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 any copyright law.
However, licensed forks of proprietary software also happen.
Etymology "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 term "fork" entered computing jargon around 1969 with
the Unix mechanism by which a process split in two by forming an identical copy of itself.
In the context of software development, the first documented use of the term "fork" in
the sense of "branch" was by Eric Allman in 1980, to describe forming branches in 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 or the BSDs; 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.
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 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:
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 The death of the original
Successful branching, typically with differentiation Distributed revision control 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 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 or LibreOffice for OpenOffice.org.
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 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, Cedega and CrossOver, EnterpriseDB, 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 References
External links Right to Fork at Meatball Wiki