Source-only distribution of MIT-licensed project which depends on GPL library

Question

I'm trying to understand exactly when a work becomes a "derivative work," and how it affects licensing of my project. In my situation, we have a scientific code that we have written which is licensed under the MIT license. It uses the GNU Scientific Library (GSL) which is licensed under the GPLv3 (specifically not the LGPL). It is obvious to me that, once compiled and statically linked, the resulting binaries (and subsequently their source) must be distributed under the more restrictive GPLv3. However, we never distribute any binaries or any source licensed under the GPL. Therefore my question is:

Can I license and distribute source code under the MIT license which requires a GPL'd library to compile if I do not include anything derived from the GPL'd library? That is, does the dependency on a GPL'd library only "kick in" once compiled/linked into a binary? Or does the dependency itself mean that all source files which need to be linked with the GPL'd library need to be under the GPL as well?

I found the following questions before but they don't seem to quite answer my question:

• Using GPL library with MIT licensed code
• GPL-3.0 library in MIT project
• What if I don't know whether my program will be linked to a GPL library or not?
• GPL-2 Derivative works
• https://opensource.stackexchange.com/questions/10287/can-an-npm-package-have-an-mit-license-on-github-if-it-depends-on-mit-packages-w
• https://lwn.net/Articles/495361/

Answer 1

While other posts here make excellent points, the FSF's line is not that your code, written to be linked to a GPL library, must be released under the GPL. They require only that (a) the combined work, if released, be released under the GPL in its entirety, and (b) your work, if released separately, be released under the GPL or a GPL-compatible licence. They have always been clear that you can give away more rights to your work if you wish to, for example here, when they write

If a library is released under the GPL (not the LGPL), does that mean that any software which uses it has to be under the GPL or a GPL-compatible license?

Yes, because the program actually links to the library. As such, the terms of the GPL apply to the entire combination. The software modules that link with the library may be under various GPL compatible licenses, but the work as a whole must be licensed under the GPL.

So while your work is (to my mind) definitely a derivative of the GPL library, when it is distributed on its own I would argue strongly that you are fine to distribute it under a permissive free licence if that is what you wish to do.

I note that there is then some danger that a third party comes along, removes the GSL calls from your code and replaces them with their own, then locks up the resulting product with a proprietary licence. If you use the GPL for your code it avoids that risk.

Answer 2

The FSF (owners of the GPL licenses) take the stand that by writing programs to the API of the libary creates a derivative work, that thus must be distributed under GPL. As far as I know (IANAL, even less one versed in copyright law and open source licensing), there are no legal precedents on the point.

Case in point is the (GPLed) readline libary. For quite some time, linking to it from non-GPLed programs was forbidden, until the drop-in replacement (BSD licenced) editline showed up.

Answer 3

The makers of GSL are clear about their intentions. GSL is licensed under GPL v3 and they explain clearly how and why they want to have it.

The way GSL is integrated and works in programs is intimate, anything that builds on GSL is a 'covered work' as defined in GPLv3. The makers of GSL have not added a 'Linking Exception' to the license.

The fact that you are not distributing the GSL yourself, but you are depending on it --in the absence of a Linking Exception-- does not provide you with a way out of your obligations. Your software will also have to be licensed under GPLv3 as soon as it is compiled. I do not see any way out, these are the terms, take it or leave it.

If you only distribute the source code of your software and you see the need for licensing it under a license other than GPL, then I suggest you dual-license it with GPL (making it clear that the compiled software would be) and another license of your choice which fits your additional needs on top of what GPL can do for you. Please be aware that this addnl license might add confusion among the recipients, and a simple GPL license statement would be much clearer.

If the code is all yours, and you are concerned about your own usage of your code after you published it under GPL, then you can be relaxed. You can re-license your own work at any time, see the questions here and here.