The GNU General Public License (GPL) is a free, open-source software license created by the Free Software Foundation (FSF) to ensure software remains freely accessible and modifiable. Its core principle is “copyleft,” which requires that any derivative work—software built using GPL-licensed code—must also be distributed under the GPL. This ensures that modifications or extensions to the original software stay open source, preventing proprietary restrictions. The GPL grants users four freedoms: to run the software for any purpose, study and modify its source code, redistribute copies, and distribute modified versions.
The GPL enforces its terms by mandating that developers who distribute GPL-licensed software (or software incorporating it) must provide recipients with the corresponding source code. For example, if you modify the Linux kernel (which uses GPLv2) and distribute your version, you must share your changes under the same license. This applies even if the GPL-licensed code is part of a larger system. A common practical consideration is dynamic linking: combining GPL code with proprietary code in a single executable often requires the entire project to be GPL-licensed. However, standalone programs communicating via interfaces (like APIs) may avoid this requirement. Projects like WordPress use the GPL, ensuring plugins and themes inherit the license, fostering a collaborative ecosystem.
There are two main versions of the GPL: GPLv2 (1991) and GPLv3 (2007). GPLv3 addresses modern issues like patent agreements and compatibility with other licenses, while GPLv2 remains widely used (e.g., in Linux). Developers must choose a version when licensing their software, and mixing code under different GPL versions can create conflicts. The Lesser GPL (LGPL) is a variant designed for libraries, allowing linking with proprietary code under certain conditions. Understanding these nuances is critical for compliance. For instance, using GPL code in a commercial product requires careful planning to avoid legal risks, as violating the license can lead to loss of distribution rights or litigation.