A copyleft license is a type of software license designed to ensure that software remains free and open for everyone to use, modify, and redistribute. It operates on a "share-alike" principle, meaning that while it allows software to be modified and re-distributed by anyone, it mandates that the same rights are also preserved in derivative works. This ensures that any new software created from or based on a copyleft-licensed program must also be distributed under the same or a compatible copyleft license, effectively preventing it from becoming proprietary.
Understanding the Core Principle of Copyleft
At its heart, copyleft is a clever legal mechanism that uses copyright law to guarantee public access to software's source code and the freedoms associated with it. Instead of restricting use, it imposes a condition that perpetuates freedom. This concept is central to the Free Software Foundation's definition of free software, emphasizing four essential freedoms:
- The freedom to run the program as you wish, for any purpose.
- The freedom to study how the program works and change it to make it do what you wish (requiring access to the source code).
- The freedom to redistribute copies so you can help your neighbor.
- The freedom to distribute copies of your modified versions to others, giving the whole community a chance to benefit from your changes (requiring access to the source code).
Copyleft licenses, like the influential GNU General Public License (GPL), enforce these freedoms by ensuring they cannot be removed in subsequent versions or distributions.
How Copyleft Licenses Work
When you receive software under a copyleft license, you gain several crucial freedoms, but with an important condition:
- Modification and Redistribution: You are free to modify the software, fix bugs, add features, or adapt it to new uses. You can also redistribute the original or your modified versions.
- Preservation of Rights: The critical stipulation is that if you distribute your modified version (or a work that incorporates the copyleft-licensed software), you must do so under the same copyleft license. This means your new version must also provide its recipients with the same freedoms to use, study, modify, and redistribute. This is often referred to as the "viral" or "hereditary" nature of copyleft, as the license "spreads" to derivative works.
Key Characteristics of Copyleft Licenses
| Feature | Description |
|---|---|
| Share-Alike | The most defining feature: any derivative work must be licensed under the same or a compatible copyleft license. |
| Source Code Access | Always requires that the source code of the software (and any derivative works) be made available to recipients. |
| Freedom to Use | Users are free to run the program for any purpose. |
| Freedom to Modify | Users have the right to study and alter the software. |
| Freedom to Distribute | Users can redistribute copies of the original or modified software. |
| Open Source | All copyleft licenses are considered open-source licenses. |
Types of Copyleft
Copyleft licenses vary in how "strong" their share-alike requirement is:
- Strong Copyleft: These licenses, such as the GNU General Public License (GPL), require that any software linked to or derived from the copyleft-licensed code must also be licensed under the GPL. This makes them highly effective at ensuring the continued freedom of entire software projects.
- Weak Copyleft (or Library Copyleft): Licenses like the GNU Lesser General Public License (LGPL) are designed for software libraries. They allow other programs (even proprietary ones) to link to and use the library without requiring the entire program to be copyleft. However, any modifications to the library itself remain under the LGPL.
- File-level Copyleft: An example is the Mozilla Public License (MPL). It applies the copyleft condition on a file-by-file basis. If you modify an MPL-licensed file, your modifications must also be MPL-licensed, but other files in the same project can use different (even proprietary) licenses.
Common Examples of Copyleft Licenses
- GNU General Public License (GPL): The most well-known strong copyleft license, used by Linux, Git, and countless other projects.
- GNU Lesser General Public License (LGPL): A weak copyleft license often used for libraries, allowing for broader adoption.
- Mozilla Public License (MPL): A file-level copyleft license used by projects like Firefox.
- Affero General Public License (AGPL): A strong copyleft license specifically designed to cover network services, ensuring that even users interacting with software over a network receive the source code.
Copyleft vs. Permissive Licenses
It's helpful to compare copyleft with permissive licenses (like MIT, Apache, or BSD licenses). While both are open source, they have a fundamental difference:
- Copyleft: Requires derivative works to maintain the same freedoms.
- Permissive: Allows derivative works to be released under different licenses, including proprietary ones, with minimal restrictions.
Permissive licenses offer maximum flexibility for developers, while copyleft licenses prioritize the preservation of software freedom for users.
When to Choose a Copyleft License
Copyleft licenses are particularly suitable for:
- Projects aiming to ensure long-term freedom: If the goal is for the software (and all its derivatives) to always remain open and freely accessible.
- Fostering community contributions: Encouraging a collaborative environment where improvements benefit everyone.
- Building upon existing copyleft code: When developing a project that incorporates components already under a copyleft license, using a compatible copyleft license for the new work is often a necessity.
In essence, a copyleft license is a powerful legal tool that uses copyright to protect and propagate the freedom of software, making it a cornerstone of the open-source movement.
