Is it legal to modify and redistribute a GPL engine, under a more restrictive license (for example closing the source code)?

2 Answers 2


GPL-licensed software cannot become closed software. If you 'fork' it, then the rights on this fork remain the same (see GNU General Public License on Wikipedia).

  • Why downvote? It's correct.
    – ferit
    Dec 29, 2015 at 12:49
  • So, there is so exception for chess engines right? It is the same?
    – ferit
    Dec 29, 2015 at 20:49
  • No exception* sorry for typo
    – ferit
    Dec 29, 2015 at 21:15
  • 1
    It is all the same. When GPL was created I do not think that they thought about chess :-) The license is only about changing and including the software. If you 'call' for example stockfish from another application then you install stockfish executable next to the application.
    – Marco
    Dec 30, 2015 at 8:42
  • 2
    There is NO exception and there is no exception needed. Once GPL, always GPL.
    – Marco
    Dec 30, 2015 at 19:00

Short answer: No. But for chess, the answer: Yes. Read on.

GPL is quite complicated. Essentially, it requires the application be open-source.

From: http://www.gnu.org/licenses/gpl-faq.html

Can I sell a GPL project?

You are allowed to sell copies of the modified program commercially, but only under the terms of the GNU GPL. Thus, for instance, you must make the source code available to the users of the program as described in the GPL, and they must be allowed to redistribute and modify it as described in the GPL.

Thus, it's ok to sell the Stockfish chess engine which is licensed as GPL.

Why open my source?

A crucial aspect of free software is that users are free to cooperate. It is absolutely essential to permit users who wish to help each other to share their bug fixes and improvements with other users.

In theory, whoever gets the source code could improve the engine and therefore benefit the whole community. However, in practice this has never worked well.

Can I change the license to something more restrictive?

No. The GPL says that your modified versions must carry all the freedoms stated in the GPL. Thus, anyone who receives a copy of your version from you has the right to redistribute copies (modified or not) of that version. You may not distribute any version of the work on a more restrictive basis.

But... ICC Chess Club bundles their software with the Stockfish engine and I don't have their source code. Is that illegal? No.


Chess engine almost always runs as a separate process from the application itself, connected to the application through pipes. This is very different to traditional programming where you bundles the GPL source code within the application.

Technically, there is no need to link (statically or dynamically) the GPL engine while compiling the GUI application. The engine itself is an executable that can run independently. Everything in the FAQ is only valid if the application links with the engine, if that isn't the case, there is no such thing as GPL license restriction.

Try to download Stockfish, do you see something like Stockfish_64bits.exe? This is a working executable that is independent to whatever chess GUI you'd like to use. You can even run it directly in the terminal. The engine is not linked to any chess GUI, thus it's always legal to bundle it with a commercial non-GPL chess GUI.

You can read more here

What does that mean?

It's perfectly legal to bundle a chess engine with a closed-source application if the engine is an independent executable. The GPL license simply don't apply here. But if you compile the engine with the application, you must provide the source code for your entire application. For example, ICC Chess Club is allowed to distribute the Stockfish engine without providing you their source code.

  • Nice effort but it's actually answer of another question. I'm asking about chess engines, not chess apps that run engines inside.
    – ferit
    Dec 29, 2015 at 12:52
  • @Saibot I answered like that because you were asking about chess application in another post and then notified me about your question. Please note chess engine by itself is totally useless and meaningless in all circumstance. If u insist on only and only engine then the answer is NO.
    – SmallChess
    Dec 29, 2015 at 12:56
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    Do not think that there is an exception clause for chess in GPL :-)
    – Marco
    Dec 30, 2015 at 8:43
  • 1
    @Marco did you even read it? Nobody said about exception.
    – SmallChess
    Dec 30, 2015 at 8:44
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    Chess is different sounds like exception. The case that you describe goes for other types of software too. BTW. The question was about modifying and changing the license, your reply not.
    – Marco
    Dec 30, 2015 at 8:47

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