Permissive free software licences
From Wikipedia, the free encyclopedia
Permissive free software licences are software licences for a copyrighted work that offer many of the same freedoms as releasing a work to the public domain. Well-known examples include the MIT license, the modified BSD license, and the X11 licence. In contrast, copyleft licences like the GNU General Public License require copies and derivatives of the source code to be made available on terms no more restrictive than those of the original licence.
[edit] 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, such licences are not actually equivalent to releasing a work into the public domain, so such a term can be considered a misnomer. But because the MIT and BSD licenses essentially retain no rights apart from attribution of the copyright owner, the purported copyright holders would not be losing rights if a court held that the works were actually in the public domain.
[edit] Advertising clauses and GPL compatibility
Some permissive free software licences contain clauses requiring you to give credit to their project in certain conditions. Examples of licences with such clauses are the original "4-clause" BSD licence, the PHP licence, and the OpenSSL licence.
Licences with the advertising clause are incompatible with the GNU General Public License.
Examples of permissive free software licences without the advertising clause are the X11 licence, the Zlib licence, the revised BSD licence, and the Expat licence.
