|Copyleft is a general method for making a program free software and requiring all modified and extended versions of the program to be free software as well.
The simplest way to make a program free is to put it in the public domain, uncopyrighted. This allows people to share the program and their improvements, if they are so minded. But it also allows uncooperative people to convert the program into proprietary software. They can make changes, many or few, and distribute the result as a proprietary product. People who receive the program in that modified form do not have the freedom that the original author gave them; the middleman has stripped it away.
In the GNU project, our aim is to give all users the freedom to redistribute and change GNU software. If middlemen could strip off the freedom, we might have many users, but those users would not have freedom. So instead of putting GNU software in the public domain, we ``copyleft'' it. Copyleft says that anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. Copyleft guarantees that every user has freedom.
Copyleft also provides an incentive for other programmers to add to free software. Important free programs such as the GNU C++ compiler exist only because of this.
Copyleft also helps programmers who want to contribute improvements to free software get permission to do that. These programmers often work for companies or universities that would do almost anything to get more money. A programmer may want to contribute her changes to the community, but her employer may want to turn the changes into a proprietary software product.
When we explain to the employer that it is illegal to distribute the improved version except as free software, the employer usually decides to release it as free software rather than throw it away.
To copyleft a program, we first state that it is copyrighted; then we add distribution terms, which are a legal instrument that gives everyone the rights to use, modify, and redistribute the program's code or any program derived from it but only if the distribution terms are unchanged. Thus, the code and the freedoms become legally inseparable.
Proprietary software developers use copyright to take away the users' freedom; we use copyright to guarantee their freedom. That's why we reverse the name, changing ``copyright'' into ``copyleft.''
Copyleft is a general concept; there are many ways to fill in the details. In the GNU Project, the specific distribution terms that we use are contained in the GNU General Public License (available in HTML, text, and Texinfo format). The GNU General Public License is often called the GNU GPL for short. There is also a Frequently Asked Questions page about the GNU GPL. You can also read about why the FSF gets copyright assignments from contributors.
An alternate form of copyleft, the GNU Lesser General Public License (LGPL) (available in HTML, text, and Texinfo format), applies to a few (but not all) GNU libraries. This license was formerly called the Library GPL, but we changed the name, because the old name encouraged developers to use this license more often than it should be used. For an explanation of why this change was necessary, read the article Why you shouldn't use the Library GPL for your next library.
The GNU Library General Public License is still available in HTML and text format although it has been superseded by the Lesser GPL.
The GNU Free Documentation License (FDL) (available in HTML, text and Texinfo) is a form of copyleft intended for use on a manual, textbook or other document to assure everyone the effective freedom to copy and redistribute it, with or without modifications, either commercially or noncommercially.
The appropriate license is included in many manuals and in each GNU source code distribution.
The GNU GPL is designed so that you can easily apply it to your own program if you are the copyright holder. You don't have to modify the GNU GPL to do this, just add notices to your program which refer properly to the GNU GPL.
If you would like to copyleft your program with the GNU GPL or the GNU LGPL, please see the GPL instructions page for advice. Please note that you must use the entire text of the GPL, if you use it. It is an integral whole, and partial copies are not permitted. (Likewise for the LGPL.)
Using the same distribution terms for many different programs makes it easy to copy code between various different programs. Since they all have the same distribution terms, there is no need to think about whether the terms are compatible. The Lesser GPL includes a provision that lets you alter the distribution terms to the ordinary GPL, so that you can copy code into another program covered by the GPL.
If you would like to copyleft your manual with the GNU FDL, please see the instructions at the end of the FDL text, and the GFDL instructions page. As with the GNU GPL, you must use the entire license; partial copies are not permitted.
From Wikipedia, the free encyclopedia.
Copyleft is the application of copyright law to force derivative works to also be released with a copyleft license. So long as all of those wanting to modify the work accept the terms, the net effect is to facilitate successive improvement by a wide range of contributors. Those who are unwilling or unable to accept the terms are prohibited from creating derivative works.
No restrictions apply to works in the public domain. They may be freely modified, and the creator of the derivative work may license any new portions of the derivative work, but not the public domain portion, under any terms, or none. The resulting derivative work may not be available to the creators of the original or may compete with them.
In copyleft, the copyright holder grants an irrevocable license to the recipient of a copy, generally permitting the free unlimited use, modification and redistribution (often including sale of media or auxiliary materials which may carry a different copyright license (e.g. documentation)) of copies. The distinctive condition to that license is that any modifications to the work, if redistributed, must carry the same permissions (i.e. license terms) and be made available in a form which facilitates modification. For software, this means in source code.
The concept of copyleft arose when Richard Stallman was working on a Lisp interpreter. Symbolics asked to use the Lisp interpreter, and Stallman agreed to supply them with a public domain version of his work. Symbolics extended and improved the Lisp interpreter, but when Stallman wanted access to the improvements that Symbolics had made to his interpreter, Symbolics refused. Stallman then, in 1984, proceeded to create a software license that would prevent this behavior which he named software hoarding. The term "copyleft" came from a message contained in Tiny BASIC, a free distributed version of Basic written by Dr. Wang in the late 1970s. The program listing contained the phrases "All Wrongs reserved" and "CopyLeft."
There are definitional problems with the term "copyleft" which contribute to controversy over it. The term originated as an amusing backformation from the term 'copyright', and was originally a noun, meaning the copyright license terms of the GNU General Public License originated by Richard Stallman as part of the Free Software Foundation's work. Thus, 'your program is covered by the copyleft'. When used as a verb (i.e. 'he copylefted his most recent version'), it is less precise and can refer to any of several similar licenses, or indeed to a notional imaginary license for discussion purposes.
Because of complications caused by use of software library routines, there developed the GNU Library General Public License (subsequently renamed the Lesser GPL), which changes the requirement of further distribution in ways which are compatible with actual library routine use.
Copyleft is one of the key features in free software/open source licences, and is the licenses' legal framework to ensure that derivatives of the licensed work stay free/open. If the licensee fails to distribute derivative works under the same license he will face legal consequences - the license is terminated, leaving the licensee without permission to copy, distribute, display publicly, or prepare derivative works of the software.
Other free software licenses, such as those used by the BSD operating systems, the X Window System and the Apache web server, are not copyleft licenses because they do not require the licensee to distribute derivative works under the same license. There is an ongoing debate as to which class of license provides a larger degree of freedom. This debate hinges on complex issues such as the definition of freedom and whose freedoms are more important. It is sometimes argued that the copyleft licenses attempt to maximize the freedom of all potential recipients in the future (freedom from the creation of proprietary software), while non-copyleft free software licenses maximize the freedom of the initial recipient (freedom to create proprietary software).
An example of a free software license that uses strong copyleft is the GNU General Public License. Free software licenses that use weak copyleft include the GNU Lesser General Public License and the Mozilla Public License. Examples of non-copyleft free software licenses include the Q Public License, the X11 license, and the BSD licenses.
Copyleft licenses for materials other than software include the Creative Commons ShareAlike licenses and the GNU Free Documentation License. The latter is being used for the content of Wikipedia. The Free Art license is a license that can be applied to any work of art.
Copyleft licenses are sometimes called viral copyright licenses , mainy by people who stand to lose much money from them, because any works derived from a copylefted work must themselves be copylefted. The term "viral" implies propagation like that of a biological virus through an entire organ of similar cells or species of similar bodies. In context of legally binding contracts and licenses, "viral" refers to anything, especially anything memetic, that propagates itself by attaching itself to something else, regardless of whether the viral assertions themselves add value to the individual work. The viral metaphor is over-used but is reasonable to help distinguish between free software and open source in software and documentation projects. Most advocates of copyleft argue that the analogy between copyleft and computer viruses does not apply. As they point out, computer viruses generally infect computers without the awareness of the user, whereas the copyleft actually grants the user certain permissions to distribute modified programs, which is not allowed under copyright law without permission of the copyright holder. Most proprietary software licenses do not allow such distribution. Furthermore, copyright itself is "viral" in this sense, since any works derived from a copyrighted work must have permission from and obey any conditions set by the original copyright holder.
The view that copyleft licenses are viral is supported by Microsoft, who say that if a product uses GPLed code, that product automatically escapes the creator's control and becomes GPLed, leaving the creator no recourse. Obviously, working for a software company will have a like effect. Advocates, including Eben Moglen, Professor of Law at Columbia University and counsel for the Free Software Foundation, note that this is not true since the GPL is a license, not a contract.
Microsoft, and others, in describing the GPL as a "viral license", may also be referring to the idea that any release of something new under the GPL would seem to create a positive feedback network effect, in which over time there will be an ever-expanding amount of copylefted code. Code reuse is of course tempting, as a way to save effort and get on with a project, especially when a perfectly sensible design and implementation has already been done and is available. In contrast, those working on non-copylefted programs will have to "reinvent the wheel" for their own programs.
Copyleft licenses are desirable and popular for shared works precisely because they are viral, and apply to all derivative works, which are thus "infected" by the requirement to re-integrate changes deemed desirable by any party down the line. This guarantee is important because it ensures uniform license terms and free access, and makes copyleft projects resistant to unnecessary forking because all maintainers, of the original work or other versions, may use any modifications released by anyone. Useful changes tend to be merged, and different versions are maintained only to the extent that they are useful. Without the "viral" license, variant terms can apply to the forks, derivative works can be controlled commercially by the parties that extend or translate them, and the project would degrade to an open source one. It is thought that Linux has not suffered the same fragmentation as Unix because it is copylefted.
Copyleft-like ideas are increasingly being suggested for patents, such as open patent pools that allow royalty-free use of patents contributed to the pool under certain conditions (such as surrendering the right to apply for new patents that are not contributed to the pool).
Copyleft is also starting to inspire the arts with movements like the Libre Society and open-source record labels emerging.
Text from gnu (General Public Licence) and wikipedia Page of