CPSC225/Notes/GPL Public License

The General Public License or GPL: The license is mainly the brainchild of Richard Stalman.. He wanted to stop software hoarding. Stallman had had a problem with software hoarding. The GPL allows everything a public domain license allows, except that derivative works must also be under the GPL. For example, if Person A makes Software B and releases it under the GPL, and later Person C changes Software B to make Software D, then Software D also has to be under the GPL.

Basic Terms and Conditions
The terms and conditions of the GPL are available to anybody receiving a copy of the work that has a GPL applied to it ("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. The FSF argues that free software should not place restrictions on commercial use, and the GPL explicitly states that GPL works may be sold at any price.

Another key aspect of the GPL is that the source code must be provided to the client upon request or in the form that the original source was distributed.

Specifics
The primary characteristics of the GPL are: 1. Any derivative work—that is, any work containing a nontrivial amount of GPLed code—must itself be distributed under the GPL. 2.No additional restrictions may be placed on the redistribution of either the original work or a derivative work. (The exact language is: "You may not impose any further restrictions on the recipients' exercise of the rights granted herein.")

Compatible Licenses

Incompatible Licenses

Terms and conditions for modifying and releasing GPL software

CopyLeft
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 "bug") be found in the copyleft statement.

It is important to note...he 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 public web portal running a modified derivative of a GPLed content management system is not required to distribute its changes to the underlying software.

Popularity
According to Freshmeat open source community nearly 70% of open source projects are licensed under the GPL.

Tagging the GPL
The General Public License copyright line is as follows:

one line to give the program's name and an idea of what it does. Copyright (C) yyyy name of author This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., etc

Legality, Controversy and Alternatives
It has been alleged that the GPL is not only impossible to uphold in court, but also unconstitutional. The grounds for this controversy stem around that the GPL permits licensees to make unlimited copies of programs it covers, while copyright law only allows a single copy to be made.However, most of these claims have proven unfounded. 

The GPL has been described as being "viral" by many of its critics, because the GPL terms require that all derived works must in turn be licensed under the GPL. This is part of a philosophical difference between the GPL and permissive free software licenses such as the BSD-style licenses, which put fewer restrictions on derived works. While proponents of the GPL believe that free software should ensure that its freedoms are preserved in derivative works, others believe that free software should give its users the maximum freedom to redistribute it as they wish.

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 freedoms of the free software definition 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.