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: 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: 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 (
e.g.,
egcs becoming "blessed" as the new version of
GNU Compiler Collection.) • 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". With a DVCS such as
Mercurial or
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 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 numbers typically used for initial versions of programs like 0.0.1, 0.1, or 1.0 even if the original software was at another version such as 3.0, 4.0, or 5.0. An exception is sometimes made 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. The
BSD licenses permit forks to become proprietary software, and
copyleft proponents say that commercial incentives thus make proprietisation almost inevitable. (Copyleft licenses can, however, be circumvented via dual-licensing with a proprietary grant in the form of a
Contributor License Agreement.) Examples include
macOS (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. ==Forking proprietary software==