This question is a follow-up on a number of questions that all mention some sort of software architecture X for creating software composites, and then goes on to ask: "Will using X to create a composite result in a derivative work?". For example:
- What are the arguments for considering dynamic links to constitute derivative works?
- What are the arguments for considering dynamic links to *not* constitute derivative works?
- Does a host application's license apply to plug-ins written for it?
- Does sharing a file format make two programs either a combined or derived work?
The assumption behind under each and every one of them is that software architecture X may be suitable to for telling the difference between derivative and aggregate composites (otherwise they would not be answerable).
I am starting to suspect that this assumption is wrong, and would like to try a new tack on this.
What do we know about composites that involve both proprietary software and free software? (And yes, this is just rhetorical - I know that this far too broad for this site.)
To turn this into a more focused question, I would like to see examples of proprietary software that somehow makes functional use (i.e. proprietary software that in some way depends on free software in order to perform a useful function).
We all know that proprietary software runs on a kernel that is under GPL (e.g. Linux), and also that this is not considered a license violation. So this is an example of the sort of composite I am asking about. I am of course not interested in being reminded about this well-known fact, but rather why this is not considered a problem.
To narrow it down further, I will say that I am looking for answers that clearly identify the components of the composite program, their interdependency, and how they communicate or are linked. The best answer to this question will be the answer that is most illuminating in pointing out a pattern of use or linking that can be used by non-lawyers to tell the difference between a derivative and an aggregate. I have a priority interest in concrete examples based upon existing and usable software. The answer may also contain references to legal theory about this topic, but please note I am not looking for "mere theory". Any legal theory need to be connected to real examples, and its relevance to the example (if not obvious), need to be pointed out in the answer.
Edit: This question did not impose any restrictions on what type of license was used for the free software. It brought to light (some of?) linking and other exceptions that exists and how they are used in real projects. (I learned a lot more about the permissive side of open source than I knew beforehand from the best answer). However, I also would welcome examples that shows when and how copylefted free software can be combined with free software, but I believe this requires a new question: Are there examples of proprietary software that functionally depends on copylefted software?