Editing Free software (section)

== Licensing ==

{{Main|Free-software license}}
{{Further|Open-source license}}
{{See also|Free and open-source software#Licensing}}

[[File:Copyleft.svg|thumb|[[Copyleft]], a novel use of copyright law to ensure that works remain unrestricted, originates in the world of free software.<ref>{{Cite journal|last=Carver|first=Brian W.|date=2005-04-05|title=Share and Share Alike: Understanding and Enforcing Open Source and Free Software Licenses|journal=Berkeley Technology Law Journal|volume=20|pages=39|ssrn=1586574}}</ref>]]

All free-software licenses must grant users all the freedoms discussed above. However, unless the applications' licenses are compatible, combining programs by mixing source code or directly linking binaries is problematic, because of [[License compatibility|license technicalities]]. Programs indirectly connected together may avoid this problem.

The majority of free software falls under a small set of licenses. The most popular of these licenses are:<ref>{{cite web |url=http://www.blackducksoftware.com/resources/data/top-20-licenses |quote=1. MIT license 24%, 2. GNU General Public License (GPL) 2.0 23%, 3. Apache License 16%, 4. GNU General Public License (GPL) 3.0 9%, 5. BSD License 2.0 (3-clause, New or Revised) License 6%, 6. GNU Lesser General Public License (LGPL) 2.1 5%, 7. Artistic License (Perl) 4%, 8. GNU Lesser General Public License (LGPL) 3.0 2%, 9. Microsoft Public License 2%, 10. Eclipse Public License (EPL) 2% |title=Top 20 licenses |publisher=Black Duck Software |access-date=19 November 2015 |date=19 November 2015 |archive-url=https://web.archive.org/web/20160719043600/http://www.blackducksoftware.com/resources/data/top-20-licenses |archive-date=19 July 2016 |url-status=dead}}</ref><ref>{{cite web|url=https://github.com/blog/1964-license-usage-on-github-com |quote="1 MIT 44.69%, 2 Other 15.68%, 3 GPLv2 12.96%, 4 Apache 11.19%, 5 GPLv3 8.88%, 6 BSD 3-clause 4.53%, 7 Unlicense 1.87%, 8 BSD 2-clause 1.70%, 9 LGPLv3 1.30%, 10 AGPLv3 1.05% |title=Open source license usage on GitHub.com |date=2015-03-09 |first=Ben |last=Balter |access-date=2015-11-21 |publisher=[[github.com]]}}</ref>
* The [[MIT License]]
* The [[GNU General Public License|GNU General Public License v2]] (GPLv2)
* The [[Apache License]]
* The [[GNU General Public License|GNU General Public License v3]] (GPLv3)
* The [[BSD licenses|BSD License]]
* The [[GNU Lesser General Public License]] (LGPL)
* The [[Mozilla Public License]] (MPL)
* The [[Eclipse Public License]]

The Free Software Foundation and the Open Source Initiative both publish lists of licenses that they find to comply with their own definitions of free software and open-source software respectively:
* [[Comparison of free and open-source software licenses|List of FSF approved software licenses]]
* [[Open-source license|List of OSI approved software licenses]]

The FSF list is not prescriptive: free-software licenses can exist that the FSF has not heard about, or considered important enough to write about. So it is possible for a license to be free and not in the FSF list. The OSI list only lists licenses that have been submitted, considered and approved. All open-source licenses must meet the [[Open Source Definition]] in order to be officially recognized as open source software. Free software, on the other hand, is a more informal classification that does not rely on official recognition. Nevertheless, software licensed under licenses that do not meet the Free Software Definition cannot rightly be considered free software.

Apart from these two organizations, the [[Debian]] project is seen by some to provide useful advice on whether particular licenses comply with their [[Debian Free Software Guidelines]]. Debian does not publish a list of {{em|approved}} licenses, so its judgments have to be tracked by checking what software they have allowed into their software archives. That is summarized at the Debian web site.<ref name="Debian" />

It is rare that a license announced as being in-compliance with the FSF guidelines does not also meet the [[Open Source Definition]], although the reverse is not necessarily true (for example, the [[NASA Open Source Agreement]] is an OSI-approved license, but non-free according to FSF).

There are different categories of free software.
* [[Public domain|Public-domain]] software: the copyright has expired, the work was not copyrighted (released without [[copyright notice]] before 1988), or the author has released the software onto the public domain with a [[waiver]] statement (in countries where this is possible). Since public-domain software lacks copyright protection, it may be freely incorporated into any work, whether proprietary or free. The FSF recommends the [[Creative Commons license#Zero / public domain|CC0]] public domain dedication for this purpose.<ref>{{cite web|title=Various Licenses and Comments about Them|url=https://www.gnu.org/licenses/license-list.html#CC0|website=GNU Operating System|date=12 January 2022}}</ref>
* [[permissive free software licence|Permissive licenses]], also called BSD-style because they are applied to much of the software distributed with the [[Berkeley Software Distribution|BSD]] operating systems. The author retains copyright solely to disclaim warranty and require proper attribution of modified works, and permits redistribution and {{em|any}} modification, even closed-source ones.
* [[Copyleft]] licenses, with the [[GNU General Public License]] being the most prominent: the author retains copyright and permits redistribution under the restriction that all such redistribution is licensed under the same license. Additions and modifications by others must also be licensed under the same "copyleft" license whenever they are distributed with part of the original licensed product. This is also known as a [[viral license|''viral'']], ''[[protective license|protective]]'', or ''reciprocal'' license.

Proponents of permissive and copyleft licenses disagree on whether software freedom should be viewed as a [[negative liberty|negative]] or [[positive liberty]]. Due to their restrictions on distribution, not everyone considers copyleft licenses to be free.<ref>{{Cite web|url=https://www.charvolant.org/doug/gpl/html/gpl.html|title=Why Not Use the GPL? Thoughts on Free and Open-Source Software|last=Palmer|first=Doug|date=2003-02-15|website=www.charvolant.org|url-status=live|archive-url=https://web.archive.org/web/20200124140248/https://www.charvolant.org/doug/gpl/html/gpl.html|archive-date=2020-01-24|access-date=2020-01-24}}</ref> Conversely, a permissive license may provide an incentive to create non-free software by reducing the cost of developing restricted software. Since this is incompatible with the spirit of software freedom, many people consider permissive licenses to be less free than copyleft licenses.<ref>{{cite web |url=https://www.gnu.org/licenses/bsd.en.html |title=The BSD License Problem |last=Stallman |first=Richard |authorlink=Richard Stallman |publisher=[[Free Software Foundation]] |date=2021-12-25 |accessdate=2024-03-29 }}</ref>