Dual license with GPL and a closed source license

Question

I've done a lot of research on this, but I'm still quite confused.

I'm currently working on an Arduino-compatible software project. The project is open source, so I want others to make use of it as well, but I don't want them to create derivative works with closed-source licenses. I would like them to acknowledge any code used by them from my project as deriving from my work and would require them to keep their derivative work open source as well.

At the same time, I may want to create works in the future that build upon my project that are closed-source - for example, I wish to be able to use my own work in a piece of private, closed-source consultancy.

I think that GPL would be the most appropriate license for the first part. My question is, what license would cover the second part? And can I dual license that license with GPL?

Answer 1

As the copyright holder, you can do whatever you wish with your own code. Nothing prevents you from closing your own source in your own projects, if you hold the copyright.

Use whatever closed-source license your lawyers say is appropriate for your needs. Your existing GPL licenses should be unaffected.

Note that you cannot close the source of any code that was contributed by someone else to your GPL'd project, since you don't hold the copyright to that code.

Answer 2

For open-sourcing your code for others to use, you're right that GPL will make sure that people can't create closed source derivatives. GPL will force them to share their derivatives under the same GPL licence.

Bear in mind though that GPL might put others off using your code. If they make any use of your GPL code in their project (for example linking to your library) GPL will force them to open-source their entire project too when they distribute it - this is why GPL known as a viral licence. LGPL may make your code more useful to others, while still protecting your work.

For your closed source work, there's nothing wrong with also licensing your code (code that you personally wrote and own the copyright on) with a private licence (e.g. as part of a consultancy contract). However, you have no right to do this with someone else's (e.g. a contributor's) code unless you've got a separate agreement with them.