Free software licence

From Wikipedia, the free encyclopedia

A free software licence is a software licence which grants recipients rights to modify and redistribute the software which would otherwise be prohibited by copyright law. A free software licence grants, to the recipients, freedoms in the form of permissions to modify or distribute copyrighted work.

[edit] History

There is no recognised "first" free software licence. Early licences include that of TeX and that of X11.

In the mid-80s, the GNU project produced individual free software licences for each of its software packages. These were all replaced in 1989 with version 1 of the GNU General Public License (GPL). Version 2 of the GPL, released in 1991, went on to become the most widely used free software licence.

In the mid- to late-90s, a trend began where companies and new projects wrote new licences. This licence proliferation led to problems of complexity and licence compatibility.^{[citation needed]} The trend slowed, and reversed in some ways, in the early 2000s.^{[citation needed]}

[edit] FSF-approved free software licences

Free Software Foundation, the group that maintains The Free Software Definition, maintains a list of free software licences.^[1] The list distinguishes between free software licences that are compatible or incompatible with the FSF licence of choice, the GNU General Public License, which is a copyleft licence. The list also contains licences which the FSF considers non-free for various reasons.

For more details on this topic, see List of FSF approved software licences.

[edit] OSI-approved "open source" licences

Another group, Open Source Initiative (OSI), also maintains a list of approved licences. OSI and FSF agree on all widely used free software licences. OSI's list is different from FSF's list because the two organisations have reviewed different sets of licences. There are a few licences that OSI have approved that the FSF has not, and vice versa, but these are licences that are used by niche projects or none at all.

For more details on this topic, see List of OSI approved software licences.

[edit] Restrictions

In order to preserve the freedom to use, study, modify, and redistribute free software, most free software licences carry requirements and restrictions which apply to distributors. There exists an ongoing debate within the free software community regarding the fine line between restrictions which preserve freedom and those which reduce it.

During the 1990s, free software licences began including clauses, such as patent retaliation, in order to protect against software patent litigation cases which had not previously existed. This new threat became the primary purpose for composing the new version 3 of the GNU GPL.^[2] In the decade 2000, tivoisation has emerged as yet another new threat^{[citation needed]} which some current free software licences do not protect users from.

[edit] Copyleft

Main article: copyleft

Since the mid 1980s, free software licences written by Richard Stallman pioneered a concept known as copyleft. Ensuing copyleft provisions stated that when modified versions of free software are distributed, they must be distributed under the same terms as the original software. Thus, all enhancements and additions to copylefted software must also be distributed as free software. This is sometimes referred to as "share and share alike" or "quid pro quo".

[edit] Patent retaliation

Main article: Patent retaliation

Most newly written free software licences since the late 1990s include some form of patent retaliation clauses. These measures ensure that one's rights under the licence (such as to redistribution), may be terminated if one attempts to enforce specific patent monopolies, under noted circumstances, as specified in the licence. As an example, the Apple Public Source License may terminate a user's rights if said user embarks on litigation proceedings against them due to patent litigation. Patent retaliation emerged in response to proliferation and abuse of software patents.

[edit] Tivoisation

For more details on this topic, see Tivoisation.

As of late 2006, no free software licences contain explicit language prohibiting additional restrictions being enforced by digital rights management (DRM). Version 3 of the GNU GPL does, however, include such specific language in order to prohibit "Tivoisation" in certain cases.

[edit] Attribution, disclaimers and notices

The majority of free software licences require that modified software not claim to be unmodified. Some licences also require that copyright holders be credited. One such example is version 2 GNU GPL, which requires that interactive programs that print warranty or licence information, may not have these notices removed from modified versions intended for distribution.

[edit] Practical problems that licences try to avoid

[edit] Licence compatibility

Main article: Licence compatibility

Licences of software packages containing contradictory requirements, render it impossible to combine source code from such packages in order to create new software packages.^[3]

For example, if one licence says "modified versions must mention the developers in any advertising materials", and another licence says "modified versions cannot contain additional attribution requirements", then, if someone combined a software package which uses one licence with a software package which uses the other, it would be impossible to distribute the combination because the two requirements cannot be simultaneously fulfilled. Thus, these two packages would be licence-incompatible.^[4]

[edit] Licence proliferation

Main article: Licence proliferation

Licence proliferation compounds the problems of licence incompatibility. It likewise burdens software developers and distributors by increasing the amount of legal documents they must read. Licence proliferation gained momentum during the late 1990s and increased into the early 2000s. By the year 2005, it was being identified as a problematic phenomenon and the gratuitous writing of new licences became more frowned upon.

[edit] Unacceptable restrictions

[edit] Purpose of use

Restrictions on private use of the software ("use restrictions") are generally unacceptable. Examples include prohibiting the software to be used for military purposes, for comparison or benchmarking, for ethically-questionable means,^[5] or in commercial organisations.^[6] For this reason, such licences are not considered free software by the standards of the FSF, OSI, Debian, or the BSD-based distributions.

The FSF's free software definition further states that development and distribution must not be restricted^[7]. Thus, commercial distribution of free software is acceptable and has become common.

[edit] The Permissive versus Copyleft controversy

Many users and developers of BSD-based operating systems have a different position on licensing. The main difference is the belief that the copyleft licences, particularly the GNU General Public License (GPL), are too complicated and have restrictions which are undesirable. The GPL requires any derivative work that is released to be released according to the GPL while the BSD licence does not. Essentially, the BSD licence's only requirement is to acknowledge the original authors, and poses no restrictions on how the source code may be used. As a result, BSD code can find its way into proprietary software that only acknowledge the authors. For instance, the IP stack in Microsoft Windows and Mac OS X are derived from BSD-licensed software.

GPL supporters claim that mandating derivative works remain free fosters the growth of free software and requires equal participation by all users. Developers who use GPL code in their product must make the source code available to anyone, but only when they share or sell the Object code. In this case, the source code must also contain any changes the developers may have made. If GPL code is used but not shared or sold, the code is not required to be made available and any changes may remain private. This permits developers and organizations to use and modify GPL code for private purposes without being required to make their changes available to the public.

Supporters of the BSD licence argue that it is more free than the GPL because it grants the right to do anything with the source code, second only to software in the public domain. The nature of BSD has encouraged the inclusion of well-developed standard code into proprietary software. In response, GPL supporters claim that this is more a form of power than a necessary freedom.^[8] The right to make closed-source code is therefore not included in the Free Software Foundation's "four freedoms of free software": using, studying, copying, and distributing modifications of the code.

Code licensed under a permissive free software licence, such as the BSD licence, can be incorporated into copylefted (e.g. GPL'd) projects. Such code is thus "GPL-compatible". There is no need to securing the consent of the original authors. In contrast, code under the GPL cannot be relicensed under the BSD licence without securing the consent of all copyright holders. Thus the two licences are compatible, but the combination as a whole must be distributed under the terms of the GPL, not the permissive licence.

Existing free software BSDs tend to avoid including software licensed under the GPL in the core operating system, or the base system, except as a last resort when alternatives are non-existent or vastly less capable, such as with GCC. The OpenBSD project has acted to remove GPL-licensed tools in favour of BSD-licensed alternatives, some newly written and some adapted from older code.

[edit] Debian

The Debian project uses the criteria laid out in its Debian Free Software Guidelines (DFSG). The only notable cases where Debian and Free Software Foundation disagree are over the Artistic License and the GNU Free Documentation License. Debian accepts the original Artistic License as being a free software licence, but FSF disagrees. This has very little impact however since the Artistic License is almost always used in a dual-licence setup, along with the GNU General Public License.

Regarding the GNU Free Documentation License, Debian decided to apply the DFSG to all content, not only software, and so documentation licences must also be examined against these guidelines. FSF say that documentation is qualitatively different from software and is subject to different requirements. The end result of a long discussion and the eventual vote in Debian ^[9] is that the works licensed under the GFDL are considered free as long as they do not contain unmodifiable sections (what the GFDL calls "Invariant Sections").

[edit] Controversial borderline cases

The vast majority of free software uses licences which are undisputedly free software licences, however there have been many debates over whether or not certain other licences are free software licences.

Examples of licences which provoked debate include the 1.x series of the Apple Public Source License, which were accepted by the Open Source Initiative but not by the Free Software Foundation or Debian, the RealNetworks Public Source License, which was accepted by Open Source Initiative and Free Software Foundation but not by Debian, and in 2007, the Common Public Attribution License, which was approved by Open Source Initiative only.^[10]