Linux has an explicit exception in its license so that user space software doesn’t fall under GPL. https://github.com/torvalds/linux/blob/master/LICENSES/excep...

> NOTE! This copyright does not cover user programs that use kernel services by normal system calls - this is merely considered normal use of the kernel, and does not fall under the heading of "derived work".

That's not an actual exception but a note clarifying what the GPLv2.0 covers.

Linux’s exception is because you need its header files to compile c code that uses the kernel. If someone created an extension that provided a json api to blendr, it seems that would need to be open source but callers would not since nothing would need to be compiled against gpl code.

Whether or not "using an API" requires your software to be GPL depends on how it's implemented.

Going by GNU's description of what does and does not extend the GPL requirement[1] and how Blender plugins works, to me at least it seems they very clearly have to be GPL. Unless someone took the route that is common in closed source Linux drivers of having a small open source module that communicates with a binary blob where the majority of the actual interesting code is, but I have not seen any Blender plugins like that.

The Linux kernel actually provides a specific license exemption[2] for the headers that makes software using syscalls not bound by the GPL.

[1] https://www.gnu.org/licenses/gpl-faq.en.html#GPLAndPlugins

[2] https://www.kernel.org/doc/html/v4.18/process/license-rules....

Actually this is exactly what we did with the renderman addon for Blender. And AFAIK how other renderer addons work.

Frankly if you want to be doing any meaningful computation in your addon it probably makes sense to go this route and have the closed source part in C while the addon "bridge" is python.

>I don't think using an API of a GPL software means your software "has to be" GPL.

We should GPL the software that is calling the Api of the GPL software.

>not all software running on Linux using Linux APIs (syscalls) is GPL

This is because linus Torvold(Other contributors) has given the syscall exception along with GPL-2.0. That's why there exists all kind of proprietary software running on top linux os.

Note: I am an open source officer I deal on this topic very often

It's an area of open legal debate, with a spectrum of what it means to be using an API and what constitutes a derived work.

From what I can see, Blender is trying to pretend it's a hard-and-fast rule, and they are requiring addon authors to label their work as open source to sell on their store.

> From what I can see, Blender is trying to pretend it's a hard-and-fast rule

Well yeah it's a debate - I guess this is their side of the debate. Arguing for something isn't 'pretending'.

I think if it did mean that, Linux would be abandoned by most major companies overnight, as you could not develop non-GPL binaries for it; I do however believe Blender is in the wrong here.

Maybe all the confusion seen in this thread about what does and doesn't fall under GPL is the reason people take issue with it, moreso than what it actually does.

I remember such story with «Blender-OSM» addon by Vladimir Elistratov.[0]

[0] https://github.com/vvoovv/blender-osm/issues/42