# LGPL License The GNU Lesser General Public License (LGPL) is a copyleft license created by the [[Free Software Foundation (FSF)]] as a compromise between the strict [[GNU General Public License (GPL)]] and permissive licenses like [[MIT License]]. Originally called the "Library GPL," it allows proprietary software to link against LGPL libraries without the copyleft requirement spreading to the proprietary code. The LGPL was designed for software libraries where the FSF wanted to encourage adoption (even by proprietary software) while still ensuring the library itself remains free. Modifications to the LGPL library must be released under LGPL, but applications merely using the library can remain proprietary; provided users can replace the library with modified versions. ## Key Characteristics - **Weak copyleft**: Only the library itself must remain open source - **Linking permitted**: Proprietary software can link against LGPL libraries - **Modification sharing**: Changes to the library must be LGPL - **User freedom**: Users must be able to replace the library ## LGPL vs GPL | Aspect | LGPL | GPL | |--------|------|-----| | Proprietary linking | Allowed | Not allowed | | Copyleft scope | Library only | Entire derivative work | | Use case | Libraries | Applications | | Freedom guarantee | Partial | Complete | ## LGPL Versions | Version | Year | Notes | |---------|------|-------| | LGPL v2 | 1991 | Original "Library GPL" | | LGPL v2.1 | 1999 | Renamed to "Lesser GPL" | | LGPL v3 | 2007 | Based on GPLv3, adds patent/anti-Tivoization | ## Notable LGPL Projects - **glibc**: GNU C Library (essential for Linux) - **GTK**: GIMP Toolkit, GUI library - **Qt** (older versions): Cross-platform framework - **[[FFmpeg]]** (parts): Multimedia libraries - **LGPL'd GNU libraries**: Many GNU utilities ## When to Use LGPL LGPL is appropriate when: - Building a library you want widely adopted - Allowing proprietary software integration - Ensuring library improvements stay open - Competing with permissively-licensed alternatives ## The "Lesser" Debate [[Richard Stallman]] has expressed reservations about LGPL, noting it provides "lesser" protection for user freedom. The FSF recommends GPL for most software, reserving LGPL for strategic cases where library adoption is prioritized over ensuring all users have freedom. ## References - https://www.gnu.org/licenses/lgpl-3.0.html - https://en.wikipedia.org/wiki/GNU_Lesser_General_Public_License ## Related - [[GNU General Public License (GPL)]] - [[GPLv2 License]] - [[GPLv3 License]] - [[Free Software Foundation (FSF)]] - [[Richard Stallman]] - [[MIT License]] - [[Open Source]]