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The strength of the copyleft license governing a work is determined by the extent to which its provisions can be imposed on all kinds of derivative works. Thus, the term "weak copyleft" refers to licenses where not all derivative works inherit the copyleft license; whether a derivative work inherits or not often depends on how it was derived.
A permissive software license, sometimes also called BSD-like or BSD-style license, [1] is a free-software license which instead of copyleft protections, carries only minimal restrictions on how the software can be used, modified, and redistributed, usually including a warranty disclaimer.
License compatibility is a legal framework that allows for pieces of software with different software licenses to be distributed together. 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.
[15] [16] Permissive licenses come from academia. [17] Copyleft licenses come from the free software movement. [18] Copyleft licenses require derivative works to be distributed with the source code and under a similar license. [15] [16] Since the mid-2000s, courts in multiple countries have upheld the terms of both types of license. [19]
Permissive licenses can be used within copyleft works, but copyleft material cannot be released under a permissive license. Some weak copyleft licenses can be used under the GPL and are said to be GPL-compatible. GPL software can only be used under the GPL or AGPL. [62]
The Copyleft sticker from an envelope Don Hopkins mailed to Richard Stallman in 1984. Copyleft licenses require source code to be distributed with software and require the source code to be made available under a similar license. [34] [60] Like the permissive licenses, most copyleft licenses require attribution. [61]
The Free Software Foundation prefers copyleft (share-alike) free-software licensing rather than permissive free-software licensing for most purposes. Its list distinguishes between free-software licenses that are compatible or incompatible with the FSF's copyleft GNU General Public License .
The FreeBSD project argues on the advantages of BSD-style licenses for companies and commercial use-cases due to their license compatibility with proprietary licenses and general flexibility, stating that the BSD-style licenses place only "minimal restrictions on future behavior" and are not "legal time-bombs", unlike copyleft licenses. [27]