Question about GPL compliance for dynamic linking in a proprietary app

[fyh333]

Hi Stockfish developers,

I want to ask about one specific GPL compliance scenario.

If a developer dynamically links Stockfish into a closed-source app and distributes that app to users, but does not release the full app source code under GPL, would the Stockfish project consider that compliant?

I’m not asking for legal advice, only for the project’s general interpretation.

Part of the reason I’m asking is that, when looking at commercial chess products from the outside, it is often hard to tell whether Stockfish is being used only on the server side, shipped as a separate component, or linked directly into a client app. For example, with products like Chess.com, it is not obvious to an outside developer which parts, if any, would be expected to be open-sourced. I’m not asking you to judge any specific company, only to clarify the general boundary so I can avoid building something in a way the project would consider non-compliant.

Thanks.

[vondele]

You are asking for legal advise, if you're asking for the interpretation.

A person distributing Stockfish, as a whole or in parts, should comply with the GPL. If it is not clear to the user that an app contains stockfish that distribution is not compliant, as that app should have presented the user with the GPL license covering SF.

SF can be called as a program by other programs of any license, SF can only be linked into programs of GPL licensed programs.

[fyh333]

Thank you for the clear explanation. That makes perfect sense regarding the linking boundary.

My challenge is specifically with iOS, which strictly prohibits spawning separate child processes (like using fork() or exec()), making standard UCI communication via OS pipes technically impossible on-device.

To respect the 'separate program' boundary you mentioned while adhering to iOS sandbox constraints, I am considering an alternative architecture: Compiling Stockfish into WebAssembly (WASM) and running it inside an isolated Web View (WKWebView) within the app. The host app would only communicate with the WASM engine via string-based message passing (simulating standard I/O), with no direct code linking.

In the project's view, would running Stockfish in a separate WASM/JS sandbox and communicating via message passing qualify as calling it 'as a separate program', or would this still be interpreted as linking?

[hitsthings]

Thanks for addressing this. I came to ask the same question. I was looking at the price tag for chess.com and wondering how far I could get building my own with AI, found out stockfish is GPL and seems to be used by chess.com, and started wondering if chess.com itself needed to be open-source!