Firstly, you have to note that all Qt open source modules are not available under LGPLv3 license. Some modules are available under GPLv3 for open source usage, and e.g. 3rd party components such as Chromium web engine are provided under LGPLv2.1.
When Qt modules and libraries under the LGPLv3, there are a couple of obligations you will need to fulfill:
When using Open source software you need to be aware of and track the licensing of each sample, source code sniplet, module and library you use in your project.
You will need to deliver the complete source code of Qt libraries you used, including all modifications you did or applied, to your users/customers. Alternatively, you need to provide a written offer with instructions on how to get the source code.
Please also note that this has to be under your control, so a link to the source code provided by the Qt Project or Qt Company is not sufficient.
The LGPL allows you to keep your own source code of your application private as long as it is “work that uses” the library. Dynamic linking is usually recommended here.
The user of your application has to be able to re-link your application against a different or modified version of the Qt library. With LGPLv3 it is also explicitly stated that the user needs to be able to run the re-linked binary on its intended target device. It is your obligation to provide the user with all necessary tools to enable this process. For embedded devices, this includes making the full toolchain used to compile the library available to users. For parts licensed under LGPLv3 you are obliged to provide full instructions on how to install the modified library on the target device (this is not clearly stated with LGPLv2.1, although running the application against the modified version of the library clearly is the stated intention of the license).
The user of an application or device using LGPL licensed software has to be notified of their rights by providing a copy of the LGPL license to the end user and displaying a prominent notice about your usage of LGPL licensed software.
The above freedoms cannot in any way be restricted by other licensing terms. If an application is not fully following all the obligations as outlined above, it is not allowed to be distributed at all.
