A GPL linking exception
modifies the GNU General Public License
(GPL) to create a new, modified license. Such modified licenses enable software projects which provide 'library' code, that is software code which is designed to be used by (in technical terms, 'linked to') other software, to distribute the software code of the library itself under terms essentially identical to the GPL without forcing others distributing code which merely uses, but therefore integrates, the code of the software library to apply the terms of the GPL to their own code.
Many free software libraries which are distributed under the GPL use an equivalent exception, although the wording of the exception varies. Notable projects include GNU Guile, the run-time libraries of GNAT,
and GNU Classpath.
Compiler runtime libraries also often use this license, e.g. the libgcc library in the GNU Compiler Collection uses a very similar linking exception, as well as all libraries of the Free Pascal project.
In 2007, Sun Microsystems released most of the code to the class libraries for the Java Standard Edition and Java Platform, Enterprise Edition projects under version 2 of the GPL license plus the Classpath linking exception, and used the same license as one possible license for their enterprise server GlassFish. and for their NetBeans Java IDE.
Version 3 of the LGPL is likewise constructed as exception to the GPL.
The classpath exception
The GNU Classpath project provides an example of the use of such a GPL linking exception. The GNU Classpath library uses the following license:
- Classpath is distributed under the terms of the GNU General Public License with the following clarification and special exception.
- Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination.
- As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obliged to do so. If you do not wish to do so, delete this exception statement from your version.
- As such, it can be used to run, create and distribute a large class of applications and applets. When GNU Classpath is used unmodified as the core class library for a virtual machine, compiler for the java language, or for a program written in the java programming language it does not affect the licensing for distributing those programs directly.
The GNU Lesser General Public License
While version 2.1 of the LGPL was a standalone licence, the current LGPL version 3 is based on a reference to the GPL, therefore qualifying as a real GPL linking exception.
Compared to the GNU Classpath license above, the LGPL formulates more requirements to the linking exception in order to guarantee the customers of the final product more freedom.
Specifically, it must be possible and legal to link the program against a newer version of the LGPL-library, and to do private modifications, reverse-engineering and debugging.