LGPL v3.0: Copyleft for Libraries

GNU Lesser General Public License v3.0

The LGPL-3.0 is a copyleft license designed specifically for software libraries. It allows proprietary programs to link against (use) the library without being subject to copyleft, while still requiring that modifications to the library itself be shared.

The Library Exception

The key difference from GPL is the "linking exception." A proprietary program can dynamically link to an LGPL library and remain proprietary. However, if someone modifies the library itself, those modifications must be released under LGPL.

Key Characteristics

Attribute	Value
SPDX Identifier	LGPL-3.0-only
Type	Weak Copyleft (Library)
Patent Grant	Yes
Copyleft	Yes (library scope)

How LGPL Works in Practice

1. Using the library unmodified — Link to it freely, your code can be any license
2. Modifying the library — Your changes to the library must be LGPL
3. Distributing the library — Users must be able to replace the library (e.g., dynamic linking)
4. Static linking — More complex; users must be able to re-link with a modified version

LGPL vs MPL-2.0

Both are weak copyleft, but they draw boundaries differently:

• LGPL — Copyleft applies to the entire library; programs linking to it are exempt
• MPL-2.0 — Copyleft applies to individual files; new files can be any license

Notable Projects

• GNU C Library (glibc)
• Qt framework (dual-licensed LGPL/commercial)
• FFmpeg libraries (LGPL for libraries, GPL for tools)
• Many GNU libraries and utilities