Would like to use Qt, but am having a license-related predicament
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While I do enjoy using Qt and applaud the addition of LGPL as a licensing option, I'm rather troubled by the clause Digia adds to it, regarding the need to open source if my program uses more than 5% of Qt. At least, that's what I'm interpreting from it.
I do regularly open source my work, but sometimes I'd prefer sharing something closed source, most likely free, with donations encouraged. Is there a way I could make some things of mine closed source without having to open my wallet and still avoid any legal action from Digia?
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Hi,
[quote author="schala" date="1403395435"]I'm rather troubled by the clause Digia adds to it, regarding the need to open source if my program uses more than 5% of Qt.[/quote]Where did you see this, and how did you reach that conclusion?
The common interpretation is that LGPL libraries can be linked to closed-source applications, no problem. (This has not been tested in court though)
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Where did you get the idea about use of "5% of Qt" from? The Qt LGPL addon works precisely the other way: it allows you not to publish the source code if your changes to Qt itself are minimal. Source.
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Sorry, here's where I read it, at the bottom: http://qt-project.org/doc/qt-5/lgpl.html
@Digia Qt LGPL Exception version 1.1
As a special exception to the GNU Lesser General Public License version 2.1, the object code form of a "work that uses the Library" may incorporate material from a header file that is part of the Library. You may distribute such object code under terms of your choice, provided that the incorporated material (i) does not exceed more than 5% of the total size of the Library; and (ii) is limited to numerical parameters, data structure layouts, accessors, macros, inline functions and templates.@
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wow, I did not know about it! You are right, this sounds really scary! Although I have never heard it being used agains anybody, throughout the years.
Header size compared to source size is small. Plus, this says about distributing object files, not executables.
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I should have given my source: "link":https://qt.gitorious.org/qt/qtbase/source/4cb03924c113c74b99e18c7347278600a011e917:LGPL_EXCEPTION.txt.
This is from a file distributed with all copies of Qt source code. Amazingly, the LGPL license text distributed with Qt does not include the Digia exception you have linked to in your previous post. Which is baffling. Right now I am really confused. I think this is a bug of some sort, maybe in the documentation.
I will ask on the dev mailing list.
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I've posted the question, you can follow the discussion (if there will be any) here: "link":http://lists.qt-project.org/pipermail/development/2014-June/017396.html.
I will update this thread if anything new springs up.
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Wow, this is quite a find. I think it is an oversight from the legal/documentation team.
Thank you schala for bringing this up, and thank you sierdzio for researching and escalating this to the dev mailing list.
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According to Thiago (maintainer of QtCore), the one that is found in source code should be followed. See the dev mailing list thread.
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For the record, I will quote Thiago's answer here:
[quote]An exception cannot restrict the rights. Both expand: both are granting you
the right to distribute binaries that incorporate a certain amount of code
from Qt into the binary. That is necessary due to the nature of inline and
template functions -- most C++-based projects carry an LGPL exception like
that.In any case, the one in the source code is the valid one.[/quote]
Hope he does not mind. Mailing lists are public anyway. Source: "link":http://lists.qt-project.org/pipermail/development/2014-June/017397.html.
