GPL linking exception


A GPL linking exception modifies the GNU General Public License in a way that enables software projects which provide library code to be "linked to" the programs that use them, without applying the full terms of the GPL to the using program. Linking is the technical process of connecting code in a library to the using code, to produce a single executable file. It is performed either at compile time or run-time in order to produce functional machine-readable code. There is a public perception, so far unsupported by any legal precedent or citation, that without applying the linking exception, a program linked to GPL library code may only be distributed under a GPL-compatible license. The license of the GNU Classpath project explicitly includes a statement to that effect.
Many free software libraries which are distributed under the GPL use an equivalent exception, although the wording of the exception varies. Notable projects include ERIKA Enterprise, GNU Guile, the run-time libraries of GNAT, GNU Classpath and the famous GCC Runtime Library Exception.
Compiler runtime libraries also often use this license modification or an equivalent one, e.g. the libgcc library in the GNU Compiler Collection, as well as all libraries of the Free Pascal project.
In 2007, Sun Microsystems released most of the source code to the class libraries for the Java SE and Java EE 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 GNU Lesser General Public License is likewise constructed as an 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:
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.
Compared to the GNU Classpath license above, the LGPL formulates more requirements to the linking exception: licensees must allow modification of the portions of the library they use and reverse engineering for debugging such modifications.