When working with open source software, selecting the right license for your project is one of the most important decisions you can make. The GNU Lesser General Public License (LGPL) is one of the key licenses within the open-source world, but many developers may not fully understand its purpose or benefits.

If you’ve ever wondered about the LGPL, how it compares to the GPL, and when to use it, you’re in the right place.

In this guide, I’ll explain the ins and outs of the LGPL in simple, friendly terms so that you can make informed decisions about using it in your projects.

What’s the GNU Lesser General Public License?

The GNU Lesser General Public License, or LGPL, is a free software license published by the Free Software Foundation. They created it as a more flexible alternative to the widely known GNU General Public License (GPL).

Both licenses fall under the umbrella of “copyleft” licenses, which ensure that software and its derivatives remain free for users to use, modify, and distribute.

The ability to combine software with proprietary (non-free) software is the key distinction between the GPL and LGPL. While the GPL requires that any software derived from GPL-licensed code also be licensed under the GPL, the LGPL is more permissive.

Developers can link to LGPL-licensed libraries within proprietary software without necessitating an open source license for the entire program.

The LGPL’s history and purpose

The LGPL was originally known as the “Library General Public License,” as it was primarily designed for libraries.

Developers who created software libraries often wanted to allow other developers, even those building proprietary software, to use their libraries without enforcing strict copyleft rules. Thus, the LGPL was born.

Over time, the LGPL’s scope expanded beyond libraries, which led to its name being changed to the Lesser General Public License. This reflected its purpose of offering more lenient terms compared to the standard GPL while still maintaining the core values of open source software freedom.

The LGPL’s key features

To understand the LGPL fully, it’s important to break down its core features:

• Freedom to Use and Modify: Like the GPL, the LGPL allows anyone to use, modify, and distribute the software. It ensures that the source code remains available to the public.
• Permissive Linking: The primary difference between the GPL and LGPL is that LGPL-licensed libraries can be linked to proprietary software without requiring the proprietary software to be open-sourced. This makes the LGPL particularly appealing for developers creating libraries or modules intended for broader use.
• Derivative Works: If you modify the LGPL-licensed library itself, you must release the modified version under the same LGPL terms. However, if you simply use the library within your proprietary software without modifying it, your proprietary code remains untouched by the LGPL’s copyleft requirements.
• Protection for the Community: The LGPL ensures that any modifications to the LGPL-licensed code itself remain free and open for the community to use, while still allowing broader compatibility with proprietary software.

When to use the LGPL

If you’re developing a project and deciding whether to use the LGPL, it’s helpful to ask yourself a few key questions:

1. Is your project a library, or a module?
   If you’re developing a software library or module that you want others to use freely, including in proprietary software, the LGPL is an excellent choice. It allows your code to be widely adopted while still protecting its open-source nature.
2. Do you want to allow integration with proprietary software?
   Unlike the GPL, which enforces strict copyleft terms, the LGPL is more lenient when it comes to proprietary integration. If you want your software to be compatible with proprietary systems without forcing them to open-source their code, the LGPL is a strong option.
3. Are you concerned about preserving user freedom?
   If you believe strongly in keeping your code open source while offering flexibility to the broader software ecosystem, the LGPL strikes a balance between freedom and flexibility.

Comparing LGPL vs. GPL

Now that we’ve covered what the LGPL is, let’s dive into how it compares to the GPL. While both licenses share many similarities, the key differences lie in their approach to proprietary software.

• GPL: The GPL enforces strict copyleft. Any derivative work that uses GPL-licensed code must also be distributed under the GPL. This means that if you link GPL code into proprietary software, you are required to open-source the entire proprietary project.
• LGPL: The LGPL offers a compromise. You can link LGPL-licensed libraries into proprietary software without triggering the copyleft requirement. However, if you modify the LGPL code itself, those changes must be open-sourced.

In essence, the LGPL provides a compromise: it safeguards libraries’ open source nature while permitting their wider use in the software industry, including in commercial, closed-source projects.

The benefits of the LGPL for developers

There are several benefits to using the LGPL for developers and organizations:

1. Broad Adoption: Since the LGPL allows linking with proprietary software, your open source library can reach a wider audience. Developers building commercial software are more likely to adopt LGPL libraries because they don’t have to open-source their proprietary code.
2. Community Contributions: By using the LGPL, you can foster a collaborative environment where others can contribute to your library. Any modifications to the LGPL-licensed code must be shared with the community, encouraging innovation and collective development.
3. Maintaining Software Freedom: The LGPL ensures that the core library remains free and open, protecting the user’s freedom to access and modify the source code while offering developers the flexibility to build proprietary systems around it.

How to apply the LGPL to your project

If you’ve decided that the LGPL is the right license for your project, applying it is straightforward. To ensure the proper licensing of your code, follow these steps:

1. Include the LGPL License: In your project’s root directory, add a file named COPYING or LICENSE and include the full text of the LGPL.
2. Add a License Notice: Indicate the LGPL license at the top of each source code file. This helps clarify the licensing terms for anyone who views your code.
3. Document the Licensing Terms: Clearly explain the licensing terms in your project’s documentation, so contributors and users understand the rules for using and modifying your code.

Conclusion: Why the LGPL Matters

The GNU Lesser General Public License offers an excellent balance between protecting software freedom and allowing broader adoption, especially in the case of libraries. If you’re a developer looking to ensure that your code remains open while still being useful in proprietary environments, the LGPL is a great option.

By offering flexibility without sacrificing the core principles of open source, you can help your project gain traction while contributing to the open-source community.

In conclusion, understanding the differences between the LGPL and other licenses, like the GPL, can guide you in making the best choice for your project.

Whether you’re building a software library or a tool that integrates with proprietary systems, the LGPL provides an ideal solution.