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I am an author of a non-GPL code that is distributed under an Apache license. There is a GPL2 library (not LGPL) that many users will commonly want to link to that provides part of our algorithm, but our repository does provide a default for the same functionality and links fine without it.

I'm trying to figure out a standard for separation. Is it OK for users to obtain and use the library? Does it matter that our code anticipates this through hooks commented out with macros or the build system? Does it matter if obtaining the code is facilitated, either using GitHub with submodules or cmake with FetchContent?

MadHatter
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Eli S
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    Can you clarify which GPL library is concerned? It makes a big difference if the license is "GPL v2 only" or "GPL v2 or later" due to compatibility issues with the Apache license. – Bart van Ingen Schenau Sep 19 '19 at 18:10
  • I don't know much about it, but JULIA employs a USE_GPL_LIBS flag to use or exclude GPL libraries (https://discourse.julialang.org/t/is-julia-suitable-for-the-development-of-proprietary-derived-softwares/7912). It's worth a closer look at how their code is structured and licensed. – apsillers Sep 19 '19 at 20:18
  • Does the hook (whether commented out or not) contain or is it derived from actual GPL code? Simply "referring" to some other code is not the code itself and does not generally require permission. This is similar to writing a book where you refer to some other other book (which is copyrighted). You do not need permission to do that. – Brandin Sep 20 '19 at 05:45
  • @Bart van Ingen Schenau Oddly, I can't clarify easily without some research. What happened was that the owner of the library copied the license and the template for the notice into the COPYING file for GitHub, but did not fill out the template or add notices to source files. It is the version 2 template. – Eli S Sep 21 '19 at 05:41
  • @Brandin Thank you for this clarification. We include a call to their routine with our arguments. I wouldn't consider it "contain" or "derived" other than that. – Eli S Sep 21 '19 at 05:43
  • @EliS, the name of the library or a link to its repository would be enough. Then we could check the fine print of the license ourselves. We probably know better what to look for anyway. – Bart van Ingen Schenau Sep 21 '19 at 08:24
  • @EliS Of course the catch is that even if you don't include the GPL software in your program, if a user downloads your software and then combines it with a GPL program, then the user needs to comply with the GPL. But I would say that's more on him. He should know that he is combining it with GPL software and must therefore follow that license (e.g. by distributing all the source code of the final product). – Brandin Sep 21 '19 at 10:37
  • @Bart van Ingen Schenau The repository is here https://github.com/gotm-model/code – Eli S Sep 22 '19 at 15:30
  • @Brandin the GPL is pretty clear that "if a user downloads your software and then combines it with a GPL program" then in fact (s)he does not need to comply with the GPL (see eg GPLv3 s2). It is only when that user redistributes the software that the GPL places obligations on him/her. I do note that you mentioned that obligations apply on redistribution, but I felt you weren't clear that they only apply on redistribution. – MadHatter Sep 23 '19 at 07:11

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Firstly note that while linking to a GPL library means you have to comply with the GPL terms, that doesn't mean that your code has to be licensed under the GPL, only that the license you use cannot prevent users complying with the GPL terms. This is where GPL compatible licenses come into play, which includes Apache 2.0 and Modified BSD (3-clause). Note in the compatible licenses list, that the version matters, you will see that Apache 2.0 is compatible with GPLv3 but not GPLv2, also the BSD mentioned is the modified BSD license which has no advertising clause, so be sure to look closely at what is used when looking at compatible licenses.

From the examples I have seen, it is considered a matter of what code is compiled/linked into the final binary rather than the method of retrieving the code that is used. I would suggest using a build option or only searching for installed libraries rather than making your build system download and build the libraries, that leaves it to the user to install and enable the use of the GPL libraries.

An example to look at would be the eigen library, where older versions were released under LGPL, it is moslty licensed under MPL now. It uses the preprocessor symbol EIGEN_MPL2_ONLY to not compile LGPL licensed code into the library. See example in v3.3 src/OrderingMethods/Ordering.h.

Similarly the quadriflow project using eigen has a build option to only use eigens MPL algorithms, you can see in src/optimizer.cpp that it chooses different classes based on the licensing choice.

sambler
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  • I agree, but have to point out that Apache 2 is not compatible with GPLv2 – amon Sep 21 '19 at 11:05
  • @amon, yes, an easy point to overlook. – sambler Sep 22 '19 at 03:25
  • If you distribute code which is linked to GPL code, then the opinion of the FSF is that you do, indeed, have to distribute the package in its entirety under GPL (see eg GPLv2 s2b, GPLv3 s5b). We have questions here that argue both for and against their position, but you seem in your first sentence to be suggesting that it's a done deal that the FSF is wrong, and without any further justification it's difficult to see how you can do that reliably. – MadHatter Sep 23 '19 at 07:07
  • @MadHatter I am not saying the FSF is wrong, I say that the license you use must not conflict with or prevent following the terms of the GPL. Yes the package as a whole would be considered a GPL package, but that doesn't mean your code has to be GPL. I am not commenting on the final package distribution, only the code internal to the project, and as in the example projects I give, if you deliberately do not use the GPL code from a library with GPL and non-GPL code, there appears to be some agreement that the end package can be non-GPL. – sambler Sep 24 '19 at 03:49
  • The FSF put a different interpretation on it, see their FAQ: you may release your code separately under a more permissive licence, but when it's being distributed as part of a larger work "the whole combined program has to be released under the GPL". – MadHatter Sep 24 '19 at 05:38