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Imagine I have an LGPL application A that has dlopen-based plugin system.

There are different parties of people that write the plugin for it.

  • Plugin B is licensed under GPL.
  • Plugin C is proprietary.

Now imagine following cases:

  1. Plugin B is shipped with some linux distribution D together with application A.
  2. Plugin C is released on their own website as binary.
  • 2a. Binary is compiled against the binary version of application A that is compiled from source code by themselves.
  • 2b. Binary is compiled against binary from this linux distribution D. (Essentially two outcomes in 2.a/2.b are same because there's not API/ABI change)

Is there any violations in the 3 cases above?

My understanding is that linux distribution that distribute application A and plugin B which essentially redistribute their own version of application A under GPL. As long as application A is GPL-compatible (LGPL in this case), there is no problem. I'm not sure about 2a and 2b, but my guess is that 2a is fine and 2b is not ok.

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Case 1 is trivial: since D can ship application A under GPL terms, the combination of A and B can legally be distributed under GPL terms.

Case 2 is almost as trivial: there's simply no (L)GPL violation in shipping something that does not contain (L)GPL code. For any Open-Source license to work, it must grant a permission for something that's barred by copyright law. According to copyright law, C is fine, so the author of A has no legal basis for a complaint.

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    I'm not convinced about GPL case you're saying. Suppose application A is GPL (not LGPL in this question). Plugin C at least is using some API interface and that will certainly show up in the compiled binary. I think this is LGPL specifically allow and GPL not. My actual question regarding 2b would then be, since A is supposed to be re-distributed under GPL instead of original LGPL because plugin B is distributed by D, would using this specific version of A a problem for plugin C that C should try to use their own compiled A?
    – csslayer
    Nov 9, 2021 at 1:40
  • @csslayer: API's aren't protected under copyright law, so again by the same logic GPL has nothing to say about that. With regard to case 2b, you cannot argue from license terms until you've shown that the license even applies. Copyright law again takes precedence.
    – MSalters
    Nov 9, 2021 at 8:10
  • "there's simply no (L)GPL violation in shipping something that does not contain (L)GPL code" - are plugins to program X considered derivative works of X?
    – user253751
    Nov 12, 2021 at 10:37
  • @user253751: The law is silent on that - just imagine trying to define in legal terms exactly what a "plugin" is. The relevant lawsuit is Google LLC v. Oracle America, Inc. which ruled that an API itself isn't subject to copyright, so a plugin API for X also isn't subject to copyright.
    – MSalters
    Nov 12, 2021 at 14:46

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