Depends on how you look at it. The short answer is technically yes, however, themes have been generally considered to be exempted from the more rigorous GPL requirements, because they represent a more artistic work instead of simply being derivatives of the WordPress codebase.
For example, the codex’s Theme List asks for themes listed there to explicitly specify their licensing scheme.
Since a theme generally defines a look and feel and may include graphics and such, these are more the property of the person who created them. Simply adding in template tags to make them WordPress compatible doesn’t really make them enough of a derivative to count. That seems to be the general consensus, although I cannot find anything explicitly stating that.
Of course, this all depends on what the WordPress devs say. But that’s what I think is the general feeling. I may be wrong.
I’d say no, it’s not a violation.
If the original software was designed to accept plug-ins or drivers using a defined mechanism, such a driver or plug-in does not form a derived work.
I’d say “designed to accept plug-ins” is the key bit here. Themes plug-in to the WordPress code without necessarily using any of it – they can just use the API.
@ceejayoz: But themes aren’t plugins. You can’t use WP without one. Sure, the backend will run just fine, and you’ll get a beautiful database full of stuff, but you won’t be able to display your posts. Since at least one theme is necessary, I tend to think of them as an integral part of the software.
However, reading the GPL FAQ, I find that this isn’t necessarily so: http://www.gnu.org/licenses/gpl-faq.html#WMS
However, the WP license doesn’t seem to have an exception for templates that link to the WP code. I suppose that the devs’ intent is to permit publishing of templates under any license, which is ok with me. However, It would be a good thing if the next versions of WP included an exception like this one.
“But themes aren’t plugins. You can’t use WP without one.”
I don’t think that’s true, for two reasons:
1. The main WP index file has a “define(‘WP_USE_THEMES’, true);” statement, which seems to suggest that it should be possible to use WP without any themes, even the “default” one. Although, setting the constant to false or commenting out the line didn’t do what I expected (present a “bare” page), I believe that was the actual intent by the developers.
2. Prior to WP-V2, there used to be a file called “wp.php”, which when renamed as “index.php” effectively acts as the “bare” theme, producing output without any stylesheet rendering. If you download this file from one of earlier versions of WP, you will see that WP still works perfectly.
So, what I’m trying to illustrate is that WP is fully functional without any themes. However, no WP theme is functional without the WP core. Therefore, WP themes cannot be seen as “derivative works” and if that’s the case, there’s absolutely no requirement for identical licensing.
My conclusion: No theme violates the WP core GPL. That of course implies that themes (a) can cost money and (2) can preclude you from modifying them.
My thoughts only, of course.
Just to note this discussion:
Pizdin: Thanks for the tip. Still, “themeless WP” is a personal modification with WP 2.x. It’s a workaround, not the normal way to use it (at least, reading the text files included in the downladable archive). If I install WP following the standard set of instructions, and delete all my themes, I get a beautiful blank screen.
As for working without the WP core: If some piece of software you write is linked to a GPL library (without a linking exception), then you must GPL it. Your software wouldn’t work (just like a standalone theme) but the library would still serve its origninal purpose (just like the WP core). However, the comparison is flawed when we analyze the scope of the pieces: a WP theme is a VERY minor contribution when compared to WP.
However, I think there still needs to be some clarification: GPLed themes CAN be sold. The FSF sells much of it’s software. Actually, something under a noncommercial license isn’t Free Software at all, as defined by the FSF, or Open Source Software, as defined by OSI.
“If I install WP following the standard set of instructions, and delete all my themes, I get a beautiful blank screen.”
Yeah, that’s right. But like I implied in point (1), I think that’s either a bug, a non-implemented feature or a silly name for a constant. Maybe I’m wrong, but to call a constant “WP_USE_THEMES” definitely suggests to me that setting it to “false” will not use themes for presentation. The constant is used in “template-loader.php” and I expect it wouldn’t be very complex to extend that code to present unstyled output, which is what I think the developers intended. So, I’m assuming that that will be something that will work in a future release. If that’s so, then an out-of-the-box installation will support non-themed output.
“If some piece of software you write is linked to a GPL library (without a linking exception), then you must GPL it.”
That’s contrary to my interpretation. The Wikipedia entry does state that “plugins” are not derivative works, so the GPL can’t be enforced on them. And strictly speaking, a theme is a plugin.
No definitive answers, just opinions. Just goes to show that GPL is still confusing after all these years.
I don’t remember what the heck WP_USE_THEMES does, it’s probably a vestigial backward compatibility thing, but rebanyo is correct that themes link and use lots of internal WordPress functions, which make them linked under the GPL and subject to being a GPL-compatible license.
If a theme (or a plugin) used no internal WP functions or APIs, then it could probably be considered independent, but that would be really really hard for a theme.
pizdin_dim: Wikipedia isn’t written by lawyers. The GPL, on the other hand, is. It is also a binding contract to which users and devs subscribe, so it’s an enforceable rule (like a written law, but of lesser hierarchy). Courts have already ruled against people using GPLed code in non-GPLed apps.
Matt: Whoa, I never thought I’d get an answer from you. Thanks for the clarification (and technical insight, for those of us who can barely read some simple PHP).
WP_USE_THEMES makes WordPress actually display the page instead of just loading the page and all the WordPress functionality. By leaving WP_USE_THEMES set to false (or unset at all), one can include the WordPress functions inside of another piece of PHP software and use it that way.
It’s not supposed to display “unstyled” output or anything. Okay, so it’s badly named, but the goal with that set to false is to make it not display *any* output, to load the functionality only.
The issue is analog to the (still disputed) issue wether or not a non-GPL software (here theme) can dynamically link (themes are normally distributed without WordPress) to a GPL software (here wordpress). So, if you want to know it for sure, sue someone who did a non-GPL theme and find out.
Additionally you can use GPL-software (let’s say gcc) to generate non-GPL software/content.
rebanyo is correct that themes link and use lots of internal WordPress functions, which make them linked under the GPL and subject to being a GPL-compatible license.
Not a problem for me – Tarski is GPL – but perhaps this should be stated somewhere slightly more prominent on WordPress.org? There are hundreds of non-GPL themes out there, and you just said they’re effectively illegal.
Like TomK32 says, that sort of thing is still disputed and has not been decided in the courts.
So, if you want to know it for sure, sue someone who did a non-GPL theme and find out.
LOL. Guess that’s one way to do it. 🙂
TomK32: Of course you can use GCC to compile non-GPL stuff. the FSF’s Freedom #0 is the freedom to use the software for any function. About linking to other non-GPL programs, that’s what the LGPL (or GPL with a linking exception) is for. The GPL I received with my WP tarball doesn’t explicitly state that I can link non-GPL software to it, so I presume it can’t be done.
As for suing people: I’d rather wait for them to sue me for copyright infringement when I modify their spammy themes (cause that’s my main issue).
Here is a relevant FAQ from the Free Software Foundation:
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