The GNU General Public License (GPL) is a free software license that guarantees users the freedom to run, study, share, and modify software. Created by Richard Stallman, it pioneered “copyleft”—using copyright law to ensure software freedom propagates.
The Four Freedoms
The GPL ensures users have:
- Freedom 0: Run the program for any purpose
- Freedom 1: Study and modify the source code
- Freedom 2: Redistribute copies
- Freedom 3: Distribute modified versions
Copyleft Innovation
The GPL’s key innovation is copyleft: derivative works must also be licensed under the GPL. This creates a “viral” effect—GPL code can only be combined with GPL-compatible code, ensuring the commons grows rather than being privatized.
Versions
The GPL has evolved through major versions:
- GPLv1 (1989): Initial version
- GPLv2 (1991): Added patent provisions, still widely used
- GPLv3 (2007): Addressed software patents and “tivoization”
Impact
The GPL changed software development:
- Linux kernel uses GPLv2
- Many GNU tools use GPL
- Influenced other copyleft licenses
- Created legal framework for free software movement
- Sparked ongoing debates about software freedom vs. permissive licensing