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Hello to the community,

I am developing a game for windows mainly and want to use QT for a lobby (the game is multiplayer) after that the game will run in another window full screen while the lobby made with QT will still be open. Can I use the open source QT for this commercial game without releasing the source code of my game? And what do I need to do to be legally correct?

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  • \$\begingroup\$ As discussed in this prior Q&A and this one, for a library under an LGPL licence like QT, you can create a stand-alone open source application that uses QT, and that your closed source program invokes as an external resource (which could be redirected to another program fulfilling the same API). Or you can purchase a non-open-source licence. See also this Q&A about Stockfish, which deals with similar issues. \$\endgroup\$
    – DMGregory
    Mar 15, 2022 at 21:02
  • \$\begingroup\$ Thanks for the reply, just to understand, because I get a headache when reading legal stuff... I will make an executable using QT and share it's source code and link my game statically or dynamically with closed source code. Correct? \$\endgroup\$
    – Mike F
    Mar 15, 2022 at 21:25
  • \$\begingroup\$ No, you will not link your game statically or dynamically. The two programs need to communicate "at arm's length", using some form of inter-process communication. It should be possible for a user to take your closed source game and, without access to the source, redirect it to use a new program that fulfills the responsibility of QT instead of the open-source QT-derived program you provided. Once you achieve that kind of interchangeability, you have a reasonably strong claim that the open source QT software is not an inseparable part of your proprietary application. \$\endgroup\$
    – DMGregory
    Mar 15, 2022 at 22:16
  • \$\begingroup\$ Note that typically users on this site, including myself, are not lawyers, and comments here do not constitute legal advice. If you want legal advice about what you can do within the bounds of a software licence, consult a lawyer, especially one who specializes in this area. \$\endgroup\$
    – DMGregory
    Mar 15, 2022 at 22:19
  • \$\begingroup\$ @DMGregory Thanks a lot for your replies. I guess I will consult a lawyer about this \$\endgroup\$
    – Mike F
    Mar 16, 2022 at 9:49

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