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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.
The strength of the copyleft governing a work is an expression of the extent that the copyleft provisions can be efficiently imposed on all kinds of derived works. "Weak copyleft" refers to licenses where not all derived works inherit the copyleft license; whether a derived work inherits or not often depends on the manner in which it was derived.
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.
Copyleft licenses require derivative works to include source code under a similar license. Permissive licenses do not, and therefore the code can be used within proprietary software. Copyleft can be further divided into strong and weak depending on whether they define derivative works broadly or narrowly. [34] [35]
FSF's free software and OSI's open-source licenses together are called FOSS licenses. There are licenses accepted by the OSI which are not free as per the Free Software Definition . The Open Source Definition allows for further restrictions like price, type of contribution and origin of the contribution, e.g. the case of the NASA Open Source ...
[48] [49] Copyleft represents the farthest that reuse can be restricted while still being considered free software. [50] Strong copyleft licenses, such as the GNU General Public License (GPL), allow for no reuse in proprietary software, while weak copyleft, such as the related GNU Lesser General Public License (LGPL), do allow reuse in some ...
The GNU Project and the Free Software Foundation sometimes differentiate between "strong" and "weak" copyleft. "Weak" copyleft programs typically allow distributors to link them together with non-free programs, while "strong" copyleft strictly forbids this practice. Most of the GNU Project's output is released under a strong copyleft, although ...
The free software movement champions copyleft licensing schema (often pejoratively called "viral licenses"). In its strongest form, copyleft mandates that any works derived from copyleft-licensed software must also carry a copyleft license, so the license spreads from work to work like a computer virus might spread from machine to machine.