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I am developing a library from scratch. If I license it under GNU GPL (or a similar license), which conditions everyone to disclose the source code, does the condition apply to my other projects?

Say I will create an android app that uses the aforementioned GPL library. Do I need to publish it under GPL aswell? Does it need to have the source disclosed even if I am the creator of both o them?

Does the situation change, when other people contribute to the library?

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Say I will create an android app that uses the aforementioned GPL library. Do I need to publish it under GPL aswell? Does it need to have the source disclosed even if I am the creator of both o them?

Of course not. There is absolutely no way whatsoever imaginable how this would be even remotely possible. The purpose of a license is to give rights to someone who otherwise wouldn't have them. You don't even need to know copyright law, it's just basic common sense.

Does the situation change, when other people contribute to the library?

Yes. If you use other people's code, you need to acquire a license for that code.

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  • Important to mention that the licensing issue with contributions can be prevented by providing contribution guidelines/conditions/agreements that grant you the necessary rights. While it might require some setup work and might prevent people from contributing, there's next to no overhead once this is done. Popular example doing this would be Unreal Engine.
    – Mario
    Jun 19, 2017 at 5:28
  • With the GPL license, the only thing that can really happen is that the copyright holder sues you for copyright infringement. Of course if I use your library without proper license, being sued for copyright infringement could be quite fatal. But as long as you are the sole copyright holder, the only legal risk is that you might sue yourself.
    – gnasher729
    Jun 19, 2017 at 11:26
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As the author of a creative work, you may choose a license by which to authorize others to redistribute your work. And you are not limited to choosing just one license.

If you choose to distribute a library under a given license, it would be inappropriate to not follow the terms of that license yourself in that distribution. So, if you created an app that uses your GPL library, it would be inappropriate to use anything other than the GPL for that app itself. Not doing so would essentially violate a demand you are placing on yourself. And it violates the spirit of the GPL that you licensed your library under.

However, if you wanted to release your app under a proprietary or different license, you can also dual-license your GPL library, and use the proprietarily licensed version for your app, and the GPL version for everyone else. Trying to single-license the library per the GPL and use it in your proprietary app is a murky land of unknown law. The complication comes from trying to identify the license of the application that has the bundled library.

Say I will create an android app that uses the aforementioned GPL library. Do I need to publish it under GPL aswell?

So, you don't really have to, but you should, and the consequences if you don't are strange and unclear.

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    What are you suggesting, that the OP write up a contract for a proprietary license and sign and counter-sign it themselves? You don't need to license your own software to yourself at all. You already have full rights to it.
    – Derek Elkins left SE
    Jun 19, 2017 at 4:07
  • @DerekElkins: while you don't have to license it to yourself, you should follow the rules of the license you choose to use. Imagine someone distributing software with a GPL license, but not making the source code available. They could conceivably do that because the GPL actually only applies to downstream users, but because the person doesn't make source available, no one else can actually redistribute it. Hence the murkiness.
    – whatsisname
    Jun 19, 2017 at 4:31
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    @whatsisname I don't see how it is a problem for the creator. It certainly defeats the point of offering the software under the GPL, but it's certainly not unlawful nor even deceptive. You know as a redistributor whether you have the source code or not. Under the GPL, you as the redistributor are obligated to provide the source code regardless of whether the creator also provides it directly. If the creator stops providing the source code themselves, that doesn't free you from your obligation. If you don't have the source code, you can't comply with the GPL. There's no ambiguity here.
    – Derek Elkins left SE
    Jun 19, 2017 at 5:57
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    You're also describing a scenario completely unrelated to the OP's. The OP is not suggesting not providing access to the source code of the GPL library. The OP is asking whether an app which uses code they (solely) own which is licensed under the GPL obligates them to also GPL a derivative app. It does not, because the OP is not a licensee of the GPL and so, as the author of the proprietary app, does not need to abide by the licensee's terms. The situation is exactly analogous to what would happen if the app author was a third-party but arranged a proprietary license with the creator.
    – Derek Elkins left SE
    Jun 19, 2017 at 6:05
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    Only the copyright holder(s) has/have the right to sue for any GPL license infringement. As long as the OP is the sole copyright holder, he is the only person who could possibly sue himself over any GPL license infringement, so he or she can do whatever they like.
    – gnasher729
    Jun 19, 2017 at 11:31

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