One of the more common misconceptions about the GPL is that it is “viral” in nature and can “infect” any software that touches it.
While it is true that the GPL does have a viral component, it is only to ensure that derivative works based upon GPLed code are also released to the GPL. It is possible, and even common, for GPL applications and proprietary ones to co-exist side by side. For example, there are many proprietary programs, including Skype and MyDropBox, that run on Linux, which is GPLed.
However, when one delves into plugins and and themes, something of a gray area begins to emerge. Though a WordPress theme, for example, might not be based upon a GPL theme, it relies upon a GPL application to function. As such, it has been widely held that they are GPL-licensed, even if they haven’t been explicitly licensed as such.
This has caused a major rift in the developer community and Automattic, the makers of WordPress, sought out the aid of the Software Freedom Law Center to determine the what the status of WordPress themes is.
However, in many ways the answer from the SFLC raised more questions than answers and may have created even more confusion.
The GPL and Themes
The reason GPL has been such an important question to the theme community is that is a large, and often controversial, market for commercial themes. Many companies have sprang up selling WordPress themes. Though many of these commercial themes are licensed under the GPL, most are not and are instead licensed under a proprietary license that forbids copying.
Though to the developers that license their themes accordingly, this is a legitimate practice. However, others consider it against the spirit of WordPress and many have wondered if it is against the GPL.
The SFLC, however, did not provide a simple answer. According to the SFLC, WordPress themes, “contain elements that are derivative works of the WordPress software as well as elements that are potentially separate works.”
As the SFLC saw it, many of the files of the themes required WordPress to run. Familiar files such as the index.php, pages.php, etc. had “hooks” that fed into WordPress and that made those filed derivative works and, as such, should be licensed under the GPL as per WordPress’ GPL license.
However, themes aren’t the only add-ons affected by this issue, plugins are as well.
Plugins and the GPL
With a theme, it is fairly easy to determine which files are likely to be non-WordPress derivative. However, with plugins the challenge becomes much greater. Without going through the plugin file by file, perhaps even line by line, and seeing what relies upon WordPress and what does not, it is almost impossible to be certain unless the plugin is deliberately licensed under the GPL.
This means that many, if not all, of the questions that surround proprietary themes also surround proprietary plugins, including many WordPress “custom installations” that are for specific uses.
Obviously, this confusion is something that Automattic and much of the WordPress community would prefer to ignore and they have come up with a solution to at least minimize the potential issue.
The best way to avoid these licensing issues would be for theme and plugin developers to license their works under the GPL. This way, the entire work would have one license. To encourage this, Automattic created both a theme directory and a plugin directory and have restricted access to these directories to only GPL-licensed works.
These directories are not only the official repositories for WordPress addons, but they are also available from within WordPress installs themselves, making them easier to install and, in the case of plugins, easier to update and maintain.
Though plugin and theme developers are free to license their works, or at least the non-derivative portions of their work, under any license they chose, not using the GPL carries with it a series of limitations that could make the extensions far less useful and far less used.
Even though some authors have decried this decision, given the potential nightmares for licensing themes and plugins, the decision seems understandable.
Though this post has focused largely on WordPress, the same is true for any other GPL bogging platform or CMS. WordPress was used as an example since Automattic used it to test the waters of these issues.
In the end, whenever you create a plugin or other add-on for a blogging platform or a CMS, it is likely that large portions of it will need to be licensed under the GPL to be compliant with the original software’s license. However, it is also very possible that other portions will not.
Though this is not necessarily a flaw within the GPL, it does highlight a limitation and a gray area that it does create, though it is largely one created by copyright law itself and the difficult definition of a derivative work. In short, almost any license that attempts to make itself viral will have a similar issue.
This, as it turns out, is part of the trade off of using a viral license, such as the GPL, versus a non-viral one, such as the BSD license.