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Ask HN: Could I use GPL licensed library in my MIT licensed project?
7 points by huxingyi 5 days ago | hide | past | web | favorite | 13 comments
I have been working on my side project for years, and I am preparing to release the 1.0 stable version. The project is on MIT license since the first day, and it’s my preferred choice. However, I introduced CGAL library to my project later, and it’s GPL licensed. Currently, there is no other choice, I have to use CGAL library.

1. Does that means I have to change my whole project to GPL license? 2. Suppose I license my project as GPL, in the future, if I found a replacement for CGAL, could I relicense my project back to MIT if I remove all GPL related code? Or all my code have been infected as GPL and I lose control of all of them? 3. If I allow user add and run Lua script in my GPL licensed software, does the Lua script forced to License on GPL also?






1. The other parts of the project can remain MIT or any other gpl compatible license. A condition for distributing the GPL library is that the whole project get covered by the GPL. A way to look at this is to think of parts only having to follow their own license, while the combined work has to follow all licenses that has clauses for combined works. In practice this mean that for example the GPL patent clause will cover the whole project and not only the library.

2. As each part has it own license you can mix and match how ever you want as long as the combined parts are compatible. If you are the author of all the project code (other than the CGAL library) then you have additional rights to change the license of future version, but in this case it is unlikely to matter as MIT is very permissive license. As the copyright author, the only way to "lose control" is that you can't retroactive change licenses of past publications of old version. New publications and new version however is fully within the control of the copyright author.

3. Plugins live in a grey area of copyright. From what I have seen the most common answer is that they are not covered by the license of the main program. Different organization makes different interpretation here, and for what it is worth FSF has published where they will enforce GPL (https://www.gnu.org/licenses/gpl-faq.html#GPLPlugins). I think Linux foundation determine the line based on GPL_ONLY flag in the kernel code, and wordpress has it own interpretation. I would however say that it is a bit of a stretch for a library authors to complain about the plugins licenses to software which uses the library unless there is very heavy and obvious 1:1 link between the plugin API and the library.


Thanks, to conclude from your detailed answer: I can still license the code other than CGAL to MIT, but license the whole project to GPL, and in the future if I removed CGAL, I can license the whole project back to MIT. And the plugin API is not related with CGAL at all, so I can let the user choose whatever the license.

1. MIT can easily be re-licensed as GPL. You can keep a branch of it right before the introduction of CGAL, so if someone really wanted to, they could replace it with something else themselves.

2. This is where it gets tricky. Yes, though you'll need the permission of all your contributors to the GPL version of your project, and replace all the GPL code ith similar functioning code. You should avoid just line-by-line rewriting of the GPL code though, you'll need to have a unique implementation.

3. Again, tricky, but MIT would probably be OK, because MIT is compatible with GPL, if it's within the GPL'd project.


Thanks, > 1. MIT can easily be re-licensed as GPL. You can keep a branch of it right before the introduction of CGAL, so if someone really wanted to, they could replace it with something else themselves.

It's too late, I introduced CGAL very early.

> 2. This is where it gets tricky. Yes, though you'll need the permission of all your contributors to the GPL version of your project, and replace all the GPL code ith similar functioning code. You should avoid just line-by-line rewriting of the GPL code though, you'll need to have a unique implementation.

So, I guess it's a Yes. If I removed CGAL in the future.

> 3. Again, tricky, but MIT would probably be OK, because MIT is compatible with GPL, if it's within the GPL'd project.

Basically, if I have to license my project on GPL because of CGAL, I would like the scripts wrote by user could license on whatever user like. But I am not sure if is possible.


> Basically, if I have to license my project on GPL because of CGAL, I would like the scripts wrote by user could license on whatever user like. But I am not sure if is possible.

It'd have to be a "compatible" license to be allowed. Here are the compatible licenses: https://www.gnu.org/licenses/license-list.en.html#GPLCompati...


Thanks, so that means the user have to open source their script also.

Kinda. If it's strictly for personal use and not shared, there's no obligation. If the user is distributing the script in any way, they would be obligated to "share" the changes to the world by having it be free software as well, yes.

I think that you can, as long as the entire project as a whole, when distributed, is licensed by GPL, even if all of your own source files use MIT license. (Since it is compatible, it can be done.) If you later remove CGAL then you do not need GPL, I think, and can distribute the version without CGAL as MIT license. (However, I am not a copyright lawyer so I don't know, but I think this is correct.)

Thanks, that's good!

This depends on the exact license they chose. https://www.cgal.org/license.html states that they use both GPLv3+ and LGPL. And they even offer a commercial license. I'd contact them and ask just to be sure. There's a mail address in the third paragraph.

Thanks, I mainly use the CGAL/Polygon_mesh_processing to do mesh boolean. I saw some thread say CGAL commercial license cost $12K, it's not affordable for a side project yet.

Take a look at this if you haven't already. https://stackoverflow.com/questions/9349220/2d-geometry-libr...

Thanks, I checked that link. The "CGAL/Polygon Mesh Processing" package is licensed on GPL: https://doc.cgal.org/latest/Manual/packages.html

And the mentioned other libraries doesn't have the features I want.




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