I have a need to emulate a 6502 in my project, but I don't really want to spend time to develop and debug something to do that when there already exist many solutions to exactly that problem.

Therefore, I scouted out and found someone else's solution, fake6502.c by Mike Chambers. That's good, I will only need to make a few small amendments to this file. So my plan is to explain what I did, in the changelog near the top, and then put a copy of the GPL somewhere in the source tree. As it is, this file has the following license:

 * LICENSE: This source code is released into the    *
 * public domain, but if you use it please do give   *
 * credit. I put a lot of effort into writing this!  *
 *                                                   *

Is it legal to redistribute the file with a few changes under the GPL?

1 Answer 1


IANAL/IANYL. That said, a licence is compatible with the GNU GPL if it imposes no requirements or restrictions that are contradicted by the GPL; such a work can be combined with code under the GPL, and the resulting whole released under the GPL (as the GPL requires). In this case, the author has released the work into the public domain. The FSF's compatibility list has this to say about public domain works:

Being in the public domain is not a license; rather, it means the material is not copyrighted and no license is needed. Practically speaking, though, if a work is in the public domain, it might as well have an all-permissive non-copyleft free software license. Public domain material is compatible with the GNU GPL.

I note the author's request for acknowledgement. Since this follows directly on from an explicit release into the public domain, I can view it only as a request, rather than a requirement, because there's no such thing as public-domain-but-must-acknowledge. Even if it were a requirement, this would still be compatible with the GPL, which has similar language in itself.

So as I see it, yes, you can.

  • I see. The file actually is unlicensed then, despite the LICENSE: . Commented Jun 14, 2019 at 11:00
  • 2
    It's a bit of a semantic edge case. Some regard PD as a licence, others (like the FSF) regard it as a formal declaration of no-licensing-conditions. It's not really an important distinction to make, IMHO. PD as a concept has problem in countries which respect the inalienability of the droits d'auteur, so CC0 was developed as a formal near-equivalent - the loosest licence possible that's still a licence.
    – MadHatter
    Commented Jun 14, 2019 at 11:02
  • I would comply, by giving credit. It is polite. And because of problems with public domain, just see it as the condition of the licence. Commented Jun 17, 2019 at 16:24
  • @ctrl-alt-delor it's definitely not a condition of the licence. But polite, yes, I would agree.
    – MadHatter
    Commented Jun 21, 2019 at 17:02
  • @MadHatter trouble with public domain is, it is not universally recognised (e.g. some legal systems). Therefore interpreting is a licence may be a better strategy. Also I think that there is also a right to be recognised as the author, in many legal systems. Commented Jun 21, 2019 at 17:14

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