Fork (software development): Difference between revisions

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{{Use dmy dates|date=December 2022}}

[[File:Linux Distribution Timeline.svg|thumb|upright|A timeline chart showing the evolution of [[Linux distribution]]s, with each split in the diagram being called "a fork"]]

In [[software engineering]], a '''project fork''' happens when developers take a copy of [[source code]] from one [[Computer software|software package]] and start independent development on it, creating a distinct and separate piece of software. The term often implies not merely a [[branching (revision control)|development branch]], but also a split in the developer community; as such, it is a form of [[schism]].<ref>"Schism", with its connotations, is a common usage, ''e.g.''
* [http://www.jwz.org/doc/lemacs.html "the Lemacs/FSFmacs schism"] {{Webarchive|url=https://web.archive.org/web/20091130093142/http://www.jwz.org/doc/lemacs.html |date=30 November 2009 }} ([[Jamie Zawinski]], 2000),
* [https://lwn.net/Articles/419822/ "Behind the KOffice split"] {{webarchive|url=https://web.archive.org/web/20130706094238/http://lwn.net/Articles/419822/ |date=6 July 2013 }} (Joe Brockmeier, ''Linux Weekly News'', 2010-12-14),
* [http://www.h-online.com/open/features/Copyright-assignment-Once-bitten-twice-shy-1049631.html "Copyright assignment – once bitten, twice shy"] {{webarchive|url=https://web.archive.org/web/20120330153250/http://www.h-online.com/open/features/Copyright-assignment-Once-bitten-twice-shy-1049631.html |date=30 March 2012 }} (Richard Hillesley, ''H-Online'', 2010-08-06),
* [http://dashes.com/anil/2010/09/forking-is-a-feature.html "Forking is a feature"] {{webarchive|url=https://web.archive.org/web/20120229032536/http://dashes.com/anil/2010/09/forking-is-a-feature.html |date=29 February 2012 }} ([[Anil Dash]], 2010-09-10),
* [http://www.linuxjournal.com/node/1000101 "The Great Software Schism"] {{webarchive|url=https://web.archive.org/web/20120106065841/http://www.linuxjournal.com/node/1000101 |date=6 January 2012 }} ([[Glyn Moody]], ''Linux Journal'', 2006-09-28),
* [http://mako.cc/writing/to_fork_or_not_to_fork.html "To Fork Or Not To Fork: Lessons From Ubuntu and Debian"] {{webarchive|url=https://web.archive.org/web/20120226160810/http://mako.cc/writing/to_fork_or_not_to_fork.html |date=26 February 2012 }} ([[Benjamin Mako Hill]], 2005).</ref> Grounds for forking are varying user preferences and stagnated or discontinued development of the original software.

[[Free and open-source software]] is that which, by definition, may be forked from the original development team without prior permission, and without violating [[copyright]] law. However, licensed forks of proprietary software (''e.g.'' [[Unix]]) also happen.

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The word "fork" has been used to mean "to divide in branches, go separate ways" as early as the 14th century.<ref>[http://www.etymonline.com/index.php?term=fork Entry 'fork' in Online Etymology Dictionary] {{webarchive|url=https://web.archive.org/web/20120525165727/http://www.etymonline.com/index.php?term=fork |date=25 May 2012 }}</ref> In the software environment, the word evokes the [[Fork (system call)|fork]] system call, which causes a running process to split itself into two (almost) identical copies that (typically) diverge to perform different tasks.<ref>"The term fork is derived from the POSIX standard for operating systems: the system call used so that a process generates a copy of itself is called fork()." {{cite conference|url=http://flosshub.org/sites/flosshub.org/files/paper_0.pdf|title=A Comprehensive Study of Software Forks: Dates, Reasons and Outcomes|first1=Gregorio|last1=Robles|first2=Jesús M.|last2=González-Barahona|conference=OSS 2012 The Eighth International Conference on Open Source Systems|year=2012|access-date=20 October 2012|url-status=live|archive-url=https://web.archive.org/web/20131202221721/http://flosshub.org/sites/flosshub.org/files/paper_0.pdf|archive-date=2 December 2013|doi=10.1007/978-3-642-33442-9_1|doi-access=free}}</ref>

In the context of software development, "fork" was used in the sense of creating a revision control "[[branching (revision control)|branch]]" by [[Eric Allman]] as early as 1980, in the context of [[Source Code Control System|SCCS]]:<ref>Allman, Eric. [http://sccs.sourceforge.net/man/sccs.me.html "An Introduction to the Source Code Control System."] {{webarchive|url=https://web.archive.org/web/20141106144859/http://sccs.sourceforge.net/man/sccs.me.html |date=6 November 2014 }} Project Ingres, University of California at Berkeley, 1980.</ref>

{{quotation|Creating a branch "forks off" a version of the program.}}

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The term was in use on [[Usenet]] by 1983 for the process of creating a subgroup to move topics of discussion to.<ref>[https://groups.google.com/group/net.misc/browse_thread/thread/b0e9f8531558b7e9/1cc726d9e9e05ebd?q=fork#1cc726d9e9e05ebd Can somebody fork off a "net.philosophy"?] ([[John Gilmore (activist)|John Gilmore]], net.misc, 18 January 1983)</ref>

"Fork" is not known to have been used in the sense of a community schism during the origins of Lucid Emacs (now [[XEmacs]]) (1991) or the [[Berkeley Software Distribution|BSDsBerkeley Software Distributions]] (BSDs) (1993–1994); [[Russ Nelson]] used the term "shattering" for this sort of fork in 1993, attributing it to [[John Gilmore (activist)|John Gilmore]].<ref>[https://groups.google.com/group/gnu.misc.discuss/browse_thread/thread/1bba70b8f8676c43/309504a5a51dd0f0 Shattering&nbsp;— good or bad?] (Russell Nelson, gnu.misc.discuss, 1 October 1993)</ref> However, "fork" was in use in the present sense by 1995 to describe the XEmacs split,<ref>[https://groups.google.com/group/cu.cs.macl.info/browse_thread/thread/ed9e5ff9cb21c359/4ce930d2fd1271eb?q=%22fork+of%22+xemacs#4ce930d2fd1271eb Re: Hey Franz: 32K Windows SUCK!!!!!] (Bill Dubuque, cu.cs.macl.info, 21 September 1995)</ref> and was an understood usage in the [[GNU]] Project by 1996.<ref>[https://groups.google.com/group/gnu.misc.discuss/browse_thread/thread/5d7529d865d4d9ca/69ebc4771f32bc45?q=fork+xemacs&utoken=jCqifzMAAABlYGNE0Z8-NzB8d_a6gkmMuNtzuV4zLfLK8bEgnt4z8g27cB9GA0FI9e6hHUM__C_J8dUPGZeQNP0WkVA49NN0 Lignux?] (Marcus G. Daniels, gnu.misc.discuss, 7 June 1996)</ref>

==Forking of free and open-source software==

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{{quotation|3. Derived Works: The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.|[[The Open Source Definition]]<ref>{{cite web|url=http://opensource.org/docs/osd|title=The Open Source Definition|date=7 July 2006 |publisher=The Open Source Initiative|access-date=15 October 2013|url-status=live|archive-url=https://web.archive.org/web/20131015144021/http://opensource.org/docs/osd|archive-date=15 October 2013}}</ref>}}

In free software, forks often result from a schism over different goals or personality clashes. In a fork, both parties assume nearly identical code bases, but typically only the larger group, or whoever controls the Webweb site, will retain the full original name and the associated user community. Thus, there is a reputation penalty associated with forking.<ref name=wheeler/> The relationship between the different teams can be cordial or very bitter. On the other hand, a ''friendly fork'' or a ''soft fork'' is a fork that does not intend to compete, but wants to eventually merge with the original.

[[Eric S. Raymond]], in his essay ''[[Homesteading the Noosphere]]'',<ref>{{cite web |last=Raymond |first=Eric S. |author-link=Eric S. Raymond |date=15 August 2002 |title=Promiscuous Theory, Puritan Practice |url=http://www.catb.org/~esr/writings/cathedral-bazaar/homesteading/ar01s03.html|title=Promiscuous Theory, Puritan Practice|date=15 August 2002|author-link=Eric S. Raymond|first=Eric S.|last=Raymond|url-status=live |archive-url=https://web.archive.org/web/20061006010031/http://www.catb.org/~esr/writings/cathedral-bazaar/homesteading/ar01s03.html |archive-date=6 October 2006 |website=catb.org}}</ref> stated that "The most important characteristic of a fork is that it spawns competing projects that cannot later exchange code, splitting the potential developer community". He notes in the [[Jargon File]]:<ref>[http://catb.org/jargon/html/F/forked.html Forked] {{webarchive|url=https://web.archive.org/web/20111108171515/http://catb.org/jargon/html/F/forked.html |date=8 November 2011 }} ([[Jargon File]]), first added to [http://magic-cookie.co.uk/jargon/jarg422/ v4.2.2] {{webarchive|url=https://web.archive.org/web/20120114192615/http://magic-cookie.co.uk/jargon/jarg422/ |date=14 January 2012 }}, 20 August 2000)</ref>

{{quotation|Forking is considered a Bad Thing—not merely because it implies a lot of wasted effort in the future, but because forks tend to be accompanied by a great deal of strife and acrimony between the successor groups over issues of legitimacy, succession, and design direction. There is serious social pressure against forking. As a result, major forks (such as the [[GNU Emacs|Gnu-Emacs]]/[[XEmacs]] split, the fissioning of the [[386BSD]] group into three daughter projects, and the short-lived GCC/EGCS split) are rare enough that they are remembered individually in hacker folklore.}}

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# The death of the fork. This is by far the most common case. It is easy to declare a fork, but considerable effort to continue independent development and support.

# A re-merging of the fork (''e.g.'', [[egcs]] becoming "blessed" as the new version of [[GNU Compiler Collection|gcc]].)

# The death of the original (''e.g.'' the [[X.Org Server]] succeeding and [[XFree86]] dying.)

# Successful branching, typically with differentiation (''e.g.'', [[OpenBSD]] and [[NetBSD]].)

[[Distributed revision control]] (DVCS) tools have popularised a less emotive use of the term "fork", blurring the distinction with "branch".<ref>''e.g.'' {{cite web|url=https://lwn.net/Articles/628527/|title=An "open governance" fork of Node.js|first=Nathan|last=Willis|work=LWN.net|date=15 January 2015|access-date=15 January 2015|quote=Forks are a natural part of the open development model—so much so that GitHub famously plasters a "fork your own copy" button on almost every page.|url-status=live|archive-url=https://web.archive.org/web/20150421055059/http://lwn.net/Articles/628527/|archive-date=21 April 2015}} See also {{cite thesis|type=PhD|page=57|first=Linus|last=Nyman|title=Understanding Code Forking in Open Source Software|publisher=Hanken School of Economics|year=2015|quote=Where practitioners have previously had rather narrow definitions of a fork, [...] the term now appears to be used much more broadly. Actions that would traditionally have been called a branch, a new distribution, code fragmentation, a pseudo-fork, etc. may all now be called forks by some developers. This appears to be in no insignificant part due to the broad definition and use of the term fork by GitHub.|hdl=10138/153135}}</ref> With a DVCS such as [[Mercurial]] or [[Git (software)|Git]], the normal way to contribute to a project, is to first create a personal branch of the repository, independent of the main repository, and later seek to have your changes integrated with it. Sites such as [[GitHub]], [[Bitbucket]] and [[Launchpad (website)|Launchpad]] provide free DVCS hosting expressly supporting independent branches, such that the technical, social and financial barriers to forking a source code repository are massively reduced, and GitHub uses "fork" as its term for this method of contribution to a project.

Forks often restart version numbering from 0.1 or 1.0 even if the original software was at version 3.0, 4.0, or 5.0. An exception is when the forked software is designed to be a drop-in replacement for the original project, ''e.g.'' [[MariaDB]] for [[MySQL]]<ref>[http://programmers.stackexchange.com/questions/31551/forked-a-project-where-do-my-version-numbers-start Forked a project, where do my version numbers start?] {{webarchive|url=https://web.archive.org/web/20110826152252/http://programmers.stackexchange.com/questions/31551/forked-a-project-where-do-my-version-numbers-start |date=26 August 2011 }}</ref> or [[LibreOffice]] for [[OpenOffice.org]].

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==Forking proprietary software==

In [[proprietary software]], the copyright is usually held by the employing entity, not by the individual software developers. Proprietary code is thus more commonly forked when the owner needs to develop two or more versions, such as a [[Window (computing)|windowed]] version and a [[command line interface|command line]] version, or versions for differing operating systems, such as a [[word processor]] for [[IBM PC]] compatible machines and [[Apple Macintosh|Macintosh]] computers. Generally, such internal forks will concentrate on having the same look, feel, data format, and behavior between platforms so that a user familiar with one can also be productive or share documents generated on the other. This is almost always an economic decision to generate a greater [[market share]] and thus pay back the associated extra development costs created by the fork.

A notable proprietary fork not of this kind is the many varieties of proprietary [[Unix]]—almost all derived from AT&T Unix under license and all called "Unix", but increasingly mutually incompatible.<ref name=moen>[http://linuxmafia.com/faq/Licensing_and_Law/forking.html Fear of forking] {{webarchive|url=https://web.archive.org/web/20121217044712/http://linuxmafia.com/faq/Licensing_and_Law/forking.html |date=17 December 2012 }} – An essay about forking in [[free software]] projects, by Rick Moen</ref> ''See'' [[UNIXUnix wars]].

==See also==