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What happens if I take a small piece of code (~400 lines) that is licensed under GPLv2, and modify and use it in my own project? Am I now obliged to license my program under GPL, or can I use a more permissive license like WTFPL, or do I have to license that particular file that has the GPL-ed code in GPL, and all other files in other license?

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You are only allowed to use the code and distribute it and its derivatives under the GPL unless you have permission of all authors to re-license it. That is true for a part of the code, too: deleting one half does not make the remaining half magically something not derived from the whole. A license applies to each line of code unless it says otherwise explicitly.

Thus if you want to take part of a GPL-licensed project to incorporate it into your own, you become bound by the GPL as a whole, if you compile it into one binary or they work together closer than "at arm's length" (that is closer than two separate programmes talking to eachother over a well-defined public API).

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  • By "the code" you mean the one file or my whole project? – MetroWind Jun 30 at 0:44
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    I amended a last paragraph to answer it also in that respect. – planetmaker Jun 30 at 8:35
  • Thanks you for the explanation! – MetroWind Jul 1 at 22:12
  • Well defined API is not enough. If the GPLed code is modified to depend on the other part, then there is one part. (the separation did not happen). – ctrl-alt-delor Jul 4 at 9:22

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