Using qt for a commercial application on windows (LGPL)
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Hi! I want to use C++/Qt to implement a closed source desktop application for Windows. I plan to use LGPL as a license.
I read the obligations that must be met at this link https://www.qt.io/licensing/open-source-lgpl-obligations
I want to make sure that I understood everything correctly. As far as I understand, in order to use C++/Qt in a commercial project for Windows with closed source code under the LGPL license, I need to:
- inform the user that the program uses LGPL modules and about the rights of this user (that he can request the source code of LGPL modules, provide his LGPL modules instead of mine, instructions on how to replace LGPL modules with custom ones)
- Link libraries dynamically (use DLL)
- Use only LGPL modules
Is this enough to use Qt for free in a commercial application for Windows so that the license is LGPL and the source code is closed?
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@Jo-Jo Yes, this sounds about right. Just one detail is missing: to enable users to replace LGPL libs with their own it might be required to provide them the object files, so they can relink the application on their own.
Keep in mind: I'm not a lawyer! -
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Would there be precedents? How did they end?
I don't often come across news about court cases regarding LGPL or other open source. Here is one that I was following a few years ago (which ended favorably for the open-source project, to my relief), but this was due to GPL violation, not LGPL: https://stockfishchess.org/blog/2022/chessbase-stockfish-agreement/
If you are interested in such news, you might want to follow (and/or join and support) eff.org or fsf.org (or both!).
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@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
inform the user that the program uses LGPL modules and about the rights of this user (that he can request the source code of LGPL modules, provide his LGPL modules instead of mine, instructions on how to replace LGPL modules with custom ones)
I am not sure what the Qt site mentions explicitly about this. However, there have been several discussions about this in this forum already. IIRC there a two options (slightly different from what you have phrased it) for distributing the source of Qt: 1) You provide a written offer to provide the source code of Qt valid for at least three months (quite close to how you said it, but it includes a written offer). 2) You distribute the source code alongside/in the same way as your own app. If you distribute your app through a CD-ROM (probably not, but trying to make a point here), the same CD-ROM should also contain the source code of Qt. If people can download your app through the internet, you need to provide a download for the source of Qt as well; it is not sufficient to link to some one elses (or the official) repository. It needs to be under your control. [1]
In any case: You need to have a copy of the Qt source code yourself so that you can give it to someone else. It is very unlikely, but theoretically Qt could vanish from the internet one day.
[1] There was recently a library under the MIT license (don't remember the name) where the original creator force pushed an empty repository to all forks of the library and deleted the original repository. He needed to make money from his library and actually this is not against the MIT license. (It is also not against the MIT license for someone who has a local copy of the repository to still create a new public repository with the full history.)
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@SimonSchroeder said in Using qt for a commercial application on windows (LGPL):
It is very unlikely, but theoretically Qt could vanish from the internet one day.
Just a thought:
If TQtC "vanishes" from the Internet probably meaning they stopped all services and selling/distributing their products, there is no one left to blame you for any future license/code violations?!
No TQtC, no license, right?!
If debugging is the process of removing software bugs, then programming must be the process of putting them in.
~E. W. Dijkstra
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@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
No TQtC, no license, right?!
Wrong. Anybody can sue you if you violate an OSS license. As far as I know FSF did this in the past for example.
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@SimonSchroeder said in Using qt for a commercial application on windows (LGPL):
You distribute the source code alongside/in the same way as your own app. If you distribute your app through a CD-ROM (probably not, but trying to make a point here), the same CD-ROM should also contain the source code of Qt. If people can download your app through the internet, you need to provide a download for the source of Qt as well; it is not sufficient to link to some one elses (or the official) repository. It needs to be under your control
Official link says :
Complete corresponding source code of the library used with the application or the device built using LGPL, including all modifications to the library, should be delivered with the application (or alternatively provide a written offer with instructions on how to get the source code)
https://www.qt.io/licensing/open-source-lgpl-obligationsThe most interesting part is "or alternatively provide a written offer with instructions on how to get the source code".
It didn't say that I should provide a link to download the Qt sources. It said that I should provide written instructions on how to get the Qt sources. This instruction could be sending a person to Mars to get the Qt sources, right? :) Or just send a written request by email to get the archive with the source code by email or stuff like this. Or am I wrong?@KH-219Design Thank you!
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@jsulm said in Using qt for a commercial application on windows (LGPL):
Wrong. Anybody can sue you if you violate an OSS license.
Probably, but I meant, if there is no Qt Company anymore... all its licenses are void?!
Because TQtC is the license "holder"?!
Am I wrong?
Or will it continue through the OpenSource / (L)GPL "community"?@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
This instruction could be sending a person to Mars to get the Qt sources, right? :)
I think they had more reasonable ways in mind :D
But interesting take...
Peter Jackson will produce a trilogy on this starting with "An unexpected journey" ;-)
If debugging is the process of removing software bugs, then programming must be the process of putting them in.
~E. W. Dijkstra
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WARNING: I am NOT a lawyer
@Pl45m4 The Qt Company is the main copyright holder (but not for everything, some files have shared copyrights and some have copyright from other parties).
Then there's the CLA that is signed by every contributor (note that the CLA does not require that you transfer copyright of your work to the Qt Company) and finally there's the licenses under which the code is made available.So technically, no, the licenses are not void. For example, you can't claim new licenses for a library (or other software) that has been made available by a now deceased developer.
One final thing is the KDE Free Qt Foundation which protects availability of Qt as Open Source software.
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@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Probably, but I meant, if there is no Qt Company anymore... all its licenses are void?!
IANAL of course!
To my understanding, software licensing is based on copyright law. The law controls if and when a copyright expires and the protected work becomes a free-for-all. For example in the United States it is life + 70 years, so for a painting the painter's estate / inheritors would keep holding the copyright for additional 70 years after death of the original author.
I'm not sure how that exactly works when the copyright holder is a corporate entity, but I suppose that if the Qt Company goes bankrupt the copyright for the parts of the Qt source code that it held will be sold off to someone by the liquidator as part of the company's intellectual property. At any rate the LGPL license is non-revocable, so anyone who got it can keep using it under the same terms even if there is a new owner.
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@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
The most interesting part is "or alternatively provide a written offer with instructions on how to get the source code".
Legally, it doesn't really matter how Qt tries to explain the LGPL. Finally, the source is put under the LGPL and that is the legal document that grants you certain rights. The LGPLv3 refers for a few things to the GPLv3 (https://www.gnu.org/licenses/gpl-3.0.html). Under section 6 there are 5 options how to distribute the source. Here are the three that I tried to refer to:
a) Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by the Corresponding Source fixed on a durable physical medium customarily used for software interchange.
b) Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by a written offer, valid for at least three years and valid for as long as you offer spare parts or customer support for that product model, [...]
d) Convey the object code by offering access from a designated place (gratis or for a charge), and offer equivalent access to the Corresponding Source in the same way through the same place at no further charge. You need not require recipients to copy the Corresponding Source along with the object code. If the place to copy the object code is a network server, the Corresponding Source may be on a different server (operated by you or a third party) that supports equivalent copying facilities, provided you maintain clear directions next to the object code saying where to find the Corresponding Source. Regardless of what server hosts the Corresponding Source, you remain obligated to ensure that it is available for as long as needed to satisfy these requirements.
Especially the last sentence means you cannot rely on Qt hosting the source code on GitHub. You neither can ensure that Qt keeps their repository there nor can you ensure that GitHub keeps existing.
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@SimonSchroeder said in Using qt for a commercial application on windows (LGPL):
Especially the last sentence means you cannot rely on Qt hosting the source code on GitHub. You neither can ensure that Qt keeps their repository there nor can you ensure that GitHub keeps existing.
Do I understand correctly that it is enough to provide a link to download the Qt source code, for example, to the GitHub repository?
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@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
Do I understand correctly that it is enough to provide a link to download the Qt source code, for example, to the GitHub repository?
No, it means the opposite, because you cannot be sure that GitHub will be there al long as you distribute your software. That's why you should have the source code of the Qt version you're using and provide it on request.
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@jsulm said in Using qt for a commercial application on windows (LGPL):
No, it means the opposite, because you cannot be sure that GitHub will be there al long as you distribute your software. That's why you should have the source code of the Qt version you're using and provide it on request.
Should it be a web hosting under my control? Even then, the hosting may stop working. Any Internet resource may eventually close down.
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