considered a strong copyleft license that’s safe to use. If you plan to combine GPL-licensed software with proprietary code, be aware that the entire project might need to be released under the GPL, which may not align with your business objectives. Always seek legal advice to ensure compliance with the license requirements.

6. What is the difference between GPLv2 and GPLv3?

There are currently two versions of the GPL license in operation: GPLv2 and GPLv3. The main differences between them are:

  • Scope. GPLv2 focuses primarily on software licensing and distribution, whereas GPLv3 includes provisions for digital rights management (DRM) and hardware restrictions.
  • Compatibility. The FSF tried to increase the compatibility of the GPLv3 with other licenses. To combine two codes into a larger work, both programs must permit it. If such rights are granted by both the programs’ licenses, they are compatible. By making the GPLv3 more compatible, the FSF expanded development options.
  • Patent provisions. GPLv3 includes explicit patent provisions to address patent-related issues. It requires any distributor of GPLv3-licensed software to license their patents that are essential to the software under terms that allow everyone to use the software freely. This provision aims to prevent patent holders from imposing restrictions on users of GPLv3-licensed software.
  • Additional permissions. GPLv3 allows copyright holders to include additional permissions in the form of extra terms or conditions. These permissions can grant users additional freedoms beyond what the GPL itself provides.
  • Internationalization. GPLv3 was written in an attempt to increase usage worldwide. GPLv3 includes improved internationalization and localization, making it easier to translate the license into different languages and ensuring that international laws will interpret it as the FSF intended, unlike the language used in GPLv2, which is considered very US-centric. GPLv3 also allows developers to add local disclaimers, which also helps increase its usage outside the U.S.

Importantly, although GPLv2 and GPLv3 differ in certain ways, they both share the core principles of free software and the four essential freedoms: the freedom to use, study, modify, and distribute software. So, the choice between GPLv2 and GPLv3 typically depends on the specific requirements and preferences of the project and its contributors.

7. Do I have to release my source code if I use a GPL library in my project?

Yes. If you use a GPL library in a project that’s released, the GPL requires that you release the source code of your entire project. This is because the GPL considers the combination of the library and your code as a derived work, subject to the copyleft requirements of the license. If you’ve modified a program’s source code for personal or internal use, there’s no need to release its source code. However, if you make the modified program available to the public, you must also make the code public.

8. Can you mix the GPL license with other licenses, or modify and distribute GPL-licensed software under a different license?

It’s possible to do this with GPLv2 and GPLv3, although there are some restrictions. Nevertheless, the FSF has stated explicitly that GPLv3 is compatible with the Apache 2.0 license. However, there’s an issue with the original BSD license as it imposes a specific requirement that is not in the GPL (the requirement on advertisements of the program).

The GPL license states that any modifications or derivative works must be licensed. So, if you modify GPL-licensed software, you must distribute your modifications under the GPL as well.

9. Can I use GPL-licensed software in a commercial product and a web application and can I link my proprietary software with a GPL library?

Yes, if you make the source code of the GPL-licensed software available to your customers and you don’t restrict the distribution or modification rights of the GPL-licensed components. And you can link your proprietary software with a GPL library.

Remember that if you modify the GPL-licensed software or combine it with your code in a web application, you must also make the modified source code available to users of your web application upon request.

Make sure you either include a copy of the GPL license with any software you develop and distribute or make it available by providing instructions on how to obtain it. If you link your proprietary software with a GPL library and then distribute it, it’s vital that the entire application is licensed under the GPL, and you must provide the corresponding source code.

10. What are LGPL and AGPL and how are they different?

LGPL stands for the Lesser General Public License, and it’s more permissive than standard GPL. AGPL stands for the Affero General Public License, a variation of the GPL that caters to programs that run on a server. Their main differences lie in their scope and requirements. 

  • Scope: LGPL is primarily designed for software libraries. It is used to license free software so that developers can use it in both open source and proprietary applications. You are only obliged to subject modifications to the original free library to the LGPL. You must also permit or prevent reverse engineering of the work that uses the library to enable debugging when the LGPL library is modified or replaced with later versions. 

The LGPL is compatible with the GPL. You can decide to “upgrade” to GPL and incorporate it in a wholly GPL-licensed project if you wish, but you can’t go the other way and re-license GPL-licensed code as LGPL.

AGPL extends the provisions of the GPL to cover software used over a network and programs running on a server. Under the AGPL, if a developer modifies a program, they must release the modified version of the program to everyone who uses the service.

  • Requirements. LGPL has weaker copyleft requirements compared to GPL or AGPL. If you modify an LGPL-licensed library, you must make the modified source code available but the application using the library can be distributed under different terms.

AGPL has stronger copyleft requirements compared to LGPL. If you modify AGPL-licensed software, you must make the modified source code available to all users interacting with the software over a network, regardless of whether it is distributed or not.

  • Distribution. When you use an LGPL-licensed library in your application, you can dynamically link to it, allowing your application to be distributed under different terms. You do not have to share the source code of your application unless you modify the library itself.

If you use AGPL-licensed software over a network, even if you modify it, you must make the modified source code available to the users who interact with it, including the ability to download the source code from the networked application.

In summary, the LGPL is primarily used for software libraries and allows greater flexibility in combining the library with proprietary applications. The AGPL focuses on networked software and ensures that modifications made to the software are shared with users who interact with it over a network.

Licensing is critical

Understanding GPL licenses is crucial for developers and businesses working with open-source software. Nevertheless, always remember to consult legal professionals or experts for specific advice related to your situation.

Are all your open source licenses compatible and compliant?

Meet The Author

Adam Murray

Adam Murray is a content writer at Mend. He began his career in corporate communications and PR, in London and New York, before moving to Tel Aviv. He’s spent the last ten years working with tech companies like Amdocs, Gilat Satellite Systems, Allot Communications, and Sisense. He holds a Ph.D. in English Literature. When he’s not spending time with his wife and son, he’s preoccupied with his beloved football team, Tottenham Hotspur.

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