The GPL can't really overwrite the legal definition of derivative work, even if it wanted to. Of course, there is little case law actually going into the weeds, so it's hard to say for sure. However, the GPL's definition seems pretty reasonable to me as to what would constitute a derived work (anything linking with the GPL work OR anything passing very very complex structures between itself and the GPL work).
In fact, I suspect that a legal definition of exactly what constitutes a derived work of a program may be more broad than the GPL's definition. For example, it's plausible to me that a program which calls `sh` as a fundamental part of its functioning could be found to represent a derivative work of `sh` per copyright laws.
I think derived work or derivative work are the right terms. However, "copyleft" is not a legal term, and it is copyright law (and associated jurisprudence) that defines what constitutes a derived work of some original. Licenses then control what the copyright holder allows you to do with their works or derivatives.
So, only copyright law itself can say whether, for example, this comment I'm making is a derived work of your own comment, or if it is an original work of my own. If it is a derived work of your comment, then you are the copyright holder for this comment I'm making and you can choose to write a license that allows me to distribute it or nor or whatever else. If it is not a derived work, then I am the copyright holder, and I don't require any license from you to distribute this comment.
So, if copyright law were to say "a computer program A is a derived work of program B if and only if it is produced from the same source code of A or textual modifications of the code of A" (very implausible, but just for the sake of argument), then there would be no difference between the GPL and the LGPL, and you could freely link to a GPL program and distribute the result under a proprietary license (you'd still have to distribute the source code of the original GPL program, of course).
> If it is a derived work of your comment, then you are the copyright holder for this comment
Is that correct? I am also not a lawyer, but this seems wrong to me.
If I make something that is a derived work of some other copyrighted work, my understanding is that I still own the copyright on the parts of the final work that I made (assuming what I made meets the thresholds for being copyrightable). But I am not permitted to distribute that work unless I receive a license to do so from the owner of the work I've derived from.
My understanding seems to dovetail with how the GPL works. If I write a program that links to a GPL library, and that does indeed cause my program to become a derived work of the GPL library, I still own the copyright to my program. In order to distribute my program, I have to abide by the terms of the GPL. But even if I were to violate the terms of the GPL (thus terminating my rights under the GPL), I still would not lose my copyright interest in the program I wrote, only my right to distribute it.
Looking this up a little bit more, I think it's more complicated than I was saying but also than you're saying.
For the "normal" cases, you're right: you can have copyright on the original parts of a derived work. You are fully right though that the original work's author does not in any circumstance get the copyright for the derived work. That was a confusion on my part.
> But even if I were to violate the terms of the GPL (thus terminating my rights under the GPL), I still would not lose my copyright interest in the program I wrote, only my right to distribute it.
As far as I understand though, this is not true. If you were not authorized to use the original work, then even the original parts of your work are in fact not protected by copyright at all. So, if, say, I take a Spiderman comic and add an extra page of fanart at the end, without any authorization from Marvel/Disney, then my extra page of fanart is not protected by copyright at all, and anyone is free to copy it (unless it also infringes on Marvel/Disney's copyright of course) [0]:
> Only the owner of copyright in a work has the right to prepare, or to authorize someone else to create, an adaptation of that work. The owner of a copyright is generally the author or someone who has obtained the exclusive rights from the author. In any case where a copyrighted work is used without the permission of the copyright owner, copyright protection will not extend to any part of the work in which such material has been used unlawfully. The unauthorized adaptation of a work may constitute copyright infringement.
So, derivative work term in copyright law defines the scope only as binary all or nothing? It's always a whole product that is considered derived, never just part of a product.
And license itself can then limit the scope of copyleft, so that it applies it to only a specific part of a work.
Note that I am not by any means a lawyer, so please don't take my understanding as authoritative.
My understanding is that "a work" and "a product" are different things. If I sell you a bundle of three books, the bundle is a product, but each book is an individual work and caries its own copyright. Even a single book may contain 5 short stories, each with a different copyright holder and license.
Similarly, I can sell you one CD that contains three different works with different licenses. Now, it is true that the GPL license does seem to say that you can't distribute a modified GPL work on the same CD as a non-GPL work, which would not fall under copyright law, it would just be an arbitrary requirement of the license that you have to accept if you want to be able to distribute the GPL work at all (by default, you're not allowed to distribute copyrighted work at all - it's only the terms of the license that allow you to).
However, the derived work problem comes more into question when you write a program that only works if it is linked with, say, glibc. The question becomes: is your program a separate work from glibc, or is it a derived work of glibc? If it is a derived work, then the GNU project is the copyright holder of the program you wrote, and you're not allowed to distribute it at all unless you get some license from them. If it is NOT a derived work, then you are at least free to distribute your program however you want, and tell your customers "just download glibc from gnu.org, put it at this path, then start my program", and the glibc license could do nothing to stop you since it doesn't apply in any way.
An analogy with books would be you writing a book that's meant as a sequel to Harry Potter. It is quite well established that even if your book is fully original, if it uses characters from Harry Potter by name, or locations or other key concepts, then your book is a derived work of the Harry Potter franchise and you're not allowed to sell it even if you're not directly using any piece of the originals.
Either way though, if you're creating a derived work, the license of the original can allow you to distribute your derived work under any terms they want. A license could say "you can distribute derived works for free on Wednesdays, but you have to pay a royalty if it's any other day of the week". Or, more interestingly, the license could say "you can distribute derived works without any conditions if they only access our original work through DLOPEN, but if they are accessing it through static linking, you must release the code and build instructions for your derived work" - this is what the LGPL does.
> Now, it is true that the GPL license does seem to say that you can't distribute a modified GPL work on the same CD as a non-GPL work
No it's not. The GPL specifically says that mere aggregation of a GPL-licensed work with another work licensed under another license does not create any sort of GPL obligation on the other work:
> A compilation of a covered work with other separate and independent works, which are not by their nature extensions of the covered work, and which are not combined with it such as to form a larger program, in or on a volume of a storage or distribution medium, is called an “aggregate” if the compilation and its resulting copyright are not used to limit the access or legal rights of the compilation's users beyond what the individual works permit. Inclusion of a covered work in an aggregate does not cause this License to apply to the other parts of the aggregate.
Here I am deferring to my company's lawyer. I believe they are interpreting (very likely incorrectly, but then they are a lawyer and I am not) this part:
> You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged.
In fact, I suspect that a legal definition of exactly what constitutes a derived work of a program may be more broad than the GPL's definition. For example, it's plausible to me that a program which calls `sh` as a fundamental part of its functioning could be found to represent a derivative work of `sh` per copyright laws.