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 latest version of the license, version 3, was released in July, 2006. The GNU Lesser General Public License (LGPL) is a modified version of the GPL, intended for some software libraries.
The GPL grants the recipients of a computer program the following rights:
- the right to run the program, for any desired purpose.
- the right to study how the program works, and modify it. (Access to the source code is a precondition for this)
- the right to redistribute copies.
- the right to improve the program, and release the improvements to the public. (Access to the source code is a precondition for this)
In contrast, the end-user licenses that come with proprietary software generally only grant the end-user the right to copy the software onto a limited number of computers. The terms and conditions of such license agreements may even attempt to restrict activities normally permitted by copyright laws, such as reverse engineering.
The primary difference between the GPL and more "permissive" free software licenses such as the BSD License is that the GPL seeks to ensure that the above points are preserved in copies and in derivative works. It does this using a legal mechanism known as copyleft, invented by Stallman, which requires derivative works of GPL-licensed programs to also be licensed under the GPL. In contrast, BSD-style licenses allow for derivative works to be redistributed as proprietary software.
By some measures, the GPL is the single most popular license for free and open source software. As of 2004, the GPL accounted for nearly 75% of the 23,479 free-software projects listed on Freshmeat, and about 68% of the projects listed on SourceForge. (These sites are owned by OSTG, a company that advocates Linux and the GPL.) Similarly, a 2001 survey of Red Hat Linux 7.1 found that 50% of the source code was licensed under the GPL, and 1997 survey of Metalab, then the largest free-software archive, showed that the GPL accounted for about half of the licenses used. Prominent free software programs licensed under the GPL include the Linux kernel and the GNU Compiler Collection (GCC). Some other prominent free software programs are licensed under multiple licenses, one of which is the GPL; Perl is a well-known example.
The GPL was written by Richard Stallman for use with programs released as part of the GNU project. It was based on a unification of similar licenses used for early versions of [GNU Emacs, the GNU Debugger and the GNU Compiler Collection. These licenses contained similar provisions to the modern GPL, but were specific to each program rendering them incompatible, despite being the same licence. Direct link to the section about the prehistory of the GPL. 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. This became the GPL version 1, released in January 1989.
By 1990, it was becoming apparent that a less restrictive license would be strategically useful for some software libraries; when version 2 of the GPL was released in June 1991, therefore, a second license - the Library General Public License, or LGPL - was introduced alongside it, and was also numbered version 2 to show that the two were complementary. The version numbers diverged in 1999 when version 2.1 of the LGPL was released, which renamed it the Lesser General Public License to reflect its place in the GNU philosophy.
According to Richard Stallman, the biggest change in GPLv2 was the Liberty or Death clause, as he calls it - Section 7. This section says that if someone has restrictions imposed that prevent them from distributing GPL-covered software in a way that respects other users' freedom (for example, if a legal ruling states that they can only distribute the software in binary form), they cannot distribute it at all.
On February 25, 2006, he said:
- Among the changes, the most important four, I will say, concern dealing with software patents, compatibility with other licences, the definition of which parts of the source code and what constitutes the source code that must be included in it, and dealing with Digital Restrictions Management.
Some important changes:
- Clauses against patents
- Compatibility with different licences
- Clauses against DRM
- Clauses against "tivoization"
- Additional restrictions
In January 2006, the Free Software Foundation began a 12-month public consultation about the possible changes to the GPL. This process is being coordinated by the Free Software Foundation, Software Freedom Law Center, and Free Software Foundation Europe.
Richard Stallman expects GPLv3 to be finalised either in October 2006 or early 2007.
The following is a colloquial summary of the terms of the GPL. The only legally precise description, however, is that of the actual text of the GPL, which is available through an external link at the end of this article.
Granting of rights
The terms and conditions of the GPL are available to anybody receiving a copy of the GPLed work ("the licensee"). Any licensee who adheres to the terms and conditions is given permission to modify the work, as well as to copy and redistribute the work or any derivative version. The licensee is allowed to charge a fee for this service, or do this free of charge. This latter point distinguishes the GPL from software licenses that prohibit commercial redistribution. Stallman has argued that free software should not place restrictions on commercial use, and the GPL explicitly states that GPLed works may be (re)sold.
The GPL additionally states that a distributor may not impose "further restrictions on the rights granted by the GPL". This forbids e.g. the distribution of the software under a non-disclosure agreement or contract. Distributors under the GPL also grant a license for any of their patents practiced by the software, to practice those patents in GPL software.
Section three of the license requires that programmes distributed as pre-compiled binaries are accompanied by a copy of the source code, a written offer to distribute the source code via the same mechanism as the pre-compiled binary or the written offer to obtain the source code that you got when you received the pre-compiled binary under the GPL.
The GPL does not give the licensee unlimited redistribution rights. The right to redistribute is granted only if the licensee includes the source code (or a legally-binding offer to provide the source code), including any modifications made. Furthermore, the distributed copies, including the modifications, must also be licensed under the terms of the GPL.
This requirement is known as copyleft, and it gets its legal teeth from the fact that the program is copyrighted. Because it is copyrighted, a licensee has no right to modify or redistribute it (barring fair use), except under the terms of the copyleft. One is only required to adhere to the terms of the GPL if one wishes to exercise rights normally restricted by copyright law, such as redistribution. Conversely, if one distributes copies of the work without abiding by the terms of the GPL (for instance, by keeping the source code secret), they can be sued by the original author under copyright law.
The copyleft thus uses copyright law to accomplish the opposite of its usual purpose: instead of imposing restrictions, it grants rights to other people, in a way that ensures the rights cannot subsequently be taken away. This is the reason the GPL has been described as a "copyright hack". It also ensures that unlimited redistribution rights are not granted, should any legal flaw (or "computer bug") be found in the copyleft statement.
Many distributors of GPLed programs bundle the source code with the executables. An alternative method of satisfying the copyleft is to provide a written offer to provide the source code on a physical medium (such as a CD) upon request. In practice, many GPLed programs are distributed over the Internet, and the source code is made available over FTP. For Internet distribution, this complies with the license.
The copyleft only applies when a person seeks to redistribute the program. One is allowed to make private modified versions, without any obligation to divulge the modifications as long as the modified software is not distributed to anyone else. Note that the copyleft only applies to the software and not to its output (unless that output is itself a derivative work of the program); for example, a web portal running a modified GPL content management system is not required to distribute its changes to the underlying software. (It has been suggested that this be changed for version 3 of the GPL.)
The GPL is a license
The GPL was designed as a license, rather than a contract. In some Common Law jurisdictions, the legal distinction between a license and a contract is an important one: contracts are enforceable by contract law, whereas the GPL, as a license, is enforced under the terms of copyright law. However, this distinction is not useful in the many jurisdictions where there are no differences between contracts and licences, such as Civil Law systems.
The way the GPL license works is simple: if you do not abide by the GPL's terms and conditions, then you do not have permission, under copyright law, to copy or distribute GPL licensed software or derivative works. It does not mean that the rules of the GPL do not apply to you and that you may use the software however you like. By default, the restrictions of copyright law are in effect, not the anarchy of the public domain.
The text of the GPL is itself copyrighted, and the copyright is held by the Free Software Foundation (FSF). However, the FSF does not hold the copyright for a work released under the GPL, unless an author explicitly assigns copyrights to the FSF (which seldom happens except for programs that are part of the GNU project). Only the individual copyright holders have the authority to sue when a license violation takes place.
The FSF permits people to create new licenses based on the GPL, as long as the derived licenses do not use the GPL preamble without permission. This is discouraged, however, since such a license is generally incompatible with the GPL. (See the GPL FAQ for more information.)
Other licenses created by the GNU project include the GNU Lesser General Public License and the GNU Free Documentation License.
- GNU General Public License v2.0
- GNU General Public License v1.0 - This version is deprecated
- GNU Lesser General Public License v2.1
- Frequently Asked Questions about the GPL
- GPLMedicine.org an advocacy site for the GPL in health-related software
- History of the GPL
- Groklaw: The GPL is a license, not a contract
- Groklaw: FSF Moves to Dismiss Wallace and for Stay on Filing Briefs on Summary Judgment Motion, June 22 2005
- Enforcing the GNU GPL by Eben Moglen, September 10, 2001
- Groklaw GPL References
- GNU General Public License and Commentaries - Edited by Robert Chassell.
- Make Your Open Source Software GPL-Compatible. Or Else. (David A. Wheeler, 7 April 2004) — why a GPL-compatible license is important to the health of a project
- "Toward True Open Source" - an article about why the GPL is allegedly too restrictive
- The German GPL Order - Translated
- Patent risks of open source software - explains the patent license grant in the GPL
- "Ballmer: 'Linux is a cancer'" by Thomas C Greene, The Register, June 2 2001
- NOVELL: The GPL: Understanding the License that Governs Linux - This article explains one view of the GPL in easy terms, talks about static vs. dynamic linking and the GPL, and discusses why companies like Microsoft may "fear" the license.
- SOFTPANORAMA: Labyrinth of Software Freedom - Nikolai Bezroukov's e-book about BSD, GPL and social aspects of free licensing
- Viral Contracts or Unenforceable Documents? Contractual Validity of Copyleft Licenses - Article looking at the GPL from an European perspective.
- GPL-Violations.org - website monitoring the net for GPL violations
- Can Technical Tricks Circumvent the GPL?, by Richard Stallman
- A pacifist version of the GPL? from Ars Technica
GPLv3 and the public consultation
- A transcript of FSF's January 16th, 2006, presentation of the changes they propose from GPLv2 to v3
- A transcript of Richard Stallman speaking in Brussels on February 25th, 2006, about version three of the GPL
- A transcript of presentation by Richard Stallman about the proposed changes to the GPL for version three, Turin, Italy, March 18th 2006
- Information about the public process for drafting version three of the GPL, from FSF-Europe
- GPLv3 presentation by Richard Stallman, April 21st 2006
- A law review article arguing that GPLv3's changes will compromise its enforceability
- Audio and video recordings of a 2-day GPLv3 event, Barcelona, June 22nd 2006