I am writing some proprietary library, at least for the time being it is proprietary, until I can get enough money to hire a lawyer to properly unwrap this (as I want to opensource, but keep a way to monetize), currently however:

  • Clause A: The library is proprietary, and it allows software built with it to be released into any license, as long as the library source itself isn't bundled.
  • Clause B: The library's functionality can be extended with plugins, plugins might be of any license, including MIT, plugins might just be a bridge for 3rd parties; the library needs nothing else to work by default.
  • Clause C: Plugins can be of any license type, MIT, GPL, unlicense; and can be written by anyone, including myself.

Does that mean that if one of the 3rd parties is GPL, only the plugin must be GPL, but not the whole library right?... after all, anyone can write a plugin; but there are some GPL parties I want to write plugins for, and since the final software can be of any license, they can release under GPL, but my library stays clear off it.

1 Answer 1


You are free in your license choice.

However not every license allows software to be distributed if it links to proprietary libraries. One of the licenses which disallow distribution of software which is not free in its entirety (thus its own source as well as sources of all libraries and dependencies used) is the GPL. That means your 'clause A' is an assumption which does not hold.

You may not distribute GPL-licensed software in a compiled form which depends on a proprietary library.

A GPL-licensed software may of course require a proprietary library. But as no-one is allowed to distribute the resulting executable, it makes such programme in all situations pointless where the ready-to-use software leaves your computer(s). That includes the case where you want to sell licenses for the library.

Clause B/C is similarily problematic when distributing it jointly in binary form. Yet in my understanding a plug-in is usually much easier to install and as such you might offer the software and the plug-ins separately for download with an installer for plug-ins (or even an integrated installer for the plug-ins).

The paragraphs where I talk about GPL don't apply for other open source license which do NOT employ the strong copyleft principle but only a weak one (like MIT, BSD,...). Programmes with such license may link to proprietary libraries and the resulting executables may be distributed.

  • Clkause B falls foul of the same problem, surely? If someone writes a plugin under a GPL-incompatible licence, then the resulting combination is still not redistributable.
    – MadHatter
    Sep 20, 2020 at 6:37
  • @MadHatter hm, likely yes. I shall amend. That case though is less of a problem due to the fact that plug-ins should be easier to distribute as a linked-in library required by the binary in the first-place Sep 20, 2020 at 6:53
  • Looks good, +1 from me!
    – MadHatter
    Sep 20, 2020 at 6:57
  • This is clearly not my strong points, as I barely understand :( so if the plugin is GPL compatible, and it's passed in to a GPL open software that uses the library which is propertary and thus is not included in the sources + the plugin which is GPL compatible; not a problem? or yes?...
    – Onza
    Sep 20, 2020 at 15:02
  • 1
    The problem is distributing a gpl program which includes a proprietary library. That is not allowed. You will have to rely on people downloading your library and building the programme from source, linking to your library in that process. No - one may distribute the ready - to - use programme with that library for other people legally. Similar argument for shipping the library with gpl-licensed plug-ins. But it's no problem in that case, if users download it separately Sep 20, 2020 at 15:34

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