Q: Regarding the GPL v3 - Where is the line when software using QT needs to be released under the GPL v3?
An example: I have two, entirely separate pieces of software I wrote. One is a closed source, proprietary, compiled C++ program; It hosts a web server that, when connected to, allows two users to play tic tac toe with each other over the internet, or allow a single user to play against a bot. This software by its self has no way of displaying anything, its simply a server that handles the logic of accepting connections, recieving user input, and sending information about game state to connected users.
The other software is open-source. It allows users to input an IP address to connect to the other, closed-source tic-tac-toe server. It can launch and connect automatically to a copy of the tic-tac-toe server software to play against a bot. Its primary function is to connect to the tic-tac-toe server, send the server user inputs, and display the game-state to the user. This open-source software uses QT via the GPL v3. In what scenarios would the closed-source software violate the GPL?
A:
In the scenario you've described, the key factor determining whether the closed-source software violates the GPL v3, due to its interaction with the open-source Qt-based client, hinges on the method of communication and integration between the two pieces of software. The GPL v3 is designed to ensure that software is free and can be modified by users, but it also respects the boundaries between separate programs.
If your closed-source server and the open-source Qt client communicate over a network without sharing code or linking at the binary level, your server likely wouldn't be considered a derivative work of the Qt client under the GPL v3. This is because network communication, especially when using common protocols or APIs, is generally seen as interaction between separate works. The GPL's requirements typically don't extend to software merely communicating with GPL-licensed software over a network.
However, if there's any form of code sharing, direct linking, or if the closed-source software relies on the Qt client in a way that it becomes a combined work or derivative of the GPL-licensed software, then the situation changes. In such cases, the GPL's requirements for open sourcing the derivative work would apply. It's essential to keep the two pieces of software functionally independent and ensure that any communication between them could reasonably be seen as interaction between separate works to avoid GPL infringement.
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