Aug 24

Compatibility between the iPhone App Store and the LGPL

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LGPL Conditions

If you're developing an iPhone application that you intend to submit to Apple's App Store and you want to make use of a third-party's software library that happens to be licensed under the GNU Lesser General Public License (LGPL), you have a couple of choices according to the license requirements:
  • You can open-source your app.  Specifically, you provide to your users the source code of your entire application under the LGPL or GPL.  That means for example all the .h and .m files.
  • You can keep your app closed-source, but you provide to your users all the object code of your  application necessary to re-link your application.  That means for example all the .o and .a files.  Most people forget that this option is in fact available to iPhone app developers.
Of course, if you modify the library itself, you have to provide these code changes in source form either way.

Dynamic/Shared Library

The above LGPL conditions can be thought to apply to the case when the LGPL library is statically linked.
But outside of the world of Apple's App Store, the LGPL would normally give you another way to use LGPL code without releasing the source code or object code for your application: compiling your application with a run-time shared library (hence, allowing users to run your application with an updated library if they choose to).  The problem for us is that the Apple iPhone developer agreement doesn't allow the bundling of shared libraries.
If you don't care about the App Store and want to release/sell your application through Ad Hoc Distribution or to jailbroken devices (e.g. via Cydia), you can actually link apps with a run-time shared library and thus satisfy the LGPL without providing source code or object code.

Static Library Exception

Some library developers are aware of these iPhone and LGPL incompatibilities and provide a "static library exception," loosening LGPL requirements for the iPhone platform.  For example, the cocos2d author intended to offer such an exception: even though he neglected to distinguish between the source code and object code requirements of the LGPL, it's fairly clear he intended to relieve the app developer from having to provide source or object code even if they linked in the LGPL library statically.
It's a good idea for you to consider contacting the author of the LGPL library you're interested in to offer a similar exception for the iPhone.  That way you don't have to worry about having to provide object code for your app.

Spirit of the LGPL

Whether you decide to release the object code for your app or take advantage of a "static library exception," the spirit of the LGPL is violated by the iPhone restrictions: it becomes very difficult for your app user to customize your app with a modified or updated version of LGPL library.
Let's say you do release all the object code and all the utilities that your app requires to build a new app based on an improved LGPL library.  How can your users install the new binary?  They are faced with the following unhappy options:
  • Jailbreak their iPhone to install any binary that they want
  • Join the iPhone Developer Program for $99 a year to be able to legally distribute "Ad Hoc" the new app to up to 100 devices.
In other words, you may end up spending a lot of time distributing object code to satisfy the LGPL and protect the source code of your app, but the object code is very unlikely to get used anyway.
In fact, even if Apple allowed apps to link with dynamic shared libraries, how could users even substitute the libraries without jailbreaking their iPhone?

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