Can I post code examples with GPL software on Stack Overflow?

Question

Suppose there is a Python library called g which is licensed under GPL v2.0 (or v3.0). Suppose further that the library is sufficiently complex that it contains some C/C++ component(s) that are similarly licensed under GPL v2.0.

Now, I'd like to write a program that uses this library:

import g
g.foo()

The import statement here links to the library g, both by loading and compiling g's Python source code, and by dynamically linking to the g's .dll/.so component. According to GPL FAQ, my program has to be licensed under the terms of GPL 2.0 whenever it is propagated:

If a library is released under the GPL (not the LGPL), does that mean that any software which uses it has to be under the GPL or a GPL-compatible license?

Yes, because the program actually links to the library. As such, the terms of the GPL apply to the entire combination. The software modules that link with the library may be under various GPL compatible licenses, but the work as a whole must be licensed under the GPL.

Obviously, publishing a program on Stack Exchange is a form of propagation under GPL's terms, so the question is: can I post this code snippet on SE as a question or answer? After all, the SE Terms of Service require that I provided that code snippet under CC BY-SA license, which is not compatible with GPL:

CC BY-SA 4.0 is one-way compatible with the GNU GPL version 3: this means you may license your modified versions of CC BY-SA 4.0 materials under GNU GPL version 3, but you may not relicense GPL 3 licensed works under CC BY-SA 4.0.

Answer 1

No, but maybe. You have correctly noted that this is a problem. You cannot paste GPL'ed code to Stack Overflow. But there are two exceptions, which depend on the copyright laws in your local jurisdiction.

• The code snippet does not fall under copyright. Copyright protects creative expression. If the code snippet is very short, or cannot be creative because there is only one way to write this code, then any license is irrelevant. Where copyrightability starts depends on your jurisdiction.

• Your usage of this snippet falls under a copyright exception. For example, fair use or a right to quote are common copyright exceptions. Again, this depends on your jurisdiction. However, some kind of right to quote will exist anywhere as it is required by the Berne Convention (a treaty about international copyright).

It is also not 100% clear whether writing code that interfaces with a library creates a derived work of that library. So there is a theoretical possibility that the code snippet is not restricted by the GPL. In practice, you should not rely on this possibility, but assume that the GPL does in fact apply.

In a different scenario, e.g. a blog post that is licensed under CC-BY-SA, you could include the GPL'ed code and specify that it is not available under the CC-BY-SA but the GPL. I.e. the text of the blog post might be CC-BY-SA licensed, but other parts of the post are not. For Stack Overflow, this is not possible because you license the posts in their entirely, and you additionally issue a non-CC-BY-SA fallback license. So in a strict reading of the ToS, you wouldn't even be able to include other CC-BY-SA works in your posts.

Personally, I feel quite comfortable with posting GPL-derived code snippets on Stack Overflow. These will often not be copyrightable because they are both short and not terribly creative. If the post containing such code is discussing the GPL'ed library, and any GPL'ed code snippets are only there to support my statements, and they are attributed properly, then I can likely rely on a right to quote.

Answer 2

Yes, you can as it is your code only (and so you can license it to SE as you like). The fact that source code contains text import g, does not make said source code derived work (as it doesn't contain any code that you yourself didn't write), and so license of g is of no importance whatsoever for the purpose of posting said source code snippet.

But if some other person compiles and links that snippet to executable, then produced executable will be derived work (as it includes binary parts of GPL'd library mixed with your binary code), and hence is subject to g's license (which in case of GPL would mean that person could only distribute that executable as GPL, not as CC-BY-SA). Which is what GPL FAQ talks about.

When you use simple interpreter (such as python) instead of compiler, it is even simpler as there is no executable containing mixed code. You can distribute your code under whatever licenses you wish, but you cannot distribute GPL'd library code.

Fair use etc (which might grant you extra rights) does not even need to come into play, as there is no derived work at all when posting snippet of your source code.

To give example for python:

1. tar zcf blah.tgz your_code/*.py and distribute as CC-BY-SA - OK, it your code only. (The fact that it won't work without library is only usability issue for end user, not legal one for you)
2. tar zcf blah.tgz gpllib/*.py and distribute as CC-BY-SA - NOT allowed, library is GPL and it is incompatible with CC-BY-SA
3. tar zcf blah.tgz your_code/*.py gpllib/*.py and distribute as CC-BY-SA - NOT allowed, library is GPL and it is incompatible with CC-BY-SA and so you cannot distribute it as part of something else.

Note however that if you did for example copy/paste part of source code from GPL'd library into your code, it would create derivative work (same as linker would do with combining binary code in binary executable example above), and so it wouldn't be just your code anymore, and you'd fall under point 3. above and be unable to distribute resulting source code as CC-BY-SA.

Another easy way to look into it is to see that import g does not need refer to popular GPL library, it might as well refer to your own reimplementation of the library with same API interface (that you wrote but choose not to publish, for example; or even published it like the LessTif did when it reimplemented Motif back in the day, for example). By looking at the posted source code snippet alone, one cannot even deduce which one it is supposed to be using. Or on SE, your snippet could likely be for example pseudocode, which is never supposed to be able to run and uses some imagined library. The copyright issue (and thus licenses that override it) really only come into play only when you mix code (either source code by hand copying, or binary mixing by the way of linker etc.) from different copyright holder.