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License compatibility is an issue that arises when licenses are applied to copyrighted works, particularly licenses of software packages (including software source code and binary representation). Licenses can contain contradictory requirements, rendering it impossible to combine source code or content from such works in order to create new ones.

Example

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Suppose a software package has a license that says, "modified versions must mention the developers in any advertising materials", and another package's license says, "modified versions cannot contain additional attribution requirements". Without direct permission from the copyright holder(s) for at least one of the two packages, it would be impossible to legally distribute a combination of the two because these specific license requirements cannot be simultaneously fulfilled. Thus, these two packages would have incompatible licenses.

Definitions

License compatibility can be defined differently around the concepts of "collective/combined/aggregated work" and "derivative work". The first "collective work" license compatibility definition allows the usage of various licensed works in a combined context:

the characteristic of two (or more) licenses according to which the codes distributed under these licenses may be put together in order to create a bigger distributable software.

— Philippe Laurent, The GPLv3 and Compatibility Issues

A stronger definition, also used by the FSF, includes the capability to change the license. As most prominent example, the copyleft licenses demand that the "derived work" combined from code under various licenses as whole is applied under the copyleft license.

License compatibility: The characteristic of a license according to which the code distributed under this license may be integrated in a bigger software that will be distributed under another license.

— Philippe Laurent, The GPLv3 and Compatibility Issues

Kinds of combined works

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A "combined work" consists of multiple differently-licensed parts (avoiding relicensing). To achieve a combined work including also copyleft licensed components (which have a viral property leading potentially to a "derived work"), proper isolation/separation needs to be applied.

With individually licensed source code files multiple non-reciprocal licenses (as permissive licenses or own proprietary code) can be separated, while the combined compiled program could be relicensed (but is not required). Such source code file separation is too weak for copyleft/reciprocal licenses (as the GPL), as they require then the complete work to be relicensed under the reciprocal license as derivative.

A slightly stronger approach is a separation on the linking stage with binary object code (static linking) with all the components living in the resulting program in the same process and address space. This satisfies "weak copyleft/standard reciprocal" combined works (as LGPL licensed ones), but not "strong copyleft/strong reciprocal" combined works. While commonly accepted that linking (static and even dynamic linking) constitutes a derivative of a strong copyleft'd work, there are individual alternative interpretations.

For "combined works" with "strong copyleft" modules, a stronger isolation is required. This can be achieved for instance by separating the programs by an own process and only communication via binary ABIs or other indirect means. 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.

While for some domains agreement exist if an isolation is suitable, there are domains in dispute and up to now untested in court. For instance, in 2015 the SFC sued VMware in an ongoing dispute whether loadable kernel modules (LKMs) are derivative works of the GPL'ed Linux kernel or not.

Compatibility of FOSS licenses

Even accepted and common open-source licenses are not necessarily compatible between each other, which can make it legally impossible to mix (or link) even open-source code if the components are available under different licenses. 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 licenses' terms by default. This is despite both licenses being approved by the Open Source Initiative and Free Software Foundation. License compatibility between a copyleft license and another license is often only a one-way compatibility. This characteristic makes the copyleft GPL (and most other copyleft licenses) incompatible with proprietary commercial licenses and also many non-proprietary licenses. This "one-way compatibility" characteristic is for instance criticized by the Apache Foundation, who provides the more permissive Apache license which doesn't have this characteristic. Non-copyleft licenses as the FOSS permissive licenses have a less complicated license interaction and often have in general better license compatibility.

The Artistic license 2.0 is here notable for an excellent license compatibility with all other FOSS licenses due to a relicensing clause which allows redistribution of the source code under any other FOSS license.

You may Distribute your Modified Version as Source (either gratis or for a Distributor Fee, and with or without a Compiled form of the Modified Version) provided that you do at least ONE of the following:

(c) allow anyone who receives a copy of the Modified Version to make the Source form of the Modified Version available to others under

(i) the Original License or

(ii) a license that permits the licensee to freely copy, modify and redistribute the Modified Version using the same licensing terms that apply to the copy that the licensee received, and requires that the Source form of the Modified Version, and of any works derived from it, be made freely available in that license fees are prohibited but Distributor Fees are allowed.

The Common Development and Distribution License, a weak copyleft license in-between GPL license and BSD/MIT permissive licenses, tries to address license compatibility problems by permitting mixing of CDDL licensed source code files with source code files under other licenses without relicensing. The resulting binary can be licensed and sold under a different license as long as the source code is still available under CDDL, enabling more use cases.

GPL compatibility

To minimize license proliferation and license incompatibilities in the FOSS ecosystem, some organizations (for instance the FSF) and individuals (for instance David A. Wheeler), argue that compatibility to the widely used GPL is an important feature of software licenses. Many of the most common free software licenses, especially the permissive licenses, such as the original MIT/X license, BSD licenses (in the three-clause and two-clause forms, though not the original four-clause form), MPL 2.0, and LGPL, are "GPL-compatible". That is, their code can be combined with a program under the GPL without conflict and the new combination would have the GPL applied to the whole (not the other license).

Copyleft licenses and GPL

When it comes to copyleft software licenses, they are not inherently GPL-compatible; even the GPLv2 itself is not compatible with GPLv3 or LGPLv3. If you tried to combine code released under both these licenses, you would violate section 6 of GPLv2, resulting in the incompatibility. However, if code is released under GPL “version 2 or later,” that is compatible with GPLv3 because GPLv3 is one of the options it permits. However, most software released under GPLv2 allows you to use the terms of later versions of the GPL as well but some have exception clauses that allow combining them with software that is under different licenses or license versions.

GFDL and GPL

Also, the FSF recommended GNU Free Documentation License is incompatible with the GPL, text licensed under the GFDL cannot be incorporated into GPL software. Therefore, the Debian project decided in a 2006 resolution to use for documentation the GPL. The FLOSS Manuals foundation followed Debian in 2007. In 2009 the Wikimedia Foundation switched also from the GFDL to a CC-BY-SA license as main license for their projects.

CDDL and GPL

Another important and controversial debated case of GPL compatibility is the CDDL licensed ZFS file system with the GPLv2 licensed Linux kernel. Despite that both are free software under a copyleft license, ZFS is not distributed with most linux distros like Debian (but with FreeBSD and even Mac OS) as the CDDL is considered incompatible to the GPL'ed linux kernel by the FSF and most other FOSS parties. There exist also alternative positions as the legal interpretation, if and when this combination constitutes a "combined work" or "derivated work" of the GPL'ed kernel, is ambiguous and controversial. In 2015 the CDDL to GPL compatibility question reemerged when the linux distribution Ubuntu announced to include OpenZFS by default. In 2016 Ubuntu announced that a legal review resulted in the conclusion that it is legally safe to use ZFS as binary kernel module in linux. Others followed Ubuntu's conclusion, for instance lawyer James E.J. Bottomley argued there can't be "a convincing theory of harm" developed making it impossible to bring the case to court. Eben Moglen, co-author of the GPLv3 and founder of the SFLC, argues that while the letters of the GPL might be violated the spirit of both licenses is unharmed, which would be the relevant aspect in court. On the other hand, Bradley M. Kuhn and Karen M. Sandler from the Software Freedom Conservancy argued that Ubuntu would violate both licenses as a binary ZFS module would be a derivative work of the linux kernel and announced their will to achieve clarity in this question, even by court.

CC BY-SA and GPLv3

On October 8, 2015 Creative Commons concluded that the CC BY-SA 4.0 is one-way compatible with the GPLv3.

Creative Commons license compatibility

The Creative Commons Licenses are widely used for content, but not all combinations of the seven recommended and supported licenses are compatible among each other. Also, this is often a one directional compatibility, requiring the complete work to be licensed under the more restrictive license of both.

Re-licensing for compatibility

Sometimes projects gets stuck in a license incompatibility situation and the only feasible way to solve it is the re-licensing of the incompatible parts. Relicensing is achieved by contacting all involved developers and parties and getting their agreement for the changed license. While in the free and open-source domain achieving 100% coverage is often impossible, due to the many contributors involved, the Mozilla relicensing project assumes achieving 95% is enough for the relicensing of the complete code base. Others in the FOSS domain, as Eric S. Raymond, came to different conclusions regarding the requirements for relicensing of a whole code base.

Relicensing examples

An early example of a project who did successfully re-license for license compatibility reasons is the Mozilla project and their Firefox browser. 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 FSF and OSI for being incompatible. Around 2001 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 achieving GPL-compatibility.

The Vorbis library was originally licensed as LGPL, but in 2001 the license was changed to the BSD license with endorsement of Richard Stallman to encourage adoption.

The VLC project has also a complicated license history due to license compatibility: in 2007 it decided for license compatibility reasons to not upgrade to the just released GPLv3. After the VLC was removed from Apple App Store in begin of 2011, in October 2011 the VLC project re-licensed the VLC library part from the GPLv2 to the LGPLv2 to achieve better compatibility. In July 2013 the VLC application could then resubmitted to the iOS App Store relicensed under the Mozilla Public License.

The GNU Free Documentation License in version 1.2 is not compatible with the widely used Creative Commons Attribution-ShareAlike license, which was a problem for instance for the Misplaced Pages. Therefore, at the request of the Wikimedia Foundation the FSF added with version 1.3 of the GFDL a time-limited section allowing specific types of websites using the GFDL to additionally offer their work under the CC BY-SA license. Following in June 2009, the Wikimedia Foundation migrated their projects (Misplaced Pages, etc.) by dual licensing to the Creative Commons Attribution-ShareAlike as main license, additional to the previously used GFDL. An improved license compatibility with the greater free content ecosystem was given as reason for the license change.

Another interesting case was the relicensing of GPLv2 licensed linux kernel header files to the BSD license by Google for their Android library Bionic. To get rid of the GPL, Google claimed that the header files were cleaned from any copyright-able work, reducing them to non-copyrightable "facts". This interpretation was challenged for instance by Raymond Nimmer, a law professor at the University of Houston Law Center.

In 2014 the FreeCAD project changed their license from GPL to LGPLv2 due to GPLv3/GPLv2 incompatibilities. Also in 2014 Gang Garrison 2 was relicensed from GPLv3 to MPL for improved library compatibility.

See also

• License proliferation
• Comparison of free and open-source software licenses (also license compatibility)
• Backward compatibility
• Forward compatibility

References

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3. Neary, Dave (2012-02-15). "Gray areas in software licensing". LWN.net. Eklektix. Retrieved 2016-02-27.
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7. ^ "Frequently Asked Questions about version 2 of the GNU GPL". GNU Project. Free Software Foundation. 2015-05-29. What constitutes combining two parts into one program? This is a legal question, which ultimately judges will decide. We believe that a proper criterion depends both on the mechanism of communication and the semantics of the communication . If the modules are included in the same executable file, they are definitely combined in one program. If modules are designed to run linked together in a shared address space, that almost surely means combining them into one program.
8. ^ "Frequently Asked Questions about the GNU Licenses". GNU Project. Free Software Foundation. 2016-05-26. 'Use a library' means linking .
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21. ^ Bezroukov, Nikolai. "Comparative merits of GPL, BSD and Artistic licences (Critique of Viral Nature of GPL v.2 - or In Defense of Dual Licensing Idea)". Archived from the original on 2001-12-22. Viral property stimulates proliferation of licenses and contributes to the 'GPL-enforced nightmare' -- a situation when many other licenses are logically incompatible with the GPL and make life unnecessary difficult for developers working in the Linux environment (KDE is a good example here, Python is a less known example).
22. Fogel, Karl. "The GPL and License Compatibility". Producing Open Source Software - How to Run a Successful Free Software Project. Retrieved 2015-11-29. The GPL and license compatibility - Because the primary goal of the GPL's authors is the promotion of free software, they deliberately crafted the license to make it impossible to mix GPLed code into proprietary programs. Any derivative work—that is, any work containing a nontrivial amount of GPLed code—must itself be distributed under the GPL. No additional restrictions may be placed on the redistribution of either the original work or a derivative work.
23. "Apache License v2.0 and GPL compatibility". Apache Software Foundation. Retrieved 2015-05-30. Apache 2 software can therefore be included in GPLv3 projects, because the GPLv3 license accepts our software into GPLv3 works. However, GPLv3 software cannot be included in Apache projects. The licenses are incompatible in one direction only, and it is a result of ASF's licensing philosophy and the GPLv3 authors' interpretation of copyright law.
24. Hanwell, Marcus D. (2014-01-28). "Should I use a permissive license? Copyleft? Or something in the middle?". Opensource.com. Retrieved 2015-05-30. Permissive licensing simplifies things One reason the business world, and more and more developers , favor permissive licenses is in the simplicity of reuse. The license usually only pertains to the source code that is licensed and makes no attempt to infer any conditions upon any other component, and because of this there is no need to define what constitutes a derived work. I have also never seen a license compatibility chart for permissive licenses; it seems that they are all compatible.
25. "Licence Compatibility". European Union Public Licence. Joinup. 2015-06-11. Retrieved 2015-05-30. The licences for distributing free or open source software (FOSS) are divided in two families: permissive and copyleft. Permissive licences (BSD, MIT, X11, Apache, Zope) are generally compatible and interoperable with most other licences, tolerating to merge, combine or improve the covered code and to re-distribute it under many licences (including non-free or "proprietary").
26. "Interview with Allison Randal about Artistic License 2.0". The CPAN blog.
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31. Chisnall, David (2009-08-31). "The Failure of the GPL". InformIT. Pearson Education. Retrieved 2016-01-24. The GPL places additional restrictions on the code, and therefore is incompatible. You can combine APSL, MPL, CDDL, Apache, and BSD-licensed code in the same project easily, but you can only combine one of these with GPLv2 code. Even the Free Software Foundation can't manage to get it right. Version 3 of the LGPL, for example, is incompatible with version 2 of the GPL. This has caused a problem recently for a few GNU library projects that wanted to move to LGPLv3 but were used by other projects that were GPLv2-only.
32. Asay, Clark D. "The General Public License Version 3.0: Making or Breaking the Foss Movement". Michigan Telecommunications and Technology Law Review. 14 (2). TheUniversity of Michigan Law School.
33. Landley, Rob. "CELF 2013 Toybox talk" (raw text). landley.net. Retrieved 2013-08-21. GPLv3 broke 'the' GPL into incompatible forks that can't share code.
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39. "2.2 What is the licensing concern?". zfsonlinux.com.
40. Xu, Aron (2014-08-28). "[zfs-discuss] Summary of ZFS on Linux for Debian (was: zfs-linux_0.6.2-1_amd64.changes REJECTED)" (email message). ZFS on Linux. Retrieved 2016-01-14. Upstream ZoL project holds the view that in this case the combination of the two in the same binary would create a derived work, so this is not acceptable for redistribution. We accept the interpretation that this last case is not acceptable for redistribution. Therefore our package does not (and never will) ship or facilitate building a custom kernel where the ZoL ZFS driver is built-in in a monolithic binary, instead of built as an independent dynamic LKM.
41. Tagliamonte, Paul Richards (2014-08-26). "Pkg-zfsonlinux-devel -zfs-linux_0.6.2-1_amd64.changes REJECTED". Our consensus was that this package appears to violate the spirit of the GPL at minimum, and may cause legal problems. Judges often interpret documents as they're intended to read, hacks to comply with the letter but not the intent are not looked upon fondly. This may be a hard thing for technical folks to accept, but in legal cases one usually isn't dealing with technical people. As such, this package has been rejected.
42. jake (2014-09-11). "Yao: The State of ZFS on Linux". LWN.net. Eklektix.
43. Jaeger, Till (2005-03-01). Die GPL kommentiert und erklärt (PDF) (in German). Institut für Rechtsfragen der Freien und Open Source Software. p. 70. ISBN 3-89721-389-3. Retrieved 2016-01-12. In der Praxis ist stark umstritten, ob ein Kernelmodul als 'derivative work' betrachtet werden muss. Die Auseinandersetzungen um Binär-Treiber für Linux werden mit Heftigkeit geführt. Man wird wohl nicht für sämtliche Kernelmodule eine einheitliche Antwort finden können: Wann ein Kernelmodul von Linux »abgeleitet« ist, hängt stark von der technischen Umsetzung ab und richtet sich nach den oben dargelegten Kriterien. Es existieren allerdings auch Kernelmodule, die älter sind als Linux, etwa das Dateisystem AFS. Dort liegt es auf der Hand, dass sie als funktional eigenständig anzusehen sind, da sie gar nicht »für Linux« geschrieben sein können. {{cite book}}: |work= ignored (help)
44. Larabel, Michael (2015-08-06). "Ubuntu Is Planning To Make The ZFS File-System A 'Standard' Offering". Phoronix.
45. Kirkland, Dustin (2016-02-10). "ZFS Licensing and Linux". Ubuntu Insights. Canonical.
46. Bottomley, James E.J. (2016-02-23). "Are GPLv2 and CDDL incompatible?". James Bottomley's random Pages. What the above analysis shows is that even though we presumed combination of GPLv2 and CDDL works to be a technical violation, there's no way actually to prosecute such a violation because we can't develop a convincing theory of harm resulting. Because this makes it impossible to take the case to court, effectively it must be concluded that the combination of GPLv2 and CDDL, provided you're following a GPLv2 compliance regime for all the code, is allowable.
47. Moglen, Eben; Choudhary, Mishi (2016-02-26). "The Linux Kernel, CDDL and Related Issues". Software Freedom Law Center.
48. Kuhn, Bradley M.; Sandler, Karen M. (2016-02-25). "GPL Violations Related to Combining ZFS and Linux". Software Freedom Conservancy. Ultimately, various Courts in the world will have to rule on the more general question of Linux combinations. Conservancy is committed to working towards achieving clarity on these questions in the long term. That work began in earnest last year with the VMware lawsuit, and our work in this area will continue indefinitely, as resources permit. We must do so, because, too often, companies are complacent about compliance. While we and other community-driven organizations have historically avoided lawsuits at any cost in the past, the absence of litigation on these questions caused many companies to treat the GPL as a weaker copyleft than it actually is. Conservancy (as a Linux copyright holder ourselves), along with the members of our coalition in the GPL Compliance Project for Linux Developers, all agree that Canonical and others infringe Linux copyrights when they distribute zfs.ko.
49. "Compatible Licenses". Creative Commons. GPLv3: The GNU General Public License version 3 was declared a 'BY-SA–Compatible License' for version 4.0 on 8 October 2015. Note that compatibility with the GPLv3 is one-way only, which means you may license your contributions to adaptations of BY-SA 4.0 materials under GPLv3, but you may not license your contributions to adaptations of GPLv3 projects under BY-SA 4.0.
50. "Frequently Asked Questions". Creative Commons. 2016-07-14. Retrieved 2016-08-01.
51. O’Riordan, Ciaran (2006-10-06). "(About GPLv3) Can the Linux Kernel Relicense?". Free Software Foundation Europe. Retrieved 2015-05-28. Someone who works with many lawyers on free software copyright issues later told me that it is not necessary to get permission from 100% of the copyright holders. It would suffice if there was permission from the copyright holders of 95% of the source code and no objections from the holders of the other 5%. This, I'm told, is how Mozilla was able to relicense to the GPL in 2003 despite years of community contributions.
52. Raymond, Eric Steven; Raymond, Catherine Olanich. "Licensing HOWTO". Retrieved 2015-11-21. Changing an existing license You can change the license on a piece of code under any of the following conditions: If you are the sole copyright holder If you are the sole registered copyright holder If you obtain the consent of all other copyright holders If no other copyright holder could be harmed by the change.
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64. Ménard, Delphine. "Why the Wikimedia projects should not use GFDL as a stand alone license for images". notablog.
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66. Moeller, Erik (2009-06-30). "Licensing update rolled out in all Wikimedia wikis". Wikimedia Blog. Perhaps the most significant reason to choose CC-BY-SA as our primary content license was to be compatible with many of the other admirable endeavors out there to share and develop free knowledge.
67. Metz, Cade (2011-03-29). "Google's 'clean' Linux headers: Are they really that dirty?". The Register.
68. Proffitt, Brian (2011-03-21). "Android: Sued by Microsoft, not by Linux". ITworld. Microsoft launches new Android suit, Linus Torvalds' take on Linux kernel headers and Android
69. Nimmer, Raymond (2011). "Infringement and disclosure risk in development on copyleft platforms". Contemporary Intellectual Property, Licensing & Information Law.
70. Prokoudine, Alexandre (2012-12-27). "LibreDWG drama: the end or the new beginning?". Libre Graphics World. Retrieved 2013-08-23. the unfortunate situation with support for DWG files in free CAD software via LibreDWG. We feel, by now it ought to be closed. We have the final answer from FSF. 'We are not going to change the license.'
71. "License". FreeCAD. Retrieved 2015-03-25. Licences used in FreeCAD - FreeCAD uses two different licenses, one for the application itself, and one for the documentation: Lesser General Public Licence, version 2 or superior (LGPL2+) Open Publication Licence
72. "License.txt". Gang-Garrison-2. GitHub. 2014-11-09. Retrieved 2015-03-23.
73. MedO (2014-08-23). "Planned license change (GPL -> MPL), Help needed" (forum post). Gang Garrison 2 Forums. Retrieved 2015-03-23. tl;dr: The current license prevents us from using certain nice and (cost-)free libraries / frameworks, so we want to change it. The new license (MPL) would be strictly more free than the old one, and is the same one that's also used by Firefox.

External links

• FLOSS Compatibility Diagram by David A. Wheeler (2007, some licenses superseded)

• Misplaced Pages articles needing copy edit from July 2016
• Copyright licenses