This question is a follow-up on: Are there examples of proprietary software that functionally depends on free software?

Please note that this is not a duplicate, as "copylefted" (and some more, see full list below) are added as criteria.

I can no longer add those criteria to the previous question, because it has already got a good answer. Adding new criteria would invalidate the accepted answer.

Here is the new question:

Are there examples of composite programs that involve both proprietary software and free software and that also meet the following six criteria:

  1. The free software component that is part of the composite is only available under a strong copyleft license (no permissive license, no dual-license).
  2. The free software component does not allow some exception to strong copyleft (i.e. no LGPL, no linking exception, classpath exception, system library exception, kernel exception, font exception, etc.)
  3. The link that exists between the two programs is not that one program interprets the other's data.
  4. The composite is not a mere aggregate, where the free program just happens to be on the same file system or same media as the proprietary software, with no functional linking between the two.
  5. The composite work is distributed under non-free terms (not only used privatly).
  6. A court of law has declared the distribution lawful, or the distribution has never been challenged in a court of law.

The last requirement would exclude WMkernel from being used as example, as this has been challenged in a court of law (the decision is currently pending).

It is well known that opinions differs between FSF, OSI and TLF about what type of linkage that triggers the copyleft clause. My goal here is not to settle that dispute, or even to come up the the definitive answer about this (I don't even think that is possible).

Why I want to do, is to locate examples meeting the six criteria listed above. Yes, such examples may be "a legal disaster waiting to happen" (to quote a critic of the original question), but that is not the point. By identifying such examples and study them, we may learn something about what types of composites exist and that (so far) has not been challenged (or has prevailed) about copyleft.

In particular, I am looking for answers that clearly identify the components of the composite program, their functional interdependency, and how they communicate or are linked. The answer may also contain references to legal theory about this topic, but please note I am not looking for "mere theory", as I already is familiar with the theoretical positions of FSF, OSI and TLF. Any theory need to be connected to real examples, and its relevance to the example (if not obvious), should be pointed out in the answer.

Caveat: Do not even think about acting on any of the examples or analysis that appear in answers as being authoritative. This area is legally tangled and if you are going to create such a composite, you must either hire a lawyer to advice you, or you must secure explicit up-front permission to combine works from the copyright holder.

2 Answers 2


Your critieria does not seem to exclude this simple answer: "All proprietary software that runs only on Linux"

If you write software that requires Linux to run but your software is proprietary, you have a case of proprietary software that relies on copyleft software and the copyleft software has no special exception etc. There's no extra clauses added to the kernel's GPLv2. And there's no reason to think this is legally problematic.

If you're not satisfied with this answer, I think the clarification has to do with how reliant software can be / how connected etc.

  • If your point is that the proprietary software makes use of system library calls, note that this use is explicitly allowed by the Linux kernel exception and the GNU GPL system library exception (see item 2 on my list). I hope this comment clarifies that the criteria that makes this answer irrelevant is already part of the question. Aug 7, 2015 at 8:16
  • Indeed, sorry for my confusion. I guess the question is what would happen with a GPL kernel that didn't have the explicit kernel exception wording.
    – wolftune
    Aug 7, 2015 at 17:08

Take a look at firmwares of most low-cost, low-end home routers.

They use tons of GPL (and other) components, while being closed source as a whole. If there are source codes of such devices provided, then only for:

  • kernel (including drivers for custom chips used in router)
  • used software in original versions
  • other source code, but only if this is convenient for some reason for the manufacturer

In such devices there exist multiple contact points between open and closed parts, and while some of them were reported as GPL violations, most of them weren't.

(Of course most of them were done intentionally, to lower manufacturing costs using GPL software and bypassing the license terms.)

Also, most reported violations concerned directly Linux kernel and custom drivers, while most types of closed-to-open-source wrappers (for example to routing protocol daemons), network bridges etc. are recognized as clear from legal point of view.

  • Can you name a specific example? I know that D-Link routers use copylefted software, but they also provide source: intellectualpropertylawblog.com/archives/… Aug 7, 2015 at 15:40
  • At least D-Link, Ubiquiti, SheevaPlug and MikroTik (however the last one is not low-end). And Linksys WRT replacements: Sveasoft and DD-WRT. And of course OEM routers, which you can order from China with your own branding. Aug 7, 2015 at 16:15

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