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Fork (software development)

In software engineering, a project fork happens when developers take a copy of source code from one software package and start independent development on it, creating a distinct and separate piece of software The term often implies not merely a development branch, but a split in the developer community, a form of schism

Free and open-source software is that which, by definition, may be forked from the original development team without prior permission without violating copyright law However, licensed forks of proprietary software eg Unix also happen


  • 1 Etymology
  • 2 Forking of free and open source software
  • 3 Forking proprietary software
  • 4 See also
  • 5 References
  • 6 External links


The word fork stems from the Latin word furca, meaning a "fork or similarly shaped instrument" "Fork" in the meaning of "to divide in branches, go separate ways" has been used as early as the 14th century In the software environment, the word evokes the fork system call, which causes a running process to split itself into two almost identical copies that typically diverge to perform different tasks

In the context of software development, "fork" was used in the sense of creating a revision control "branch" by Eric Allman as early as 1980, in the context of SCCS:

Creating a branch "forks off" a version of the program

The term was in use on Usenet by 1983 for the process of creating a subgroup to move topics of discussion to

"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 BSDs 1993–1994; Russ Nelson used the term "shattering" for this sort of fork in 1993, attributing it to John Gilmore However, "fork" was in use in the present sense by 1995 to describe the XEmacs split, and was an understood usage in the GNU Project by 1996

Forking of free and open source software

Free and open source software may be legally forked without prior approval of those currently developing, managing, or distributing the software per both The Free Software Definition and The Open Source Definition:

The freedom to distribute copies of your modified versions to others freedom 3 By doing this, you can give the whole community a chance to benefit from your changes Access to the source code is a precondition for this

— The Free Software Definition

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

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 Web site, will retain the full original name and the associated user community Thus, there is a reputation penalty associated with forking The relationship between the different teams can be cordial or very bitter

Eric S Raymond, in his essay Homesteading the Noosphere, 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:

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/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

— Eric S Raymond, Jargon File

David A Wheeler notes four possible outcomes of a fork, with examples:

  1. 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
  2. A re-merging of the fork eg, egcs becoming "blessed" as the new version of gcc
  3. The death of the original eg the XOrg Server succeeding and XFree86 dying
  4. Successful branching, typically with differentiation eg, OpenBSD and NetBSD

Distributed revision control DVCS tools have popularised a less emotive use of the term "fork", blurring the distinction with "branch" With a DVCS such as Mercurial or Git, the normal way to contribute to a project is to first branch the repository, and later seek to have your changes integrated with the main repository Sites such as GitHub, Bitbucket and 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 01 or 10 even if the original software was at version 30, 40, or 50 An exception is when the forked software is designed to be a drop-in replacement for the original project, eg MariaDB for MySQL or LibreOffice for OpenOfficeorg

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 windowed version and a command line version, or versions for differing operating systems, such as a word processor for IBM PC compatible machines and 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 See UNIX wars

The BSD licenses permit forks to become proprietary software, and some say that commercial incentives thus make proprietisation almost inevitable Examples include Mac OS X based on the proprietary NeXTSTEP and the open source FreeBSD, Cedega and CrossOver proprietary forks of Wine, though CrossOver tracks Wine and contributes considerably, EnterpriseDB a fork of PostgreSQL, adding Oracle compatibility features, Supported PostgreSQL with their proprietary ESM storage system, and Netezza's proprietary highly scalable derivative of PostgreSQL Some of these vendors contribute back changes to the community project, while some keep their changes as their own competitive advantages

See also

  • List of software forks
  • Source port
  • Downstream software development


  1. ^ "Schism", with its connotations, is a common usage, eg "the Lemacs/FSFmacs schism" Jamie Zawinski, 2000, "Behind the KOffice split" Joe Brockmeier, Linux Weekly News, 2010-12-14, "Copyright assignment - once bitten, twice shy" Richard Hillesley, H-Online, 2010-08-06, "Forking is a feature" Anil Dash, 2010-09-10, "The Great Software Schism" Glyn Moody, Linux Journal, 2006-09-28, "To Fork Or Not To Fork: Lessons From Ubuntu and Debian" Benjamin Mako Hill, 2005
  2. ^ See, eg, "furca" from Etymological Dictionary of Latin by Michiel de Vaan PhD 2002
  3. ^ Entry 'fork' in Online Etymology Dictionary
  4. ^ "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" Robles, Gregorio; González-Barahona, Jesús M 2012 A Comprehensive Study of Software Forks: Dates, Reasons and Outcomes PDF OSS 2012 The Eighth International Conference on Open Source Systems Retrieved 20 Oct 2012 
  5. ^ Allman, Eric "An Introduction to the Source Code Control System" Project Ingres, University of California at Berkeley, 1980
  6. ^ Can somebody fork off a "netphilosophy" John Gilmore, netmisc, 18 January 1983
  7. ^ Shattering — good or bad Russell Nelson, gnumiscdiscuss, 1 October 1993
  8. ^ Re: Hey Franz: 32K Windows SUCK!!!!! Bill Dubuque, cucsmaclinfo, 21 September 1995
  9. ^ Lignux Marcus G Daniels, gnumiscdiscuss, 7 June 1996
  10. ^ a b c Why Open Source Software / Free Software OSS/FS, FLOSS, or FOSS Look at the Numbers!: Forking David A Wheeler
  11. ^ Stallman, Richard "The Free Software Definition" Free Software Foundation Retrieved 2013-10-15 
  12. ^ "The Open Source Definition" The Open Source Initiative Retrieved 15 October 2013 
  13. ^ Raymond, Eric S 15 August 2002 "Promiscuous Theory, Puritan Practice" 
  14. ^ Forked Jargon File, first added to v422, 20 Aug 2000
  15. ^ eg Willis, Nathan 15 January 2015 "An "open governance" fork of Nodejs" LWNnet Retrieved 15 January 2015 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  See also Nyman, Linus 2015 "Understanding Code Forking in Open Source Software" PhD Hanken School of Economics p 57 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 
  16. ^ Forked a project, where do my version numbers start
  17. ^ Fear of forking - An essay about forking in free software projects, by Rick Moen
  18. ^ EnterpriseDB
  19. ^ Fujitsu Supported PostgreSQL
  20. ^ Netezza

External links

  • Right to Fork at Meatball Wiki
  • A PhD examining forking: Nyman, 2015 "Understanding Code Forking in Open Source Software - An examination of code forking, its effect on open source software, and how it is viewed and practiced by developers"

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