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I am considering using QT for a commercial hardware product. The product will have a Raspberry Pi Compute Module and will run standard Raspberry Pi Linux OS. It will load our application, which will dynamically link to standard QT library on the OS.

The issue is that one of the requirements of the LGPL is for us to provide the instructions on how to replace the QT library. We can do that, but it would require the user to kind of disassemble the product and potentially damage the casing to do that.

We don't mind doing that, but would that comply with the license requirements? Do we even have to do it? Can we simply provide the source of the QT library and let the user know that if they were to replace it it would work, and they can do so by changing these files on the RPi.

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  • as a user, I'd be really glad to have these instructions, even if I have to fix the casing with duct tape afterwards. Who cares about a little broken plastic.
    – user253751
    Jun 29 at 9:54
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but it would require the user to kind of disassemble the product and potentially damage the casing to do that.

As far as the LGPL is concerned, if you can have a fully functioning device after updating the Qt libraries, then it doesn't matter to what degree the user needed to damage the device to gain access.

However, there might be other requirements, like product safety, based on which you may want to limit the amount of damage that would be done when opening the case.

But cosmetic damage or a broken "warranty void if broken" seal definitely complies with the LGPL requirements.

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I think you've interpreted the requirements correctly.

When you consider the intent of LGPL, it makes sense: users should have the freedom to replace the library with a better implementation. Without the means to transfer the improved implementation to the device, then they would not have this freedom, so the LGPL really needs to mandate it.

Obviously, it's better if you can provide a connector (e.g. USB type B socket) over which the updates can be transferred without having to disassemble the product. You probably want that for your own field updates, anyway.

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