Copyleft Licenses

Open source licenses that require derivative works to be distributed under the same or compatible license terms, ensuring that modifications remain freely available to the community.

What are Copyleft Licenses?

Copyleft licenses are a category of open source software licenses that use copyright law to ensure that derivative works of the licensed software remain free and open. Unlike permissive licenses, copyleft licenses require that any modified versions or derivative works be distributed under the same or compatible license terms as the original work.

This licensing approach creates a "viral" effect, requiring that the freedoms granted by the original license be preserved in derivative works. The core philosophy is to create an expanding commons of freely available software that cannot be incorporated into proprietary, closed-source products without making the entire product open source.

Types of Copyleft Licenses

Strong Copyleft

Licenses with expansive scope:

GNU General Public License (GPL): The canonical strong copyleft license
GNU Affero General Public License (AGPL): Extends copyleft to network services
Common Development and Distribution License (CDDL): Strong copyleft with file-level scope
Eclipse Public License (EPL): Strong copyleft with commercial-friendly provisions
Open Software License (OSL): Strong copyleft covering distribution and network use

Weak Copyleft

Licenses with more limited scope:

GNU Lesser General Public License (LGPL): Permits linking with non-free software
Mozilla Public License (MPL): File-based copyleft allowing mixed licensing
Eclipse Public License (EPL): Sometimes considered weak due to linking exceptions
Common Public License (CPL): Allows certain types of combining with proprietary code
GNU Classpath Exception: Modification of the GPL for certain libraries

Network Copyleft

Licenses addressing the "SaaS loophole":

GNU Affero General Public License (AGPL): Extends GPL to network service deployment
Open Software License (OSL): Includes network use as triggering distribution
European Union Public License (EUPL): Includes network service provisions
Commons Clause: Add-on restricting commercial hosting (not OSI-approved)
Server Side Public License (SSPL): MongoDB's controversial network copyleft (not OSI-approved)

Core Copyleft Provisions

Source Code Requirements

Requirements for source code availability:

Complete Source: Requirement to provide complete and corresponding source code
Preferred Form: Source must be in the preferred form for making modifications
Complete Machine-Readable Copy: Requirements for complete machine-readable copies
Build Instructions: Requirements for build scripts and instructions
Availability Duration: How long source code must remain available

Distribution Triggers

What actions trigger copyleft obligations:

Conveying Covered Works: Defining what constitutes distribution
Propagation vs. Conveying: Different types of sharing under GPL
Network Use: When network use triggers obligations
Internal Use Exception: When internal use is exempt
Sublicensing: How sublicensing relates to distribution

Derivative Works

Determining what constitutes a derivative work:

Derivative Work Definition: How licenses define derivative works
Modification Scope: What constitutes a modification
Collective Works: Distinguishing collective works from derivatives
Combined Works: Specific provisions for combined works
Aggregation: When separate programs are merely aggregated

License Compatibility

Compatible Licenses

Understanding license compatibility:

GPL Compatibility: Licenses compatible with the GPL
One-Way Compatibility: When code can flow in only one direction
Permissive to Copyleft: Using permissively licensed code in copyleft projects
Copyleft to Copyleft: Compatibility between different copyleft licenses
Compatibility Charts: Resources for determining license compatibility

License Exceptions

Special exceptions to copyleft requirements:

System Library Exception: Exception for system libraries
Classpath Exception: Java-specific linking exception
Runtime Library Exception: Exception for runtime libraries
Linking Exceptions: General exceptions for various forms of linking
Special Purpose Exceptions: Other specialized exceptions

Multi-licensing

Managing multiple licenses:

Dual Licensing: Making code available under multiple licenses
License Selection: User's right to choose among available licenses
Commercial Dual Licensing: Business models based on dual licensing
Contributor Agreements: Managing contributions in dual-licensed projects
License Compatibility in Multi-licensed Works: Handling compatibility with multiple licenses

Technical Implications

Linking and Integration

How linking affects license obligations:

Static Linking: License implications of static linking
Dynamic Linking: License implications of dynamic linking
Inter-Process Communication: Using IPC to connect components
Plugins and Extensions: License considerations for plugin architectures
APIs and Interfaces: Copyright status of APIs and interfaces

System Boundaries

Defining technical boundaries:

Separate Programs: When programs are considered separate
Aggregation vs. Combination: Technical distinctions between aggregating and combining
Module Boundaries: Using modules to define license boundaries
Process Isolation: Using process boundaries to limit license scope
Network Separation: Using network boundaries to limit license scope

Language-Specific Considerations

How programming languages affect copyleft:

Interpreted Languages: Special considerations for interpreted languages
JVM and CLR: Considerations for virtual machine environments
Header Files and Libraries: Treatment of headers and libraries
Template Code: Handling of C++ templates and similar constructs
Web Technologies: JavaScript, WebAssembly, and other web technologies

Compliance Management

License Identification

Identifying and managing copyleft code:

License Scanning: Detecting copyleft code in projects
License Header Requirements: Requirements for source code headers
Code Provenance Tracking: Tracking the origin of code
Version Control Integration: Using version control for compliance
Bill of Materials: Creating and maintaining software BOMs

Compliance Programs

Establishing organizational compliance:

Compliance Policies: Creating organizational policies
Developer Guidelines: Guidelines for developers
Review Processes: Establishing review workflows
Training Programs: Educating teams about copyleft
Audit Procedures: Regular compliance audits

Compliance Tools

Tools for managing copyleft compliance:

License Scanners: Tools for detecting licenses
Dependency Analyzers: Tools for analyzing dependencies
Compliance Automation: Automating compliance tasks
Documentation Generators: Tools for generating compliance documentation
SBOM Integration: Including license information in SBOMs

Business Considerations

Incorporation in Products

Using copyleft code in commercial products:

Copyleft-Compatible Business Models: Business models that work with copyleft
Product Architecture: Designing products with license boundaries in mind
Mixed-License Products: Managing products with mixed licensing
Proprietary Add-ons: Creating proprietary additions to copyleft products
Service-Based Models: Service models around copyleft software

Contributor Management

Managing contributions to copyleft projects:

Contribution Policies: Policies for accepting contributions
Contributor License Agreements (CLAs): Using CLAs to manage rights
Copyright Assignment: When and how to use copyright assignment
Developer Certificate of Origin (DCO): Alternative to CLAs
Corporate Contribution Policies: Corporate policies for contributing to copyleft projects

Compliance Costs

Understanding the business impact:

Compliance Resource Requirements: Resources needed for compliance
Risk Assessment: Assessing non-compliance risks
Cost-Benefit Analysis: Weighing the costs and benefits of using copyleft code
Compliance ROI: Measuring return on compliance investment
Outsourcing vs. In-house: Deciding between outsourced and in-house compliance

Legal Aspects

Enforcement and Litigation

Understanding enforcement mechanisms:

Enforcement History: Notable copyleft enforcement cases
Enforcement Organizations: Groups that enforce copyleft licenses
Litigation Precedents: Legal precedents in copyleft litigation
Settlement Patterns: Common settlement approaches
Compliance Remediation: Approaches to remediate compliance issues

Jurisdictional Considerations

Legal variations across jurisdictions:

International Applicability: How copyleft applies internationally
Regional Variations: Regional differences in interpretation
Legal Traditions: Impact of different legal traditions
Copyright Law Variations: How copyright law variations affect copyleft
Contract vs. License: License as contract or unilateral permission

Interpretation Challenges

Resolving ambiguities in copyleft licenses:

Derivative Work Analysis: Determining derivative work status
Distribution Definition: Defining what constitutes distribution
Proprietary Claims: Addressing proprietary claims over open code
License Text Ambiguities: Resolving textual ambiguities
Intent vs. Text: Balancing license intent with actual text

Community and Governance

Foundation Governance

Role of foundations in copyleft:

Free Software Foundation: Role in copyleft development and enforcement
Software Freedom Conservancy: Approach to compliance and enforcement
Other Foundations: Other foundations' roles in copyleft governance
License Stewardship: Ongoing maintenance of license texts
Community Support: Foundation support for compliance efforts

Community Norms

Community expectations beyond legal requirements:

Reciprocity Expectations: Community expectations of reciprocity
Contribution Norms: Norms for contributing to copyleft projects
Fork Ethics: Ethical considerations when forking projects
Upstream First: Expectations for contributing upstream
Attribution Practices: Community norms for attribution

License Evolution

How copyleft licenses evolve:

License Versioning: Process for updating license versions
Community Input: How community input shapes license evolution
Addressing New Technologies: Adapting to technological changes
Compatibility Improvements: Efforts to improve compatibility
Simplification Efforts: Making copyleft licenses more accessible

Future of Copyleft

Emerging Challenges

New challenges for copyleft:

Cloud Services: Addressing the "SaaS loophole"
Containerization: Impact of container technologies
Web Applications: Challenges with web-based distribution
AI and ML: Implications for AI training and models
IoT and Embedded: Challenges in embedded environments

License Innovation

New approaches to copyleft:

Network Service Licenses: Evolution of network copyleft
Ethical Licenses: Adding ethical restrictions to copyleft
Simplified Copyleft: Efforts to simplify copyleft compliance
Data Copyleft: Extending copyleft concepts to data
Non-Software Copyleft: Application to non-software domains

Policy and Advocacy

Influencing the broader ecosystem:

Regulatory Approaches: Government regulation of open source
Procurement Policies: Public procurement policies and copyleft
Educational Initiatives: Educating about copyleft principles
Standards Organizations: Role of standards bodies in copyleft
Digital Commons Advocacy: Advocacy for expanding digital commons