ShareAlike compatibility analysis: GPL
GPLv3 is a copyleft license primarily used for software, stewarded by the Free Software Foundation. Published in 2007, it is one of the most widely-used free software licenses, and the most widely used copyleft, as well as one of the influences on the Creative Commons licenses. It requires users of licensed material to share alike and to provide attribution to the author(s), and it does not violate the definition of Free Cultural Licenses. Thus, it satisfies the minimum criteria for compatibility set forth in our ShareAlike compatibility criteria.
GPL v3 and BY-SA 4.0 are similar licenses with similar aims. But because GPLv3 was written specifically for licensing software, it does have some differences from BY-SA, which are explained in more detail below.
Summary of comparison: GPLv3 and BY-SA 4.0
- The scope of licensed rights in the two licenses is similar but not identical. There are a few key differences: unlike BY-SA, GPL explicitly licenses patent rights where owned by the copyright holder, and addresses "copyright-like rights" (specifically including semiconductor mask rights), and does not explicitly mention sui generis database rights and neighboring rights. Moral rights are also not explicitly addressed in GPLv3.
- The specific attribution and marking requirements vary slightly, including that GPLv3 addresses authorship information by requiring a copyright notice, while BY-SA requires authorship information even when a work has no copyright notice.
- Source code, defined as the preferred form for modification of a work, must be conveyed with GPL works. Source must be sufficient to install, use, and modify the work. BY-SA has no source requirement.
- Both licenses contain a reinstatement mechanism; GPLv3 is slightly more complicated. In some situations, reinstatement of rights may happen under BY-SA but not GPLv3 and vice versa.
|Features||BY-SA 4.0||GPL 3.0|
|License scope||Copyright, neighboring rights and sui generis database rights (SGDRs) (§1(d))||Copyright, copyright-like laws (including semiconductor masks) (§2), patent rights if owned by copyright holder (§11)|
|Attribution trigger||If there is applicable copyright in work, then when work or adaptation of work is shared. (§3(a))
||Action that implicates copyright in a manner that enables other parties to make or receive copies (§§4-6), except if giving to other party solely so they can run the works for you on their facilities or make modifications for you (§2)|
|Attribution removal clause||Yes (§3(a)(3))||No|
|Special marking requirement if work is modified||Yes (indicate if you modified and retain indication of previous modifications) (§3(a)(1)(B))||Yes (notice that you modified the work, with the relevant date; special notices for interactive user interfaces)(§5)|
|ShareAlike trigger||When adaptation is shared (§3(b))||When the modified work is conveyed (§5)|
|ShareAlike scope||Must ShareAlike contributions to adaptations (§3(b));
||Must share entire work under GPL terms if copyright permission was needed to create the modified work.
|Source requirement||No||Yes; source required if work not already distributed in preferred form for modification, may be provided in varying ways as specified by license. (§6)|
|Effective technological measures||May not be applied by licensees if they restrict access to the work or adaptations (§2(a)(5)(C))
||No explicit mention, so implicitly permitted on non-source forms, though ability to forbid circumvention is waived (§3); Requirement for source to be conveyed in a publicly documented format without technical restrictions, which ensures no lockup by ETMs (§6).|
|Moral rights||Waived only to the extent necessary to allow the license to function. (§2(b)(1))||No explicit mention|
|Termination||Automatic upon breach (§6(a))||Automatic upon breach (§8)|
|Reinstatement after termination||Requires express permission unless cured within 30 days of discovery of violation (§6(b))||Automatic upon cure of breach within 30 days of notice by licensor unless not first violation; also automatic upon cure if copyright holder fails to notify you of violation within 60 days of cure (§8)|
|Reps & warranties||Expressly disclaimed (§5)||Expressly disclaimed (§§15-16)|
|Choice of law||None||None|
|Option to comply with other license versions||Yes, but limited. Licensees may choose to comply with a later version of BY-SA if such version is applied to an adaptation of the work.
|Compliance with any version automatically permitted if no version specified; but licensor may specify "or any later versions" or may select a proxy to decide which later versions are allowed. No additional obligations are imposed on authors where a reuser follows a later version. (§§14)|