Copyleft - Applying Copyleft

Applying Copyleft

Common practice for using copyleft is to codify the copying terms for a work with a license. Any such license typically gives each person possessing a copy of the work the same freedoms as the author, including (from the Free Software Definition):

Freedom 0 – the freedom to use the work,

Freedom 1 – the freedom to study the work,

Freedom 2 – the freedom to copy and share the work with others,

Freedom 3 – the freedom to modify the work, and the freedom to distribute modified and therefore derivative works.

(Note that the list begins from 0 due to a coding tradition—the first array element in C and many other programming languages is numbered as 0.)

These freedoms do not ensure that a derivative work will be distributed under the same liberal terms. In order for the work to be truly copyleft, the license has to ensure that the author of a derived work can only distribute such works under the same or equivalent license.

In addition to restrictions on copying, copyleft licenses address other possible impediments. These include ensuring the rights cannot be later revoked and requiring the work and its derivatives to be provided in a form that facilitates modification. In software, this requires that the source code of the derived work be made available together with the software itself.

Copyleft licenses necessarily make creative use of relevant rules and laws. For example, when using copyright law, those who contribute to a work under copyleft usually must gain, defer or assign copyright holder status. By submitting the copyright of their contributions under a copyleft license, they deliberately give up some of the rights that normally follow from copyright, including the right to be the unique distributor of copies of the work.

Some laws used for copyleft licenses vary from one country to another, and may also be granted in terms that vary from country to country. For example, in some countries it is acceptable to sell a software product without warranty, in standard GNU GPL style (see articles 11 and 12 of the GNU GPL version 2), while in most European countries it is not permitted for a software distributor to waive all warranties regarding a sold product. For this reason the extent of such warranties are specified in most European copyleft licenses. Regarding that, see the European Union Public Licence EUPL, or the CeCILL license, a license that allows one to use GNU GPL (see article 5 of the EUPL and article 5.3.4 of CeCILL) in combination with a limited warranty (see article 7 and 8 of the EUPL and 9 of CeCILL).