Using qt for a commercial application on windows (LGPL)
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@Jo-Jo I'm not sure what kind of answer you expect to get in a user forum like this? Nobody here is a lawyer.
wrote on 18 Apr 2025, 10:41 last edited by@jsulm said in Using qt for a commercial application on windows (LGPL):
I'm not sure what kind of answer you expect to get in a user forum like this?
I wonder why some people advise to contact a lawyer while others advise to listen to certain people on the internet. Who is right?
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A bit late to the party!
The most comprehensive and useful presentation that I know on this topic is one from Burkhard Stubert, entitled "Making Qt Systems Comply To LGPL Version 3" https://www.youtube.com/watch?v=bwTlCBbB3RY It also mentions a few cases towards the end where companies did not comply properly at first.
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
After all the messages, unfortunately, I still couldn't get a concrete answer about whether it is possible to use Qt for Windows app with the LGPL license for free and safely or not.
The presentation above discusses how to do it safely on embedded systems. Everything that you learn there can be applied to Windows too (and it's arguably even simpler on desktop systems).
wrote on 18 Apr 2025, 11:29 last edited by@JKSH said in Using qt for a commercial application on windows (LGPL):
Now i am watching this video and can`t understand this detail: At the 6:30, video says: "display copyright notices of Qt libraries and license texsts lf LGPLv3 and GPL in Application's GUI (4c)".
- I am familiar with LGPLv3 and GPL text, but where i can found "copyright notices of Qt libraries"?
- What does 4c mean?
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@JonB said in Using qt for a commercial application on windows (LGPL):
If you have "in-depth understanding on how to use LGPL'ed software" then you already have your answers.
No, i dont have "in-depth understanding on how to use LGPL'ed software". I think you've lost the context of this phrase.
@JonB said in Using qt for a commercial application on windows (LGPL):
Nobody here is going to give you any cast iron guarantees about interpretations, legal issues, how The Qt Company might or might not act, or similar.
I understand that. The main thing for me is to understand how to approach this issue correctly. Some users of this forum suggest listening to what some people say, others recommend contacting a lawyer. There is no standard way, and there is no example of successful use of LGPL for commercial closed source software for Windows.
@Jo-Jo I pointed you to Blackmagic Design.
As for who is right ? Both are. Inform yourself with people having real experience in managing software and projects using open source licenses and consult a lawyer to ensure you are doing things correctly. Especially when working in fields like medical.
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@Jo-Jo I pointed you to Blackmagic Design.
As for who is right ? Both are. Inform yourself with people having real experience in managing software and projects using open source licenses and consult a lawyer to ensure you are doing things correctly. Especially when working in fields like medical.
wrote on 18 Apr 2025, 14:42 last edited by@SGaist said in Using qt for a commercial application on windows (LGPL):
As for who is right ? Both are. Inform yourself with people having real experience in managing software and projects using open source licenses and consult a lawyer to ensure you are doing things correctly
I got you, thanks! But I don't really understand what I should talk about with a lawyer. If, for example, I ask him whether it is necessary to link dynamically with Qt libraries, he probably won't understand me, because he most likely doesn't know programming. How should I talk to him then?
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@SGaist said in Using qt for a commercial application on windows (LGPL):
As for who is right ? Both are. Inform yourself with people having real experience in managing software and projects using open source licenses and consult a lawyer to ensure you are doing things correctly
I got you, thanks! But I don't really understand what I should talk about with a lawyer. If, for example, I ask him whether it is necessary to link dynamically with Qt libraries, he probably won't understand me, because he most likely doesn't know programming. How should I talk to him then?
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There are law firms specialized on IT/software development, licensing, eCommerce, data privacy etc...
You don't talk to a lawyer's office that would also help you with your divorce ;-)wrote on 19 Apr 2025, 11:13 last edited by@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
There are law firms specialized on IT/software development, licensing, eCommerce, data privacy etc...
Thank you!
https://www.youtube.com/watch?v=bwTlCBbB3RY
This video says to "display copyright notices of Qt libraries and license texsts lf LGPLv3 and GPL in Application's GUI". Where can I find copyright notices of Qt libraries? -
@JKSH said in Using qt for a commercial application on windows (LGPL):
Now i am watching this video and can`t understand this detail: At the 6:30, video says: "display copyright notices of Qt libraries and license texsts lf LGPLv3 and GPL in Application's GUI (4c)".
- I am familiar with LGPLv3 and GPL text, but where i can found "copyright notices of Qt libraries"?
- What does 4c mean?
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
@JKSH said in Using qt for a commercial application on windows (LGPL):
Now i am watching this video and can`t understand this detail: At the 6:30, video says: "display copyright notices of Qt libraries and license texsts lf LGPLv3 and GPL in Application's GUI (4c)".
- I am familiar with LGPLv3 and GPL text, but where i can found "copyright notices of Qt libraries"?
- What does 4c mean?
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See here for a definition of a "copyright notice": https://copyrightalliance.org/faqs/what-is-copyright-notice/
Then, in a Qt Widgets application, callQApplication::aboutQt()and you'll see this line near the bottom of the dialog: "Copyright (C) The Qt Company Ltd. and other contributors." -- just add the year of release and you'll have a notice that meets the definition given in the link above. -
"(4c)" in the presentation slide refers to section 4c of the actual LGPLv3 license (visit https://www.gnu.org/licenses/lgpl-3.0.en.html and scroll down to "4. Combined Works"). Ditto for the other tags in the slide ("4a", "4d1", etc.)
The presenter is rephrasing/simplifying/summarizing different parts of the license to make them easier to understand. He's citing the original parts so that viewers can check the actual license (for example, so that you can satisfy yourself that his statements align with the actual license)
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@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
@JKSH said in Using qt for a commercial application on windows (LGPL):
Now i am watching this video and can`t understand this detail: At the 6:30, video says: "display copyright notices of Qt libraries and license texsts lf LGPLv3 and GPL in Application's GUI (4c)".
- I am familiar with LGPLv3 and GPL text, but where i can found "copyright notices of Qt libraries"?
- What does 4c mean?
-
See here for a definition of a "copyright notice": https://copyrightalliance.org/faqs/what-is-copyright-notice/
Then, in a Qt Widgets application, callQApplication::aboutQt()and you'll see this line near the bottom of the dialog: "Copyright (C) The Qt Company Ltd. and other contributors." -- just add the year of release and you'll have a notice that meets the definition given in the link above. -
"(4c)" in the presentation slide refers to section 4c of the actual LGPLv3 license (visit https://www.gnu.org/licenses/lgpl-3.0.en.html and scroll down to "4. Combined Works"). Ditto for the other tags in the slide ("4a", "4d1", etc.)
The presenter is rephrasing/simplifying/summarizing different parts of the license to make them easier to understand. He's citing the original parts so that viewers can check the actual license (for example, so that you can satisfy yourself that his statements align with the actual license)
wrote on 20 Apr 2025, 13:23 last edited byThank you so much!
@JKSH said in Using qt for a commercial application on windows (LGPL):
just add the year of release and you'll have a notice that meets the definition given in the link above.
QApplication::aboutQt()Shows following text: "Copyright (C) 2023 The Qt Company Ltd and other contributors."
Should I leave 2023 or should I specify the year that was in effect when the current version of the app was released?
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Thank you so much!
@JKSH said in Using qt for a commercial application on windows (LGPL):
just add the year of release and you'll have a notice that meets the definition given in the link above.
QApplication::aboutQt()Shows following text: "Copyright (C) 2023 The Qt Company Ltd and other contributors."
Should I leave 2023 or should I specify the year that was in effect when the current version of the app was released?
You're most welcome!
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
QApplication::aboutQt()Shows following text: "Copyright (C) 2023 The Qt Company Ltd and other contributors."
Should I leave 2023 or should I specify the year that was in effect when the current version of the app was released?
Depending on the version of Qt that you're using, 2023 might be incorrect: https://bugreports.qt.io/browse/QTBUG-121906
Personally, I would use the year that the particular version of Qt was released (because this particular copyright notice is about Qt, not about your app)
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You're most welcome!
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
QApplication::aboutQt()Shows following text: "Copyright (C) 2023 The Qt Company Ltd and other contributors."
Should I leave 2023 or should I specify the year that was in effect when the current version of the app was released?
Depending on the version of Qt that you're using, 2023 might be incorrect: https://bugreports.qt.io/browse/QTBUG-121906
Personally, I would use the year that the particular version of Qt was released (because this particular copyright notice is about Qt, not about your app)
wrote on 20 Apr 2025, 14:28 last edited by@JKSH said in Using qt for a commercial application on windows (LGPL):
Personally, I would use the year that the particular version of Qt was released (because this particular copyright notice is about Qt, not about your app)
I got you, thanks again!
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wrote on 22 Apr 2025, 10:29 last edited by
Sorry, i have another question. In this video https://www.youtube.com/watch?v=bwTlCBbB3RY at the 18:20 he talking about "Displaying License and Copyright in GUI": It looks like for each module I need to specify the copyright (and not just the Qt copyright, but all the copyrights from all the source files in the module), the GPL/LGPL license text, modifications (if any), the used-license (what is that? Should we just show the LGPLv3 text when clicking on this menu or something else?). So I need to duplicate this information for each Qt module I will use and it is not enough to provide the common text for all modules once?
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wrote 28 days ago last edited by
Can anyone answer?
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A bit late to the party!
The most comprehensive and useful presentation that I know on this topic is one from Burkhard Stubert, entitled "Making Qt Systems Comply To LGPL Version 3" https://www.youtube.com/watch?v=bwTlCBbB3RY It also mentions a few cases towards the end where companies did not comply properly at first.
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
After all the messages, unfortunately, I still couldn't get a concrete answer about whether it is possible to use Qt for Windows app with the LGPL license for free and safely or not.
The presentation above discusses how to do it safely on embedded systems. Everything that you learn there can be applied to Windows too (and it's arguably even simpler on desktop systems).
wrote 26 days ago last edited by@JKSH said in Using qt for a commercial application on windows (LGPL):
The most comprehensive and useful presentation that I know on this topic is one from Burkhard Stubert, entitled "Making Qt Systems Comply To LGPL Version 3" https://www.youtube.com/watch?v=bwTlCBbB3RY It also mentions a few cases towards the end where companies did not comply properly at first.
In that vein, Burhard's blog post on the Qt license might be relevant here: https://burkhardstubert.substack.com/p/do-not-sign-the-qt-license-agreement
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@JKSH said in Using qt for a commercial application on windows (LGPL):
The most comprehensive and useful presentation that I know on this topic is one from Burkhard Stubert, entitled "Making Qt Systems Comply To LGPL Version 3" https://www.youtube.com/watch?v=bwTlCBbB3RY It also mentions a few cases towards the end where companies did not comply properly at first.
In that vein, Burhard's blog post on the Qt license might be relevant here: https://burkhardstubert.substack.com/p/do-not-sign-the-qt-license-agreement
wrote 25 days ago last edited by@Asperamanca said in Using qt for a commercial application on windows (LGPL):
In that vein, Burhard's blog post on the Qt license might be relevant here: https://burkhardstubert.substack.com/p/do-not-sign-the-qt-license-agreement
Found another one of his posts: https://burkhardstubert.substack.com/p/episode-29-better-built-by-burkhard
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Sorry, i have another question. In this video https://www.youtube.com/watch?v=bwTlCBbB3RY at the 18:20 he talking about "Displaying License and Copyright in GUI": It looks like for each module I need to specify the copyright (and not just the Qt copyright, but all the copyrights from all the source files in the module), the GPL/LGPL license text, modifications (if any), the used-license (what is that? Should we just show the LGPLv3 text when clicking on this menu or something else?). So I need to duplicate this information for each Qt module I will use and it is not enough to provide the common text for all modules once?
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
the used-license (what is that? Should we just show the LGPLv3 text when clicking on this menu or something else?).
Keep the video running a tiny bit longer and he shows you an example (~20:00). Notice that the "used-license" view shows some Qt-specific text, that is not part of the LGPLv3 license text.
So I need to duplicate this information for each Qt module I will use and it is not enough to provide the common text for all modules once?
First, you must realize that Qt is not a single library; it consists of many libraries (or "modules"/"packages"). And the principle is: Each library that you use in your project should be clearly "attributed" (unless their license allows you to avoid attributing them).
The other important thing to realize is that different libraries in Qt can be offered under different licenses. For example:
- Qt Multimedia is offered under LGPLv3 OR GPLv3 OR Commercial
- Qt Graphs is only offered under GPLv3 OR Commercial (no LGPL)
Finally, the screens that Burkhard shows are just one example of how to arrange the license and copyright information (it's optimized for embedded devices). To see some examples optimized for desktop apps, open Mozilla Firefox and enter "about:license" as the URL, and open Google Chrome/Microsoft Edge and enter "chrome://credits/" as the URL. Notice that the Firefox arrangement does not require duplicating license text, while Chrome's does.
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@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
the used-license (what is that? Should we just show the LGPLv3 text when clicking on this menu or something else?).
Keep the video running a tiny bit longer and he shows you an example (~20:00). Notice that the "used-license" view shows some Qt-specific text, that is not part of the LGPLv3 license text.
So I need to duplicate this information for each Qt module I will use and it is not enough to provide the common text for all modules once?
First, you must realize that Qt is not a single library; it consists of many libraries (or "modules"/"packages"). And the principle is: Each library that you use in your project should be clearly "attributed" (unless their license allows you to avoid attributing them).
The other important thing to realize is that different libraries in Qt can be offered under different licenses. For example:
- Qt Multimedia is offered under LGPLv3 OR GPLv3 OR Commercial
- Qt Graphs is only offered under GPLv3 OR Commercial (no LGPL)
Finally, the screens that Burkhard shows are just one example of how to arrange the license and copyright information (it's optimized for embedded devices). To see some examples optimized for desktop apps, open Mozilla Firefox and enter "about:license" as the URL, and open Google Chrome/Microsoft Edge and enter "chrome://credits/" as the URL. Notice that the Firefox arrangement does not require duplicating license text, while Chrome's does.
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