Using qt for a commercial application on windows (LGPL)
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@Jo-Jo While not using Qt (AFAIK), the firmware from the Garmin GPS uses multiple open source libraries that are listed alongside their licenses for example.
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So, guys, what should I do then? We don't have an example on hand that would prove that all these messages in this topic work effectively in practice. I'm tormented by the question: what if it turns out that following all these rules is not enough to use qt for free? After all, we don't have real examples of anyone who did this and didn't have any problems
wrote on 7 Apr 2025, 07:03 last edited by@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
After all, we don't have real examples of anyone who did this and didn't have any problems
I think we don't have any real example where someone did have problems. It is quite rare that companies get sued over open source licenses. Most of the time it is because they are trying to hide that they are using open source components because they don't want to comply with open source licenses (e.g. tivoization). It is also mostly large companies that get sued because they don't think they have to follow the rules. I don't think you'll get sued if you are trying your best to fulfill open source licenses (though legally speaking "doing your best" is not enough). The major problem in correctly answering your question is that there is no legal precedent how to comply with the LGPL. If you ask Qt they will always tell you that they don't know (it's because there is no legal precedent) and they'll offer you a commercial license to be on the safe side.
I'd always choose the LGPL (even for a commercial application). The way I am using Qt I don't see any advantage in a commercial license. The only good thing about the commercial license is peace of mind. But, it is a little expensive for that (especially if you have multiple developers on the project; even more so, if some developers just occasionally program a small part of the software like two weeks a year).
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wrote on 13 Apr 2025, 13:12 last edited by Jo Jo
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. Only at your own risk or buy a license.
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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. Only at your own risk or buy a license.
wrote on 13 Apr 2025, 18:10 last edited by Pl45m4@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
whether it is possible to use Qt for Windows app with the LGPL license for free and safely or not
100% you can. But you have to follow some rules/comply with the obligations. Everything that was said above.
Still: None of us is a lawyer. Most people speak from their personal experience.Only at your own risk
Almost everything you do (on- and offline) is at your own risk :)
If you mess up, no one else is responsible for it but yourself. You can't blame people here in the forum for that :)
That's why it's tricky to ask for legal advices here. -
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
whether it is possible to use Qt for Windows app with the LGPL license for free and safely or not
100% you can. But you have to follow some rules/comply with the obligations. Everything that was said above.
Still: None of us is a lawyer. Most people speak from their personal experience.Only at your own risk
Almost everything you do (on- and offline) is at your own risk :)
If you mess up, no one else is responsible for it but yourself. You can't blame people here in the forum for that :)
That's why it's tricky to ask for legal advices here.wrote on 14 Apr 2025, 13:57 last edited by Jo Jo@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
100% you can
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
is at your own risk
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Most people speak from their personal experience.
Very interesting :)) Anyway, nobody in this topic can provide some successfully example of LGPL qt usage under Windows
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@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
100% you can
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
is at your own risk
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Most people speak from their personal experience.
Very interesting :)) Anyway, nobody in this topic can provide some successfully example of LGPL qt usage under Windows
wrote on 14 Apr 2025, 18:34 last edited by Pl45m4@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
Very interesting :)) Anyway, nobody in this topic can provide some successfully example of LGPL qt usage under Windows
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Almost everything you do (on- and offline) is at your own risk :)
If you mess up, no one else is responsible for it but yourself. You can't blame people here in the forum for that :)Consult a laywer if you want to be 100% sure.
What I meant above is that what you are planning to do is possible in general, but there are some restrictions and obligations that come with it...
whether you choose to follow them or not is up to you :)Nobody will give you legal advices here in the forum. Nobody else is responsible for your license.
What you need to do depends on your actual case... the "frame" (what you must and can do) was provided in detailed answers by @SimonSchroeder , @IgKh, @jsulm, @KH-219Design and @SGaist -
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
100% you can
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
is at your own risk
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Most people speak from their personal experience.
Very interesting :)) Anyway, nobody in this topic can provide some successfully example of LGPL qt usage under Windows
@Jo-Jo under Windows ? Krita for example. There's a whole lot of KDE based software that are available on Windows.
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@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
Very interesting :)) Anyway, nobody in this topic can provide some successfully example of LGPL qt usage under Windows
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Almost everything you do (on- and offline) is at your own risk :)
If you mess up, no one else is responsible for it but yourself. You can't blame people here in the forum for that :)Consult a laywer if you want to be 100% sure.
What I meant above is that what you are planning to do is possible in general, but there are some restrictions and obligations that come with it...
whether you choose to follow them or not is up to you :)Nobody will give you legal advices here in the forum. Nobody else is responsible for your license.
What you need to do depends on your actual case... the "frame" (what you must and can do) was provided in detailed answers by @SimonSchroeder , @IgKh, @jsulm, @KH-219Design and @SGaistwrote on 15 Apr 2025, 09:13 last edited by Jo Jo@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Consult a laywer if you want to be 100% sure.
How hard is it to find a lawyer with C++ skills? Who would explain the nuances at a programmer level, all the details? And what if the lawyer makes a mistake? Who will be held responsible?
@SGaist said in Using qt for a commercial application on windows (LGPL):
Krita for example
Interesting example, but Krita is open source and free (in my case i am looking for closed source and paid app)
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@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Consult a laywer if you want to be 100% sure.
How hard is it to find a lawyer with C++ skills? Who would explain the nuances at a programmer level, all the details? And what if the lawyer makes a mistake? Who will be held responsible?
@SGaist said in Using qt for a commercial application on windows (LGPL):
Krita for example
Interesting example, but Krita is open source and free (in my case i am looking for closed source and paid app)
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
How hard is it to find a lawyer with C++ skills?
Why do you need a lawyer with C++ skills?! It is not about a programming language.
It is actually simple: you have to fulfill involved OSS licenses. In this case LGPL. Read it and also some explanations like https://www.qt.io/licensing/open-source-lgpl-obligations or https://connect.hyland.com/t5/alfresco-blog/tldr-the-lgpl-license-explained/ba-p/123690 or whatever else you can find.
Nobody here will give you any guarantees! It is your job to make sure you do not violate LGPL (or whatever other OSS licenses are involved).
If you want to write commercial software you should really ask a real lawyer, there are also some specialising on this topic. -
@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Consult a laywer if you want to be 100% sure.
How hard is it to find a lawyer with C++ skills? Who would explain the nuances at a programmer level, all the details? And what if the lawyer makes a mistake? Who will be held responsible?
@SGaist said in Using qt for a commercial application on windows (LGPL):
Krita for example
Interesting example, but Krita is open source and free (in my case i am looking for closed source and paid app)
@Jo-Jo If memory serves well Blackmagic Design uses Qt for some of their software.
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@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
Consult a laywer if you want to be 100% sure.
How hard is it to find a lawyer with C++ skills? Who would explain the nuances at a programmer level, all the details? And what if the lawyer makes a mistake? Who will be held responsible?
@SGaist said in Using qt for a commercial application on windows (LGPL):
Krita for example
Interesting example, but Krita is open source and free (in my case i am looking for closed source and paid app)
wrote on 15 Apr 2025, 09:26 last edited by Pl45m4@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
what if
What if an asteroid strikes the earth tomorrow or a solar flare destroys all IT infrastructure next monday?
That's bs ;-)in my case i am looking for closed source and paid app)
To add to @jsulm 's answer (this is what anybody else would also suggest you)
How do you want to find the sources/files of a closed source app to check what they've done, where they mention their licensing and so on?!
[Edit: they need to mention their (L)GPL license, but you are still not able to look at their code... well because it's not open. In most cases you have the binaries. What strategies they use to fulfill Qt's LGPL part might not be invisible unless you actually buy the program and request their license]IIRC BlackMagic Design as mentioned by @SGaist costs like couple hundred dollars to be able to have full support and to "use" it... I don't know if there is a demo or something.
Also IIRC they also have to provide the objects for re-linking for customers only. And not to anyone who's just curious -
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
How hard is it to find a lawyer with C++ skills?
Why do you need a lawyer with C++ skills?! It is not about a programming language.
It is actually simple: you have to fulfill involved OSS licenses. In this case LGPL. Read it and also some explanations like https://www.qt.io/licensing/open-source-lgpl-obligations or https://connect.hyland.com/t5/alfresco-blog/tldr-the-lgpl-license-explained/ba-p/123690 or whatever else you can find.
Nobody here will give you any guarantees! It is your job to make sure you do not violate LGPL (or whatever other OSS licenses are involved).
If you want to write commercial software you should really ask a real lawyer, there are also some specialising on this topic.wrote on 15 Apr 2025, 09:29 last edited by@jsulm said in Using qt for a commercial application on windows (LGPL):
Why do you need a lawyer with C++ skills?
Because someone should tell me that for example I should link dynamically with Qt libraries or something like that. Compliance with the license terms of the software requires this.
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@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
what if
What if an asteroid strikes the earth tomorrow or a solar flare destroys all IT infrastructure next monday?
That's bs ;-)in my case i am looking for closed source and paid app)
To add to @jsulm 's answer (this is what anybody else would also suggest you)
How do you want to find the sources/files of a closed source app to check what they've done, where they mention their licensing and so on?!
[Edit: they need to mention their (L)GPL license, but you are still not able to look at their code... well because it's not open. In most cases you have the binaries. What strategies they use to fulfill Qt's LGPL part might not be invisible unless you actually buy the program and request their license]IIRC BlackMagic Design as mentioned by @SGaist costs like couple hundred dollars to be able to have full support and to "use" it... I don't know if there is a demo or something.
Also IIRC they also have to provide the objects for re-linking for customers only. And not to anyone who's just curiouswrote on 15 Apr 2025, 09:32 last edited by@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
How do you want to find the sources/files of a closed source app to check what they've done, where they mention their licensing and so on?!
So an LGPL compatible application should indicate somewhere that Qt is used and about the user's rights. This would be enough to understand that the application uses Qt
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@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
How do you want to find the sources/files of a closed source app to check what they've done, where they mention their licensing and so on?!
So an LGPL compatible application should indicate somewhere that Qt is used and about the user's rights. This would be enough to understand that the application uses Qt
@Jo-Jo The link I gave you above says that already: https://www.qt.io/licensing/open-source-lgpl-obligations
"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." -
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
what if
What if an asteroid strikes the earth tomorrow or a solar flare destroys all IT infrastructure next monday?
That's bs ;-)in my case i am looking for closed source and paid app)
To add to @jsulm 's answer (this is what anybody else would also suggest you)
How do you want to find the sources/files of a closed source app to check what they've done, where they mention their licensing and so on?!
[Edit: they need to mention their (L)GPL license, but you are still not able to look at their code... well because it's not open. In most cases you have the binaries. What strategies they use to fulfill Qt's LGPL part might not be invisible unless you actually buy the program and request their license]IIRC BlackMagic Design as mentioned by @SGaist costs like couple hundred dollars to be able to have full support and to "use" it... I don't know if there is a demo or something.
Also IIRC they also have to provide the objects for re-linking for customers only. And not to anyone who's just curiouswrote on 15 Apr 2025, 09:35 last edited by@Pl45m4 said in Using qt for a commercial application on windows (LGPL):
unless you actually buy the program and request their license
In most cases paid software give you trial version and you can see everything what you want, this is not a problem
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@Jo-Jo The link I gave you above says that already: https://www.qt.io/licensing/open-source-lgpl-obligations
"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."wrote on 15 Apr 2025, 09:36 last edited by@jsulm said in Using qt for a commercial application on windows (LGPL):
The link I gave you above says that already
This is the interpretation of Qt company, not a legal statement.
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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).
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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 16 Apr 2025, 17:49 last edited by@JKSH said in Using qt for a commercial application on windows (LGPL):
A bit late to the party!
The members of this forum recommend contacting a lawyer to be 100% sure that all LGPL requirements are met, while you recommend listening to a random person from YouTube. So who is right? You have completely confused me.
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@JKSH said in Using qt for a commercial application on windows (LGPL):
A bit late to the party!
The members of this forum recommend contacting a lawyer to be 100% sure that all LGPL requirements are met, while you recommend listening to a random person from YouTube. So who is right? You have completely confused me.
@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
The members of this forum recommend contacting a lawyer to be 100% sure that all LGPL requirements are met, while you recommend listening to a random person from YouTube. So who is right? You have completely confused me.
- If you want to obtain in-depth understanding on how to use LGPL'ed software in your project (and why each step is needed), watch the video
- If you want to obtain strong legal protection for your project, talk to a lawyer
These paths are not mutually exclusive. They serve different purposes (although they do overlap); both are valuable.
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@Jo-Jo said in Using qt for a commercial application on windows (LGPL):
The members of this forum recommend contacting a lawyer to be 100% sure that all LGPL requirements are met, while you recommend listening to a random person from YouTube. So who is right? You have completely confused me.
- If you want to obtain in-depth understanding on how to use LGPL'ed software in your project (and why each step is needed), watch the video
- If you want to obtain strong legal protection for your project, talk to a lawyer
These paths are not mutually exclusive. They serve different purposes (although they do overlap); both are valuable.
wrote on 17 Apr 2025, 10:28 last edited by Jo Jo@JKSH said in Using qt for a commercial application on windows (LGPL):
These paths are not mutually exclusive.
Thank you anyway, but i don't agree with you. If i am already have in-depth understanding on how to use LGPL'ed software in my project, i don`t need some legal protection and vice versa
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