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Thread: Qt GPL or LGPL Problem

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    Default Qt GPL or LGPL Problem

    GPL or LGPL don't prohibit commercial behaviors. Do GPL or LGPL Qt also?

    On other word, can I write a program which dynamicaly links LGPL Qt, and sell it without distribute my source code?

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    Default Re: Qt GPL or LGPL Problem

    Yes. You must dynamically link to the Qt DLLs. If you build the application linking Qt statically then the result is governed by the LGPL. There are other restrictions, e.g. you cannot prohibit reverse engineering in your licence.

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    Default Re: Qt GPL or LGPL Problem

    Quote Originally Posted by ChrisW67 View Post
    Yes. You must dynamically link to the Qt DLLs. If you build the application linking Qt statically then the result is governed by the LGPL. There are other restrictions, e.g. you cannot prohibit reverse engineering in your licence.
    Thank you.

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    Default Re: Qt GPL or LGPL Problem

    If you build the application linking Qt statically then the result is governed by the LGPL.
    No.
    If you choose the GPL then you can't "sell" your application because you have to ship the sources with the application and everybody has free access to them. So nobody will buy it.
    If you choose LGPL, then you can have your code closed. But all changes to Qt itselft must be made public. And you must link dynamically.
    If you want to link statically you have to buy a commercial licence. (There are some opinions that you can link statically under the LGPL, but I don't think so. In that case you have to ask a real lawer in your country because id differes from country to country...)

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    Default Re: Qt GPL or LGPL Problem

    Thats not how I read the license. You can still sell GPL licensed applications, and don't have to ship your sources with the application. You don't have to put the sources on your website either.

    Secondly, you can't ask for the sources unless you have the binary (Which you are entitled to sell and refuse to give out for free).

    But here's where it all falls down: The first person (and of course others) to buy your application CAN ask for the sources, and you must either send them or provide a link of where to download them. They can then give out or publish these sources under the GPL license.

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    Default Re: Qt GPL or LGPL Problem

    If you dinamically link your proprietary application against a library released under LGPL then you must provide the object code also to allow the user of your application to recompile your application against a modified version of the LGPL-ed library. Please note that I don't have any legal expertise and I could be wrong.

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    Default Re: Qt GPL or LGPL Problem

    There is no need to recompile an application to take advantage of a different version of the library. You can just change the dynamic library and the OS will take care of the rest. The library is linked to at run-time.

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    Default Re: Qt GPL or LGPL Problem

    LGPL licence on Qt libraries, allows you to link them to a non GPL or LGPL program. That program maybe be free or proprietary software, regardless of what licence you have on that particular program you created. So if you want to produce proprietary soft then you need the LGPL licence. This is the main difference of the 2 licences, they refer to how you can use the Qt libraries and affect also your product (if you choose GPL). At least this is what I understand by reading the licences.
    Anyway don't confuse the concept of free software. It has nothing to do whether or not you charge money.
    Last edited by Archimedes; 11th March 2010 at 13:56.
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