IFI7105 – Principle of copyleft

Copyleft is the ability of a contributor or distributor to add restrictions to a work. 
While most open source software allows you to use and modify the code as you want, open source software incorporating copyleft provisions prevent a contributor from making the future software proprietary.

Copyleft is a general method for making a program or other work free, and requiring all modified and extended versions of the program to be free as well.

There are no limits to how software distributed under a free licence may be used or modified, as long as the software remains within the user’s organization.
However, if the software is redistributed outside of this organization, the extent to which the initial licence is to be preserved, gives rise to three distinct types of free licence:
Strong copyleft-type licence – The software may be redistributed with or without modification, but always under the initial licence. In addition, any components that may be combined with the software in any way to form a new and larger software development will also be covered by the initial licence. For example, since the Linux kernel is under a GPL licence, the new Ext4 file system currently being developed will also be covered by the GPL licence.
Weak copyleft-type licence – Although the software must still be redistributed, with or without modification, under the original licence, code under other and even  proprietary licences may be added to provide new functions. For example, for OpenOffice.org, which is under an LGPL licence, Sun distributes StarOffice, which although still under an LGPL licence has been enhanced with proprietary add-ons.
Non-copyleft licence – The software may be redistributed, with or without modification, under another licence. For example, components of the FreeBSD operating system under a BSD free licence are used in the Mac OS X operating system, which is in turn redistributed under a proprietary licence.

Some free and open source software licences are based on the principle of copyleft, a kind of reciprocity: any work derived from a copyleft piece of software must also be copyleft itself. The most common free software license, the GNU GPL, is used for the Linux kernel and many of the components from the GNU project.

List of most popular free software licenses:
Non-copyleft
CeCILL-B
Common Development and Distribution License
Apache License, 2.0
New BSD license
Common Development and Distribution License
MIT license
Common Development and Distribution License

Weak copyleft
Mozilla Public License 1.1 (MPL)  
Common Public License 1.0
Eclipse Public License
GNU Library or “Lesser” General Public License (LGPL)
CeCILL-C

Strong copyleft
GNU General Public License (GPL) 
European Union Public Licence (EUPL)
CeCILL V2

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