The mobile market for applications is booming. Apple, Android, and many other mobile platforms are feeding a nearly insatiable demand for apps with developers struggling to keep up. Many developers are turning to open source, which can speed up development, but app developers and store owners need to be aware of the licensing responsibilities that come with distributing open source code.

Why are mobile apps any different than desktop applications or enterprise applications? In truth, they're really not — the only difference is that the mobile application market has undergone tremendous growth in the past few years. The mobile gold rush is bringing in a lot of developers and companies that may not be experienced dealing with open source software and licensing.  Developers need to take special care to comply with the open source license provisions.

The distribution method, however, differs slightly. Most mobile applications are delivered via carrier or vendor application stores. iOS users, which includes the iPhone, iPad, and iPod Touch devices, get their applications via Apple's App Store. Android users turn, primarily, to the Android Marketplace — though Android offers the option of using third-party stores as well. Nokia, Research in Motion, Palm, Microsoft, and others also offer application stores of some sort. While this offers some convenience for users and a new revenue source for the mobile vendors, it also introduces a new wrinkle for distributing software under an open source license.

Specifically, the app store End User License Agreements (EULAs) may pose a problem when distributing any software carrying a reciprocal or strong copyleft license like the GPL family of licenses. The GNU General Public License (GPL) specifically forbids distributing any software carrying the license with "further restrictions."

Problems in Paradise

The problem has cropped up twice this year alone with the Apple App Store. The first instance was reported in May by the Free Software Foundation (FSF). A port of GNU Go, was being distributed via the Apple App store.

Unfortunately, the iTunes Store EULA provides restrictions that run counter to the GPL. Apple changes the EULA frequently, but the version at the time of the report had Usage Rules that required users to comply with Apple's terms rather than the GPL. Some of the restrictions include limiting the software to five devices, which is obviously counter to the GPL.

After being notified of the violation, Apple responded quickly to remove the GNU Go port from the App Store. Application developers should take note that vendors are unlikely to revise the terms of an application store in order to comply with the GPL or other reciprocal licenses. Apple, in particular, has a history of removing all manner of application from the App Store for various violations of its terms of service.

This does not mean Apple is entirely unfriendly to open source licenses. Its terms of service forbid copying, decompiling, reverse engineering, etc. However, it does have an exception for open source software, allowing users to examine software if the license allows. The sticking point here is that the FSF doesn't agree that Apple's concessions go far enough. It's also worth noting that this may not be the only case of the FSF pursuing a GPL violation in an app store. However, the organization went out of its way to call attention to the enforcement — something that is unusual for GPL enforcement. The FSF was specifically attempting to send a message about Apple's policies, which have been perceived as more restrictive than most.

The second publicized instance of a GPL'ed application in the App Store is the game Battle for Wesnoth. Wesnoth is a popular game for Linux, Windows, and Mac OS X that's also licensed under the GNU GPLv2. Wesnoth's copyright is held by individual contributors, and some of the Wesnoth contributors have specifically worked towards providing an iPhone port. This has sparked a debate within the Wesnoth developer community between the contributors who have worked actively to port to the iPhone and iOS, and those who believe that the terms restrict the freedoms that the GPL is supposed to protect.

For now, the consensus within the developer community seems to be that Wesnoth will remain in the App Store, perhaps with some sort of modification on payment or with a GPL exception that allows distribution via the App Store. It's also recognized within the Wesnoth developer community that Apple's App Store is not unique.

Though Apple's App Store is the most popular app store at the moment, and Apple notorious for its developer restrictions, other app store terms of service are likely to fall afoul of the GPL and other reciprocal licenses like the Open Software License.

Lessons Learned

It's not surprising to see a few stumbles with open source licenses and app stores, though they could have easily been avoided.

Any application developer should be aware of the terms of distribution for any open source applications being used, and the terms of distribution for the platform that they're developing on. Software carrying a permissive license like the BSD and MIT licenses should pose no problem for app stores, just as they pose no problem for inclusion in other proprietary software. Developers and app store owners do need to make sure they abide by the terms of the license, but doing so is generally straightforward.

A reciprocal license, such as the GPL, does not mean that it's impossible to port an application or use software in an mobile app, but it does mean that developers may need to obtain an exception from the copyright owners. Some projects, like the Free Software Foundation, are unlikely to provide an exception — but many commercial-backed projects are open to licensing deals.

The amount of open sourced software for application stores is only likely to increase. Ideally, app store owners would find a way to comply with open source licenses that work with their business model. Providing an exception for GPL'ed software should not prove a threat to Apple or any other provider's model while still encouraging the open source community to develop for their devices.

Failing that, vendors need to be aware of the issues posed by reciprocal licenses and should work with developers to educate them on the licenses that are (and aren't) compatible with the terms of the app store.

Mobile applications are an increasingly popular and profitable area for development. By applying the lessons learned for open source development for more traditional applications, developers and vendors can avoid costly mistakes.