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I would like to develop an addon/extension plugin for an existing WordPress plugin, to add some premium features, but the base plugin's source-code is under GPLv2, which makes me doubtful...

Is any plugin developed for a product which is published under GPLv2 forced to be under GPL too? Or can we choose our own license?

I mean, forcing GPLv2 on plugins would be not very practical, no one would be able to create any plugin for any of such products (except when they do also use GPL as their plugin license)

for example, does that mean, we are legally NOT even allowed to develop any plugin for WordPress in the first place, except when we do publish the plugin under GPL too!!!

(I could not find any plugin exception in WordPress's license)

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You intend to make a "plugin for a plugin" for Wordpress, where the plugin you're writing code to connect to is itself GPLv2, and you wish to know what your licensing obligations are.

The short answer is that GPLv2 s2b says that

You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License

There's no ambiguity or wiggle room here. If your program is a derivative of the existing wordpress plugin, you are bound by the original plugin's conditions for making derivative works, and so you must license the complete new work, when you distribute it, under GPLv2.

The question of "when is my code a derivative of another piece of code" is a matter of copyright law, not of licensing, and is harder to answer. For one thing it's jurisdictionally-dependent, and for another a number of jurisdictions lack significant case law on the subject. While courts are in no way obliged to respect it, the normal community line is to look to the FSF's interpretation, which frames the question in terms of the idea of the single combined work. It says that "It depends on how the main program invokes its plug-ins", boiling the analysis down to how your plugin and the GPL plugin interoperate. Many, including myself, have written more on that elsewhere on this site, and though you should read it we need not discuss it further here.

You write that "it will be depending on interpretation - that of judges", which is true, but not very helpful. The GPL has already been litigated a number of times, in a number of jurisdictions, as have some other copyleft licences, and they've stood up in court. If you're prosecuted for stealing a book from a shop, the outcome also depends on the interpretation of judges on the day. Historically, that interpretation has usually been that stealing is against the law, and you would be unwise to rely on it being different in your case. You also make the point that this will only apply if it goes to court, which is true, but are you sure you'd like to be the expensive test case?

  • you are right, my answer leaves too much hope on the interpretation of judges to be against GPL (which really will not be the case), but I will not delete it yet since your answer does kind of refer to it ;-) – Top-Master Mar 18 at 10:36
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    @Top-Master judges are not likely either to be for or against GPL; they're likely to be for copyright law. If you're using someone else's work to make your own, and it sounds from the question as if this is the case because your plugin can't stand on its own, then it seems reasonable to me to expect you to honour the conditions they've placed upon the reuse of their work. As for your answer, I think you should leave it for the reasons you've stated - but perhaps you might choose to accept an answer other than your own. – MadHatter supports Monica Mar 18 at 10:57
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Since you are modifying an existing plugin, then there isn't much room to play.

But if you would create plugin from scratch, then you wouldn't need to release it under GPL.

Using libraries or creating your own plugins from scratch are not derivative work since you aren't modifying existing code.

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    +1 for short and painless answer, but note that, even using libraries under GPL license is counted as derivative work, unless there is an exception in the license, like LGPL license; IANYL, but the only other exceptions are frameworks, like WordPress, where you do not need to be afraid (from being sued to bankruptcy, by their owners, at least not till now), but even there are some cases where you sure need to be afraid from being sued, like the Qt-framework – Top-Master Apr 20 at 10:41
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I am not a lawyer, and below is my conclusions after reading another Answer:

Although the GPL license gives legally anyone the right of forcing us to change our plugins license to GPL, I think we will never receive claims or complaints from the WordPress community itself...

And also, in the end, it will be depending on interpretation - that of judges if it ever goes to court.

(If your program makes calls back and forth and shares "data structures", then they could be considered the same program. But if the judges interpret that they are two completely separate programs, then you're ok)

Edit: please refer to MadHatter's answer, since under other this answer leaves too much hope on the interpretation of judges to be against copyright laws set by GPL (which really will not be the case)

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    "If your program makes calls back and forth and shares 'data structures', then they could be considered the same program." - They would not be considered "the same program" but possibly one would be considered a derivative work of the other. And if your program is a derivative work of a GPL-licensed program, then you must comply with the GPL license to publish your program. – Brandin Apr 17 at 5:29

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