How does the GPL apply when you link libraries


The GPL (General Public License) allows developers to make use of source code easily , but once they modify and distribute the executable, they must provide the source code of the modified version as well. Section 2 of the GPLv2 states this as follows: ” 2. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions: a) … b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.

Problems arise when GPLed and additionally proprietary software is actually mixed, in particular when a proprietary application uses GPL libraries or vice versa. In these constellations

the question is whether even the proprietary code has to be distributed with source code as required by the GPL.

Mixing software programs When you operate application type software on the GPLed operating system, you use operating-system functions. However, the application need not publish its source code. This has

been the general consensus and the GPLv3 states in its section 1 in this regard:

The “System Libraries” of an executable work include anything, other than the work as a whole, that (a) is included in the normal form of packaging a Major Component, but which is not part of that Major Component, and (b) serves only to enable use of the work with that Major Component, or to implement a Standard Interface for which an implementation is available to the public in source code form. A “Major Component”, in this context, means a major essential component (kernel, window system, and so on) of the specific operating system (if any) on which the executable work runs, or a compiler used to produce the work, or an object code interpreter used to run it.

The GPL then states that system libraries will not need to be published.

Related to this issue is whether you might want to open-source a program which statically links GPLed libraries into an executable. The result is that, yes, certainly, such a developer ought to publish the origin code of his or her program. Before everyone panics, however, remember that most GNU libraries are not licensed under the GPL but under the LGPL, the “Lesser GPL”. The lesser GPL will allow a non-(L)GPLed application to be linked to the LGPL library without any sort of requirements for distributing source code. This non-(L)GPLed program may then be distributed using any chosen terms whether it is not a derivative work. A developer improving on a LGPLed library, thereby creating a derivative work, has to distribute the improved version under the LGPL.

A standalone executable that will dynamically links to your library is generally accepted as not being a derivative work (in LGPL). It would be considered a “work that uses the library” and paragraph 5 of the LGPL applies.

A program that contains no derivative of any portion of the Library, but is designed to work with the Library by being compiled or linked with it, is called a “work that uses the Library”. Such a work, in isolation, is not a derivative work of the Library, and therefore falls outside the scope of this License.

Dynamic Linking out
Tricky issues show up when proprietary and GPLed software usually are dynamically linked. Under the GPL the legal issue is, whether the new software is a “derivative work” of the GPLed portions or an independent work.

Some believe that intent of the developer is vital to answering this question: was the application intended as a library or did linking occur to unmodified off-the-shelf programs. In either case the resulting software will not need to be GPLed.

The Free Software Foundation distinguishes between statically linked libraries (yes the complete software ought to be GPLed) and dynamically linked libraries which share data structures with the calling application and call back (they are considered one work and ought to be GPLed if either the library or the necessary paperwork are GPLed).

If the dynamically linked library (aka shared library) does not share datastructures and is only called to its main function, then it might not need to be GPLed.

Fascination and Frustration are tight: on the one hand I am amazed how to analyze technical details consistent with legal concepts, on the other hand I am frustrated that the results are unpredictable.

In any event, this will remain an area involving uncertainty where all parties will tread lightly.

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