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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.
The concrete effect of strong vs. weak copyleft has yet to be tested in court. [26] Free-software licenses that use "weak" copyleft include the GNU Lesser General Public License and the Mozilla Public License. The GNU General Public License is an example of a license implementing strong copyleft.
Sometimes the word "permissive" is considered too ambiguous, because all free software licenses are "permissive", in the sense that they all allow to modify and redistribute the source code. In most cases the real opposition is between copyleft licenses and non-copyleft ones, thus some authors prefer to use the term "non-copyleft" instead of ...
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]
In the mid-1980s, the GNU project produced copyleft free-software licenses for each of its software packages. An early such license (the "GNU Emacs Copying Permission Notice") was used for GNU Emacs in 1985, [5] which was revised into the "GNU Emacs General Public License" in late 1985, and clarified in March 1987 and February 1988.
The two main categories of free and open-source licenses are permissive and copyleft. [14] Both grant permission to change and distribute software. Typically, they require attribution and disclaim liability. [15] [16] Permissive licenses come from academia. [17] Copyleft licenses come from the free software movement. [18]
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]
For instance, copyleft licenses impose a duty on licensees to share their modifications to the work with the user or copy owner under some circumstances. No such duty would apply had the software in question been in the public domain .