static linking Qt with open source version

I'm developing an application with the Qt open source edition. As I don't want to deliver the Microsoft Visual Visual C(++) redistributables, I'd like to use a static-linked version of Qt. The Qt licensing information says that I "should" develop with a commercial Qt license if I want to use static linking in my application, although when I use the configure -static command for building a static Qt library the command prompt asks me if I use the commercial or open source version of Qt.

So it is possible to build Qt with the open source version? Do I need the commercial edition to use static linking? I won't sell the application.

Answers


EDIT April 2016

Actually, I have recently been reading in depth about LGPL, and asking some experts on the subject. Turns out that the inability to use static linking for Qt in closed source applications is more of a cultivated legend and has nothing to do with reality.

What LGPL requires is the possibility for the end user to relink the application against a different version of the library. Using dynamic linking is one way to achieve that, but you can just as easily provide your object files, this way you don't have to open your source and the LGPL requirement is still satisfied.

On the Qt website there are 2 legal FAQs and in neither of them is it directly stated that you can't do it. Just as it is not stated that you can. However, there are at least several instances of implying a vague legal threat in case that you do. I think there is a good explanation for all of those - they can't say that you can't do it without publishing a practical lie which may as well have negative legal repercussions for them, and they are willing to discourage doing that rather than encourage it, as it has the potential to force more people into purchasing a commercial license.

So in short, yes you can, and you most definitely should as lately Qt has become a living deployment hell, plus as of Qt 5.7 in a static build, QML files are tucked neatly in the executable rather than being out on the file system, for anyone to tamper with. Just make sure that:

  • your Qt build only contains modules, licensed under LGPL, and nothing GPL
  • the about section of your app mentions that it is using Qt and contains a link to where you can download the application's object files
  • include all the respective license files with your application

Lastly, your application actually has to be "relinkable", that is, it must be able to work with a compatible library version that provides the necessary functionality. Which means that if you have made modifications to Qt before building it, you must provide those in the form of source code as well, but only the modifications to Qt, not your application's source code.

Update:

Here is an excerpt directly from the GNU FAQ:

For the purpose of complying with the LGPL (any extant version: v2, v2.1 or v3):

(1) If you statically link against an LGPL'd library, you must also provide your application in an object (not necessarily source) format, so that a user has the opportunity to modify the library and relink the application.

That states it pretty clear.


The old, original answer:


It is possible to build Qt statically as long as your application is open-source and you provide the source. If you want to keep your source closed, you either need an expensive commercial license, or you need to use dynamic linking.

BTW using a static build of Qt is pretty nice, for Qt5 I get about 7-8 MB executable with no external dependencies, which is much better than the 20+ MB of additional dll's you have to ship with a dynamically linked app.

For more information, you can take a look at this video: Making the correct license choice when developing with Qt

All in all, can it be done? 100% yes. Should it be done? It depends, for personal/testing/learning purposes it is 100% OK, however if you plan to distribute production grade software, be that commercially or not, open source or not, you'd better first consult with a lawyer. The whole subject is unnecessarily complicated, subject to interpretation, so that the consultation with a lawyer becomes more expensive than a pricey commercial license.


The answer is Yes, if you are willing to open source your application.

According to the Qt-Project's own interpretation of the licenses they use, if you dynamically link to the Qt libraries your application can be either closed source or open. If you statically link, however, your application is subject to the terms of the LGPL.

The exact language the Qt Project uses is this:

In case of dynamic linking, it is possible, but not mandatory, to keep application source code proprietary as long as it is “work that uses the library” - typically achieved via dynamic linking of the library. In case of static linking of the library, the application itself may no longer be “work that uses the library” and thus become subject to LGPL. It is recommended to either link dynamically, or provide the application source code to the user under LGPL.

(http://qt-project.org/legal.html)

The suggestion in some of the other answers that the situation is "not at all clear" is simply untrue - The Qt Project has tried to be abundantly clear about in what circumstances usage of the LGPL license is permissible, and static linking is one of those as long as the app is also LGPL.


Since the original question specifies non-commercial (not necessarily open source), the asker will need to decide whether they can allow distribution under the LGPL (or GPL by extension, as the above page also says "LGPL can be converted to GNU General Public License").


As long as its a closed source application, you need to buy a license from Digia, see Qt: Making the right licensing decision.


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