License compatibility

compatibleincompatiblecompatibilitylicense incompatibilitylicenseGPL compatibilityGPL-compatible licensesGPL-incompatibleincompatibilityincompatible viral licenses
License compatibility is a legal framework that allows for pieces of software with different software licenses to be distributed together.wikipedia
158 Related Articles

Software relicensing

relicensedrelicensere-license
A combined work consists of multiple differently-licensed parts (avoiding relicensing). An example of a license that has excellent compatibility with other FOSS licenses is the Artistic License 2.0, due to its re-licensing clause which allows redistribution of the source code under any other FOSS license.
Software relicensing is applied in open-source software development when software licenses of software modules are incompatible and are required to be compatible for a greater combined work.

Viral license

viralviral licencesviral" licensing terms
To achieve a combined work including copyleft licensed components (which have a viral property leading potentially to a derived work), proper isolation/separation needs to be maintained.
The "virality" can force a license change of free software, e.g. when software is derived from two or more sources having incompatible viral licenses in which the derivative work could not be re-licensed at all.

Mozilla Public License

MPLMPL 2.0MPL 1.1
For example, software that combined code released under version 1.1 of the Mozilla Public License (MPL) with code under the GNU General Public License (GPL) could not be distributed without violating one of the terms of the licenses; this despite both licenses being approved by both the Open Source Initiative and the Free Software Foundation. The source code of Netscape's Communicator 4.0 browser was originally released in 1998 under the Netscape Public License/Mozilla Public License but was criticised by the Free Software Foundation (FSF) and OSI for being incompatible with the GNU General Public License (GPL). Also in 2014, Gang Garrison 2 was re-licensed from GPLv3 to MPL for improved library compatibility.
It has undergone two revisions, a minor update to version 1.1, and a major update to version 2.0 with the goals of greater simplicity and better compatibility with other licenses.

Software license

licenselicensedlicenses
License compatibility is a legal framework that allows for pieces of software with different software licenses to be distributed together.
The FSF distinguishes additionally between free software licenses that are compatible or incompatible with the FSF license of choice, the copyleft GNU General Public License.

Bionic (software)

BionicBionic libclibbionic
Examples are Android's kernel space-to-user space separation via Bionic, or Linux distros which have proprietary binary blobs included despite having a strong copyleft kernel.
The permissive BSD license is a non-copyleft license that is compatible in both directions. A BSD-licensed glibc substitute could act as an isolation layer between the copyleft core (kernel) and the non-copyleft applications, and was therefore chosen by Google for its Bionic as a glibc substitute.

License proliferation

licence proliferationMIT-like licenseproliferation of open-source licenses
To minimize license proliferation and license incompatibilities in the FOSS ecosystem, some organizations (the Free Software Foundation, for instance) and individuals (David A. Wheeler), argue that compatibility with the widely used GPL is an important feature of software licenses.
License proliferation affects the whole FOSS ecosystem negatively by the burden of increasingly complex license selection, license interaction, and license compatibility considerations.

MIT License

MITMIT/X11X11
The Common Development and Distribution License (CDDL)—a weak copyleft license in-between the GPL license and BSD/MIT permissive licenses—tries to address license compatibility problems by permitting, without re-licensing, the mixing of CDDL-licensed source-code files with source-code files under other licenses by providing that the resulting binary can be licensed and sold under a different license as long as the source code is still available under CDDL.
As a permissive license, it puts only very limited restriction on reuse and has, therefore, an excellent license compatibility.

Common Development and Distribution License

CDDLSun Studio product licenseCDDL 1.
The Common Development and Distribution License (CDDL)—a weak copyleft license in-between the GPL license and BSD/MIT permissive licenses—tries to address license compatibility problems by permitting, without re-licensing, the mixing of CDDL-licensed source-code files with source-code files under other licenses by providing that the resulting binary can be licensed and sold under a different license as long as the source code is still available under CDDL.
The Free Software Foundation (FSF) considers it a free software license, but one which is incompatible with the GNU General Public License (GPL).

Apache License

Apache 2.0ApacheApache License 2.0
This "one-way compatibility" characteristic has been criticized by the Apache Foundation, which licenses under the more permissive Apache license, such non-copyleft licenses being often less complicated and making for better license compatibility.
The stated goals of the license included ''making the license easier for non-ASF projects to use, improving compatibility with GPL-based software, allowing the license to be included by reference instead of listed in every file, clarifying the license on contributions, and requiring a patent license on contributions that necessarily infringe a contributor's own patents''.

BSD licenses

BSDBSD license3-clause BSD license
The Common Development and Distribution License (CDDL)—a weak copyleft license in-between the GPL license and BSD/MIT permissive licenses—tries to address license compatibility problems by permitting, without re-licensing, the mixing of CDDL-licensed source-code files with source-code files under other licenses by providing that the resulting binary can be licensed and sold under a different license as long as the source code is still available under CDDL.
In addition, the clause presented a legal problem for those wishing to publish BSD-licensed software which relies upon separate programs using the GNU GPL: the advertising clause is incompatible with the GPL, which does not allow the addition of restrictions beyond those it already imposes; because of this, the GPL's publisher, the Free Software Foundation, recommends developers not use the license, though it states there is no reason not to use software already using it.

Open-source license

open sourceopen-sourceopen source license
Licenses common to free and open-source software (FOSS) are not necessarily compatible with each other, and this can make it legally impossible to mix (or link) open-source code if the components have different licenses.
An open-source license is a type of license for computer software and other products that allows the source code, blueprint or design to be used, modified and/or shared under defined terms and conditions.

Permissive software licence

permissivepermissive licensepermissive free software license
The Common Development and Distribution License (CDDL)—a weak copyleft license in-between the GPL license and BSD/MIT permissive licenses—tries to address license compatibility problems by permitting, without re-licensing, the mixing of CDDL-licensed source-code files with source-code files under other licenses by providing that the resulting binary can be licensed and sold under a different license as long as the source code is still available under CDDL. This "one-way compatibility" characteristic has been criticized by the Apache Foundation, which licenses under the more permissive Apache license, such non-copyleft licenses being often less complicated and making for better license compatibility.
Permissive licenses offer more extensive licence compatibility than copyleft licenses, which cannot always be freely combined and mixed.

JSON

JavaScript Object Notation (JSON)JSON-likedictionary
JSON developer Douglas Crockford, inspired by the words of then President Bush, formulated the "evil-doers" JSON license ("The Software shall be used for Good, not Evil.") in order to open-source the JSON libraries but also to force (troll) corporate lawyers (or those who are overly pedantic) to pay for a license from the state.
On the other hand, this clause led to license compatibility problems of the JSON license with other open-source licenses.

GNU Free Documentation License

GFDLGNU FDLGNU Free Documentation License (GFDL)
The Free Software Foundation-recommended GNU Free Documentation License is incompatible with the GPL license, and text licensed under the GFDL cannot be incorporated into GPL software.
The GNU FDL is incompatible in both directions with the GPL—material under the GNU FDL cannot be put into GPL code and GPL code cannot be put into a GNU FDL manual.

Artistic License

ArtisticArtistic 2.0Clarified Artistic License
An example of a license that has excellent compatibility with other FOSS licenses is the Artistic License 2.0, due to its re-licensing clause which allows redistribution of the source code under any other FOSS license.
The Artistic license 2.0 is also notable for its excellent license compatibility with other FOSS licenses due to a relicensing clause, a property other licenses like the GPL are missing.

VLC media player

VLClibVLCVLC for iOS
The VLC project has a complicated license history due to license incompatibility, and in 2007 the project decided, for license compatibility, to not upgrade to the just released GPLv3.
In 2007 the VLC project decided, for license compatibility reasons, to not upgrade to the just released GPLv3.

Multi-licensing

dual-licenseddual licensetri-license
Around 2001 Time Warner, exercising its rights under the Netscape Public License, and at the request of the Mozilla Foundation, re-licensed all code in Mozilla that was under the Netscape Public License (including code by other contributors) to an MPL 1.1/GPL 2.0/LGPL 2.1 tri-license, thus achieving GPL-compatibility.
The two usual motivations for multi-licensing are license compatibility and market segregation based business models.

Creative Commons

CCiCommonsCC-BY-2.0
In 2009, the Wikimedia Foundation switched from the GFDL to a Creative Commons CC-BY-SA license as the main license for their projects.
"By failing to take any firm ethical position and draw any line in the sand, CC is a missed opportunity. ... CC has replaced what could have been a call for a world where 'essential rights are unreservable' with the relatively hollow call for 'some rights reserved. He also argued that Creative Commons worsens license proliferation, by providing multiple licenses that are incompatible.

GNU General Public License

GPLGNU GPLGPLv2
For example, software that combined code released under version 1.1 of the Mozilla Public License (MPL) with code under the GNU General Public License (GPL) could not be distributed without violating one of the terms of the licenses; this despite both licenses being approved by both the Open Source Initiative and the Free Software Foundation. To minimize license proliferation and license incompatibilities in the FOSS ecosystem, some organizations (the Free Software Foundation, for instance) and individuals (David A. Wheeler), argue that compatibility with the widely used GPL is an important feature of software licenses. The Common Development and Distribution License (CDDL)—a weak copyleft license in-between the GPL license and BSD/MIT permissive licenses—tries to address license compatibility problems by permitting, without re-licensing, the mixing of CDDL-licensed source-code files with source-code files under other licenses by providing that the resulting binary can be licensed and sold under a different license as long as the source code is still available under CDDL. The VLC project has a complicated license history due to license incompatibility, and in 2007 the project decided, for license compatibility, to not upgrade to the just released GPLv3.
FSF maintains a list of GPL-compatible free software licenses containing many of the most common free software licenses, such as the original MIT/X license, the BSD license (in its current 3-clause form) and the Artistic License 2.0.

Netscape Public License

NPL
The source code of Netscape's Communicator 4.0 browser was originally released in 1998 under the Netscape Public License/Mozilla Public License but was criticised by the Free Software Foundation (FSF) and OSI for being incompatible with the GNU General Public License (GPL).
Time Warner, exercising its rights under the Netscape Public License, and at the request of the Mozilla Foundation, relicensed all code in Mozilla that was under the Netscape Public License (including code by other contributors) to an MPL 1.1/GPL 2.0/LGPL 2.1 tri-license, thus removing the GPL-incompatibility.

Gang Garrison 2

Also in 2014, Gang Garrison 2 was re-licensed from GPLv3 to MPL for improved library compatibility.
The game was originally licensed as GPLv3, but was relicensed in 2014 due to license incompatibilities.

FreeCAD

In 2014, the FreeCAD project changed their license from GPL to LGPLv2, due to GPLv3/GPLv2 incompatibilities.
As FreeCAD (and also LibreCAD) has dependencies on Open Cascade, which prior to version 6.7.0 was only compatible with GPLv2, it couldn't use the GNU LibreDWG library as GPLv2 and GPLv3 are essentially incompatible.

Comparison of free and open-source software licenses

free software licensescomparisonfree and open source software licenses
Comparison of free and open-source software licenses (also license compatibility)
This table lists for each license what organizations from the FOSS community have approved it – be it as a "free software" or as an "open source" license –, how those organizations categorize it, and the license compatibility between them for a combined or mixed derivative work.

Source code

codesourcesource file
The source code of Netscape's Communicator 4.0 browser was originally released in 1998 under the Netscape Public License/Mozilla Public License but was criticised by the Free Software Foundation (FSF) and OSI for being incompatible with the GNU General Public License (GPL). The need for such a framework arises because the different licenses can contain contradictory requirements, rendering it impossible to legally combine source code from separately-licensed software in order to create and publish a new program.

Derivative work

derivative worksderivativederivatives
License compatibility can be defined around the concepts of "collective/combined/aggregated work" and "derivative work".