STRONG COPYLEFT LICENSES
Copyleft is defined by the Free Software foundation as: “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“. The philosophy of Free Software is based on FREE AS FREEDOM and not Free as Beer. So it is not about price, it is about getting the freedom to use, distribute and modify to develop your own software, without monopolistic limitations.
In legal terms, Copyleft is an optional method to protect the Four Free Software freedoms. This means The copyleft forces derivative, extended and even combined works, to be licensed under the terms of the original license.
– The GNU General Public License. Is the most popular FOSS license. It was created in 1989 by the Free software Foundation. The origin of the GPL license was the Emacs general public license . The most populars are the GPL v2 , and the GPL v3. The GPL license is the most popular FOSS license. Very popular Software such as The Linux kernel, WordPress, or My SQL, are licensed under the GPL license.
The Copyleft has a considerable impact in terms of derivative works(For basic permissions check Section 2 of the GPL v3 license). In theory, this works in the following way: (1) When you create derivative works of a GPL’ed licensed program, they will also have to be licensed under the GPL license terms. (2) If you create an original program, all components that form a “whole” with your program will have to be licensed under the GPL license, or a compatible license(even shared libraries in most cases).
The GPL license compatibility can become a very complex task. Software is never build from zero, it is mostly based in third party code, and links to third party shared libraries in order to obtain functionality( check section 5 of the GPL v3 license).
A recommended lecture on dynamic linking libraries controversies and the GPL license is here: dynamic linked libraries paradigms of the GPL license in contemporary software.
– The European Union License (EUPL). It was created by the european commission, and fully synchronized with European Union Law. It is the first FOSS license created by a government agency. The EUPL version 1.1 is still the newest. The EUPL includes a strong copyleft clause in section 5. However, there is a list compatible licenses included in the appendix. Despite the fact of being the first official pubic license issued by a government agency, the popularity of the EUPL license is very small.
ADVANTAGES OF COPYLEFT (focusing on the GPL license): (1)the purpose of copyleft is to create a Free software world, “Free as freedom, and not free as free beer“. This means freedom to use and develop your own technology, a huge benefit for users, society in terms of free knowledge, transparency and security. (2) There is a big Free Software community who believes in FREE AS FREEDOM as and appropriate method for protecting Free Software. They will always help you.
DISADVANTAGES(focusing on the GPL license): (1) Developers are not free to choose other licenses, when creating extended, combined or derivative works of a GPL licensed work. (2) GPL License compatibility is a complex legal issue. (3) the GPL v3 license is very long(6 pages), and difficult to understand for non lawyers.
WEAK COPYLEFT LICENSES
Some free software licenses don’t establish an obligation of licensing derivative work in the same terms of the original work license. But they come with some requirements such as distribution of the source code, or dual licensing.
Some of the most popular weak copyleft licenses are:
The Lesser GPL license. The LGPL is a permissive license, created by the Free Software Foundation. This license is focused on program libraries, and it has gained a lot of popularity among developers. LGPL v3 is synchronized with the GPL v3, by adding extra permissions. these permissions are relevant in the field of Combined works, allowing being combined with GPL non compatible licenses. However, dervative works of the LGPL v3 license must allow for “modification for the customer’s own use and reverse engineering for debugging such modifications.
The Mozilla public license. Well known license created by The Mozilla Foundation. The MPL v2 is a weak copyleft license with some special particularities. It only applies to MPL licensed code. This generates a dual(or multi) licensing model as someone can create derivative works under other license(even proprietary), but the MPL parts of it will still be licensed under the MPL license terms.
ADVANTAGES OF WEAK COPYLEFT LICENSES: (1)They allow license compatibility, which solves complex compatibility controversies such as the Dynamic libraries paradigms in the GPL. (2) They allow multi-licensing, and this benefits free and commercial development.
WEAKNESS : They are in between strong copyleft licenses and non copyleft licenses. if someone likes copyleft, they would just choose the GPL license. If someone does not like the Copyleft, It might be easier to just use a non-copyleft license, with no restrictions.