This confuses me, the GPLv2 doesn't say anything about commercial use, it merely says that you need to redistribute the source code to anyone that you distribute your software to.
Have they placed additional restrictions on the GPL (which I believe you are prohibited from doing)? or are they just trying to highlight that they are dual-licensing it under a license that isn't copyleft if you pay them (since lots of companies have no-gpl policies)?
https://github.com/mongoose-os-libs/mqtt/blob/master/LICENSE
Not a lawyer, but I don't see how that flies. They could make their own GPL-like license with that clause added in. But using it as-is means it's open for commercial use, subject of course to the GPL terms. https://www.gnu.org/licenses/gpl-faq.html#GPLCommercially
It would be the equivalent of writing a GUI plugin for WordPress and trying to sell it as something like Squarespace.
Not that I agree with this possibility of Mongoose OS's approach. I don't at all, and think it would completely negate their "open source" claim, from a semantical standpoint.
In practice everybody acts under the assumption that the GPL affects all code that is statically linked together, but nothing else. Some people extend the interpretation to dynamic linking, but consensus is that the GPL does not affect code interoperating over other interfaces like web APIs.
I assume what they mean is that you can purchase a special license which allows you to use moongoose without GPL (typically for proprietary use), for example, x264 and x265 (well known GPL licensed encoders for h264 and h265) does exactly this.
A workaround is a plugin mechanism with dynamic loading of .dll or .so libraries. But that won't fly on a microcontroller.
If you are building a library and you want it to be open source but also usable by products that are not open source, you can use LGPL. If you are building a proprietary library and you want to keep it proprietary, but link it into a GPL application, I think there is nothing is stopping you from doing that.
You still have an open-source application, it just won't work without the proprietary library.
(This sounds like it should be against the letter of the GPL, but as a thought experiment, pretend someone else wrote the library. Say, Nvidia, and you are Linus. You have no control over how their library gets licensed, and there is no person or body which can stop you from releasing your own code as GPL, even if it may have branches that depend on proprietary code.)
Doing OSS this way is most certainly against the spirit of the GPL, but I think it is within the letter.
> .dll won't help with GPL, only with LGPL (IANAL)
Was pretty much totally ambiguous about what exactly was meant.
At that point, doesn't the trivial defense just become "It's not GPL. It's my own custom license which happens to have all the requirements of the GPL plus $my_awesome_additions"?