Permissive software licence

Software licenses in context of copyright: permissive licenses second from left as "non-protective FOSS licenses" (adapted from Mark Webbink).^[1]

A permissive software license, sometimes also called BSD-like or BSD-style license,^[2] is a free software license with minimal requirements about how the software can be redistributed. Examples include the MIT License, BSD licenses, Apple Public Source License and the Apache license. (As of 2016) the most popular free software license is the permissive MIT license.^[3][4]

Definitions

The Open Source Initiative defines a permissive software license as a "non-copyleft license".^[5] GitHub's choosealicense website described the MIT permissive license as, "lets people do anything they want with your code as long as they provide attribution back to you and don’t hold you liable."^[6] California Western School of Law's newmediarights.com defined them as follows: "The ‘BSD-like’ licenses such as the BSD, MIT, and Apache licenses are extremely permissive, requiring little more than attributing the original portions of the licensed code to the original developers in your own code and/or documentation."^[2]

Other terms

Copycenter

Copycenter is a term originally used to explain the modified BSD licence, a permissive free software license. The term was presented by computer scientist and Berkeley Software Distribution (BSD) contributor Marshall Kirk McKusick at a BSD conference in 1999.^{[citation needed]} It is a word play on copyright, copyleft and copy center.

The liberty to 'make as many copies as you want' is in fact also provided by all copyleft licenses. However, unlike both copyleft licenses and copyright law, permissive free software licenses do not control the license terms that a derivative work falls under.

Comparison to public domain

Computer Associates Int'l v. Altai used the term "public domain" to refer to works that have become widely shared and distributed under permission, rather than work that was deliberately put into the public domain. However, permissive licenses are not actually equivalent to releasing a work into the public domain.

Permissive licenses often do stipulate some limited requirements, such as that the original authors must be credited (attribution). If a work is truly in the public domain, this is usually not legally required, but a United States copyright registration requires disclosing material that has been previously published,^[7] and attribution may still be considered an ethical requirement in academia.

Comparison to copyleft

A major difference between the set of permissive and copyleft free software licenses is that when the software is being redistributed (either modified or unmodified), permissive licenses do not force the redistributor to open the modified source code. Copyleft licenses enforce the publication of the source code under the copyleft license.^[8][9] Some people argue that copyleft licenses see the world as "evil" and therefore enforce "freedoms" such as the availability of source code, while permissive licenses see the world as "good", therefore just allowing good actions and hoping for giving back in form of source code.^[10] Permissive licenses do not try to guarantee that future generations of the software will remain free and publicly available, in contrast to licenses which have reciprocity requirements which try to enforce this.

The FreeBSD project argues on the advantages of permissive licenses for companies and commercial use-cases: they say that they place only "minimal restrictions on future behavior" and argue that copyleft licenses are "legal time-bombs".^[11]

Permissive licenses offer more extensive licence compatibility than copyleft licenses, which cannot always be freely combined and mixed.^{[12][13][14][15]}

License compatibility

License compatibility between common free and open-source software (FOSS) licenses according to David A. Wheeler (2007): the vector arrows denote a one directional compatibility, therefore better compatibility on the left side ("permissive licenses") than on the right side ("copyleft licences").^[16]

In general permissive licenses show a good licence compatibility with most other software licenses in most situations.^[12][13]

Due to their non-restrictiveness most permissive software licenses are even compatible with copyleft licenses, which are incompatible with most other licenses. Copyleft licenses don't allow the addition of additional restrictive clauses which would be often required in a combined work made from copyleft code and other licensed code. Only some older permissive licenses have clauses requiring advertising materials to credit the copyright holder which made them incompatible with copyleft licenses, for instance the 4-clause BSD license, the PHP Licence, and the OpenSSL License. Popular modern permissive licenses, as the MIT Licence, the 3-clause BSD licence, and the Zlib Licence, don't include advertising clauses and are compatible with many copyleft licenses.

Some licenses do not allow derived works to add a restriction that says a redistributor cannot add more restrictions. Examples include the CDDL and MsPL. However such restrictions also make the license incompatible with permissive free software licenses.

Some licenses are permissive but do not qualify as free software licenses as defined by the Free Software Foundation.

Reception and adoption

While always an important part of the free and open-source software (FOSS) license landscape, since around 2010, several authors noted a raising popularity of the permissive licenses in contrast to the copyleft license.^{[17][18][19][20]}

(As of 2015) the MIT licence, a permissive license, is the most popular license in the FOSS domain before a copyleft one, the second placed GPLv2.^[3][4]

See also

• License-free software
• Public domain equivalent license
• Free software license
• Comparison of free and open-source software licenses

References

External links

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