What is copyleft?

Copyleft is a licensing tool unique to free software. It is designed to encourage the proliferation of free software and protect free software from being incorporated into non-free works. This works by giving you not only the right to share your improvements, but the obligation to share your improvements under some conditions. It is very important to understand these obligations when re-using copyleft software in your own work.

The copyleft spectrum

Different free software licenses exist along a spectrum from permissive to copyleft, based on the degree to which they emphasize copyleft in their license terms. Permissive licenses tend to allow generous reuse with relatively few and non-onerous obligations, such as simple attribution requirements. In contrast, copyleft licenses impose the obligation to share your changes and derived works under the same license terms.

Why choose a copyleft license?

It is common for permissively-licensed free software to be incorporated into non-free works. This is often done in the name of greater profits by denying the four freedoms to users who receive the non-free work, making profitable use of the software without giving anything back to the free software community.

Copyleft licenses address some of these problems:

1. Copyleft promotes the proliferation of free software and the four freedoms by ensuring that work built on top of free software grows and benefits the free software ecosystem.
2. Copyleft ensures that those who improve or re-use free software share their changes with the community, so that all users can benefit from their improvements.

Copyleft software can be sold, like all other free software, but requiring that commercial improvements remain free ensures the four freedoms are upheld by all participants. Furthermore, it is difficult to change the license of copyleft software if the copyright is held in aggregate, which serves as a strong promise for the future of the software as free software.

Weak and strong copyleft

Copyleft licenses differ in how strongly their copyleft clauses affect re-use of the software. For example, the weak copyleft Mozilla Public License is file-based, such that the copyleft clause covers individual source code files, and not the project as a whole: you can drop one of these files into any project without having to relicense the larger project, so long as you re-publish any changes to those specific files.

A somewhat stronger example is the GNU Lesser General Public License, which deals specifically with software libraries. These libraries are compiled into an aggregate software artifact, such as a shared object or static archive, and the copyleft terms applies to this entire artifact. However, when this is linked with a third-party program, the copyleft clause is not invoked. Stronger still is the GNU General Public License, which treats the completed program as the software artifact to which the copyleft clause applies.

On the far end of the copyleft spectrum are licenses like the GNU Affero General Public License, which extends the GPL to apply to software used over a network, such as databases, and considers users of that software “recipients” of the software, who are thus entitled to receive the source code.

How to re-use copyleft works

The simplest way to re-use copyleft works is to apply its license to your own work and distribute it accordingly.

If you do not want to do this, you can only use a copyleft work under the conditions permitted by its license, and will likely be limited to the use of weak copyleft works. For example, if your software depends on a library which uses the LGPL, you may use any license for your own work but need to share changes you make to the library itself. If the software uses the GPL or AGPL, you will be more constrained in your approach. Read the license terms carefully and consult a lawyer if you are unsure how to proceed.

For more details, consult our page on re-using free software.