Using qt for a commercial application on windows (LGPL)

SGaist · 4 Jan 2025, 11:02

Yes, with the software. Also ~~QMainWindow~~ QApplication has an aboutQt method that shows this information.

Jo Jo · 4 Jan 2025, 10:31

@SGaist
I can't find the QMainWindow::aboutQt method in the documentation. Can you provide a link? I found QApplication::aboutQt but it gives us general information about Qt licensing (not specific to LGPL)

One more question: can anyone provide me with a link to a commercial Windows application that uses Qt under the LGPL license? I need such a program as an example

SGaist · J Jo Jo 1 Apr 2025, 10:26

@Jo-Jo My bad it was QApplication::aboutQt

zvoopz · 4 Jan 2025, 11:13

@Jo-Jo said in Using qt for a commercial application on windows (LGPL):

can anyone provide me with a link to a commercial Windows application that uses Qt under the LGPL license? I need such a program as an example

I dont know which type of license does Viber use. I cant find any information about licenses inside this application but I see a bunch of Qt`s dlls here
C:\Users\username\AppData\Local\Viber
No Qt mention as well

Jo Jo · 2 Apr 2025, 11:39

@zvoopz said in Using qt for a commercial application on windows (LGPL):

No Qt mention as well

Probably they are using commercial license but I am interested in example of app which releases under LGPL (no payment for Qt license, but commercial app)

Pl45m4 · 4 Feb 2025, 11:42

@Jo-Jo said in Using qt for a commercial application on windows (LGPL):

I am interested in example of app which releases under LGPL (no payment for Qt license, but commercial app)

I think it might be hard to tell, because to know this, you have to get the source code, which commercial app developers would not publish.
And AFAIK commercial Qt users can link Qt statically while keeping the code hidden. So as "customer" you are not able to tell whether such app even uses Qt.
Meanwhile, commercial LGPL developed apps you also have to buy first to inspect the files that are shipped with it.
(I don't have any apps I paid for, that were developed using Qt)

You could check the Windows App Store. Maybe there you find apps tagged with "Qt" (if there is such feature).
Or look into the Showcase subforum here. Maybe you find links to commercial apps that were developed by open source users here in the forum.

Jo Jo · P Pl45m4 2 Apr 2025, 11:39

@Pl45m4 ok, thank you!

SimonSchroeder · 3 Apr 2025, 06:56

BTW, any app needs to comply with the same rules when using Qt under the GPL or LGPL. It doesn't matter if it's commercial or not. As soon as you distribute a binary (even if it is open source) you need to follow these rules. So, open source apps using Qt will suffice as example. Just note that just because someone else isn't doing something (e.g. not providing a download link for the Qt sources) doesn't mean this is the correct way to do it.

jsulm · 3 Apr 2025, 07:18

@SimonSchroeder said in Using qt for a commercial application on windows (LGPL):

It doesn't matter if it's commercial or not

I disagree with this statement. As far as I know as commercial user you do not have to follow GPL/LPLG as you get Qt under commercial license, not GPL/LGPL (dual licensing).

SGaist · J jsulm 3 Apr 2025, 06:56

@jsulm said in Using qt for a commercial application on windows (LGPL):

@SimonSchroeder said in Using qt for a commercial application on windows (LGPL):

It doesn't matter if it's commercial or not

I disagree with this statement. As far as I know as commercial user you do not have to follow GPL/LPLG as you get Qt under commercial license, not GPL/LGPL (dual licensing).

I think @SimonSchroeder meant that when using Qt under GPL/LGPL, the fact that the application is commercial or open source does not change anything with regard to respecting these licenses constraints.

Jo Jo · 6 Apr 2025, 18:13

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

Pl45m4 · J Jo Jo 6 Apr 2025, 11:48

@Jo-Jo

That's because we aren't lawyers and why you may consult one if you want to be 100% safe.

SGaist · wrote on 6 Apr 2025, 19:01

@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.

SimonSchroeder · J Jo Jo 6 Apr 2025, 11:48

@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).

Jo Jo · 13 Apr 2025, 18:10

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.

Pl45m4 · 14 Apr 2025, 13:57

@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 · 14 Apr 2025, 18:34

@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

Pl45m4 · 15 Apr 2025, 09:13

@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

SGaist · J Jo Jo 14 Apr 2025, 13:57

@Jo-Jo under Windows ? Krita for example. There's a whole lot of KDE based software that are available on Windows.

Jo Jo · 15 Apr 2025, 09:19

@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)