GPL

The GNU General Public License (GPL) is a widely used free software license published by the Free Software Foundation. It is a copyleft license designed to preserve users’ freedom to run, study, modify, and share software. Under the GPL, distribution requires providing access to the source code and licensing any modified versions under the same terms, so downstream recipients retain the same freedoms.

The GPL exists as a family of licenses with several versions. GPLv1 was released in 1989, GPLv2 in 1991, and GPLv3 in 2007. Many projects specify “GPLv2 or later” or “GPLv3 only.” The licenses differ in details such as patent grants, anti-tivoization provisions, and license compatibility. GPLv3 introduces explicit patent license terms and protections against hardware restrictions that prevent running modified versions (tivoization), as well as updated terms for DRM scenarios and internationalization.

Key requirements include that source code be made available when distributing binaries, that notices and license terms accompany the distribution, and that modified works carry the same license. When GPL software is combined with non-GPL code, the licensing implications depend on the nature of the combination, leading to discussions about the scope of copyleft and linking.

The GPL is foundational to many free software projects. The Linux kernel is licensed under GPLv2 (without the “or later” clause), while many GNU tools and libraries use GPLv3 or GPLv2. Its strong copyleft design aims to prevent proprietary enclosure of improvements, fostering openness and collaboration. Licenses’ compatibility and the implications for project mixing are common considerations for developers.