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The GNU General Public License (GNU GPL or simply GPL) is a widely used free software license, originally written by Richard Stallman for the GNU project. The GPL is the most popular and well-known example of the type of strong copyleft license that requires derived works to be available under the same copyleft. Under this philosophy, the GPL grants the recipients of a computer program the rights of the free software definition and uses copyleft to ensure the freedoms are preserved, even when the work is changed or added to. This is in distinction to permissive free software licenses, of which the BSD licenses are the standard examples.

The GNU Lesser General Public License (LGPL) is a modified, more permissive, version of the GPL, originally intended for some software libraries. There is also a GNU Free Documentation License, which was originally intended for use with documentation for GNU software, but has also been adopted for other uses, such as the Wikipedia project.

The Affero General Public License (GNU AGPL) is a similar license with a focus on networking server software. The GNU AGPL is similar to the GNU General Public License, except that it additionally covers the use of the software over a computer network, requiring that the complete source code be made available to any network user of the AGPLed work, for example a web application. The Free Software Foundation recommends that this license is considered for any software that will commonly be run over the network.
The GPL was written by Richard Stallman in 1989 for use with programs released as part of the GNU project. The original GPL was based on a unification of similar licenses used for early versions of GNU Emacs, the GNU Debugger and the GNU C Compiler. These licenses contained similar provisions to the modern GPL, but were specific to each program, rendering them incompatible, despite being the same license. Stallman's goal was to produce one license that could be used for any project, thus making it possible for many projects to share code.

As of August 2007, the GPL accounted for nearly 65% of the 43,442 free software projects listed on Freshmeat, and as of January 2006, about 68% of the projects listed on SourceForge.net. Similarly, a 2001 survey of Red Hat Linux 7.1 found that 50% of the source code was licensed under the GPL and a 1997 survey of MetaLab, then the largest free software archive, showed that the GPL accounted for about half of the software licensed there in. One survey of a large repository of open-source software reported that in July 1997, about half the software packages with explicit license terms used the GPL. Prominent free software programs licensed under the GPL include the Linux kernel and the GNU Compiler Collection (GCC). Some other free software programs are dual-licensed under multiple licenses, often with one of the licenses being the GPL.

Some observers believe that the strong copyleft provided by the GPL was crucial to the success of Linux, giving the programmers who contributed to it the confidence that their work would benefit the whole world and remain free, rather than being exploited by software companies that would not have to give anything back to the community.

The second version of the license, version 2, was released in 1991. Over the following 15 years, some members of the FOSS (Free and Open Source Software) community came to believe that some software and hardware vendors were finding loopholes in the GPL, allowing GPL-licensed software to be exploited in ways that were contrary to the intentions of the programmers. These concerns included tivoization (the inclusion of GPL-licensed software in hardware that will refuse to run modified versions of its software); the use of unpublished, modified versions of GPL software behind web interfaces; and patent deals between Microsoft and Linux and Unix distributors that may represent an attempt to use patents as a weapon against competition from Linux.

Version 3 was developed to attempt to address these concerns. It was officially released on June 29, 2007.

From now on any program can be run as service