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I want all the people who link to my software should also release their code under GPL. But what if someone forked it and attempted to relicense it under the MIT, BSD, Apache, or LGPL licenses? Can I ensure this can't happen?

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    Then they do so in violation of the license; they may not do so and if they did, you can sue them for license violation. – planetmaker Apr 26 at 21:15
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If your rights to a piece of software derive solely from the GPL -- and this is the usual case for most people who are redistributing GPL software -- then you cannot legally relicense under MIT, BSD, Apache or LGPL, because the GPL does not allow that.

Special cases are possible, of course. If you own the copyright on a piece of software and have not somehow limited your rights to it, then you could simultaneously offer it to some people under the GPL and others under, say, the MIT license -- or more likely, under a commercial license.

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    FTR, what's much more common is to offer it freely under GPL and additionally commercially under a proprietary license. Permissive licenses like MIT and BSD don't really make much sense to combine with others, because they're close to “anybody can do anything” already so nobody would have much interest in choosing the alternative license. – leftaroundabout Apr 27 at 10:55
  • @leftaroundabout Updated answer to reflect your thoughts. – door_number_three Apr 27 at 16:53
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    @leftaroundabout GPL/BSD combo licenses are used for compatibility with other projects. One cannot distribute GPL code under a BSD license, that would be violating the "viral" aspects of the GPL. And one cannot distribute BSD code under a GPL license, that would be adding restrictions BSD licenses do not allow. But a "pick either one" license can work with both. – Schwern Apr 27 at 18:40
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