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QT Commercial vs Open Source



  • Hello Moderators or whoever used the Commercial Liscenses in QT,

    I understand that this is a repeated question in this forum. I read through the post and unable to come to a firm understanding. Hence, this post is created

    I want to know the Main Differences between the OpenSourced and Commercial License. Maybe links to the comparison tables between the licenses? Or a detailed explanation of what are the essential differences between the opensourced license and Commercial LIcense.


  • Moderators

    @Stevendragoes said in QT Commercial vs Open Source:

    Alright. So, It is possible for me to get a confirmation from the QT sales team that they will not come after me if I released a proprietary software that I developed on my own?

    Ask Qt Company then :-) I'm not their employee. But I can say that if you make sure you don't break LGPL, Qt Company won't have any basis to "chase" you.

    What do I have to worry here? I heard that the Open Source license can be copied by anyone?

    Yes, but LGPL applies only to Qt not to your code. That's why you have to inform your users that you are using Qt and that they can replace Qt with their own version if they want to.

    How can i check my QT license? from the Update Manger?

    If you have installed Qt without paying the commercial license, it is safe to assume you are using the open source version ;-)

    The choice between LGPL and GPL is arbitrary - you choose which license you want Qt to be under (unless you chose some module which is not available under LGPL, like QtCharts, Lottie etc.).

    Please pardon me for being a total ignorant newbie here

    No problem, that's what this forum is for :-)

    EDIT: I think I have to make my software with the source code public for 3 years. Am I right?

    No, this is only about Qt. Your code is not bound by LGPL license (unless you decide to ship your code under LGPL as well).

    So you have to provide Qt source code to your clients if they ask for it. In practice it means you should have a copy of Qt source code somewhere on your PC/ server for 3 years (just in case that official Qt archive goes offline or something). And if you patch Qt (for example, change something in QString code), these patches need to be under LGPL and you have to make them available to your clients.

    That's one more point - to your clients. LGPL does not apply to every human being in existence :-) You don't have to publish Qt and your patches online for everybody. LGPL requires you to do it only for your clients (people using your app).


  • Moderators

    Qt Company's page has a summary of the differences here: https://www.qt.io/download-open-source

    In terms of features - FOSS and Commercial are the same on desktop and mobile platforms. On embedded devices, Qt comes with some extra stuff (base Qt code is the same but there are extra tools provided which make deployment and development easier).

    The main difference - of course - is the license. Most of Qt is available under LGPL, but some modules are GPL only. If you are making an application under a different license (for example, a commercial application), you have to be careful to meet the requirements of LGPL and/or GPL. On some platforms (Android, iOS) this is very hard or even impossible. On others (Windows, Linux) it is quite easy.



  • @sierdzio
    I am developing on desktop platform. Hence, a Free Open Sourced License will do? What if I want to sell my invention/personal project? will QT charge? Because I believe that there exists a commercial license for a reason. Hence, I don't want to make any mistakes.


  • Moderators

    As long as you use LGPL Qt (no GPL modules like QtCharts) and meet LGPL requirements you do not need the commercial license. You can sell your app, no problem there. Even the GPL does not forbid selling of stuff.



  • Alright. So, It is possible for me to get a confirmation from the QT sales team that they will not come after me if I released a proprietary software that I developed on my own?

    What do I have to worry here? I heard that the Open Source license can be copied by anyone?

    How can i check my QT license? from the Update Manger?

    Please pardon me for being a total ignorant newbie here

    EDIT: I think I have to make my software with the source code public for 3 years. Am I right?
    5b30257c-c9c9-492a-a735-d0a0ecc0313b-image.png



  • @Stevendragoes said in QT Commercial vs Open Source:

    Alright. So, It is possible for me to get a confirmation from the QT sales team that they will not come after me if I released a proprietary software that I developed on my own?

    You would have to contact The Qt Company if you really want to ask a question like that. This is a user forum.

    EDIT: I think I have to make my software with the source code public for 3 years. Am I right?

    Where does that come from?



  • @JonB said in QT Commercial vs Open Source:

    Where does that come from?

    column 3 row 3


  • Moderators

    @Stevendragoes said in QT Commercial vs Open Source:

    Alright. So, It is possible for me to get a confirmation from the QT sales team that they will not come after me if I released a proprietary software that I developed on my own?

    Ask Qt Company then :-) I'm not their employee. But I can say that if you make sure you don't break LGPL, Qt Company won't have any basis to "chase" you.

    What do I have to worry here? I heard that the Open Source license can be copied by anyone?

    Yes, but LGPL applies only to Qt not to your code. That's why you have to inform your users that you are using Qt and that they can replace Qt with their own version if they want to.

    How can i check my QT license? from the Update Manger?

    If you have installed Qt without paying the commercial license, it is safe to assume you are using the open source version ;-)

    The choice between LGPL and GPL is arbitrary - you choose which license you want Qt to be under (unless you chose some module which is not available under LGPL, like QtCharts, Lottie etc.).

    Please pardon me for being a total ignorant newbie here

    No problem, that's what this forum is for :-)

    EDIT: I think I have to make my software with the source code public for 3 years. Am I right?

    No, this is only about Qt. Your code is not bound by LGPL license (unless you decide to ship your code under LGPL as well).

    So you have to provide Qt source code to your clients if they ask for it. In practice it means you should have a copy of Qt source code somewhere on your PC/ server for 3 years (just in case that official Qt archive goes offline or something). And if you patch Qt (for example, change something in QString code), these patches need to be under LGPL and you have to make them available to your clients.

    That's one more point - to your clients. LGPL does not apply to every human being in existence :-) You don't have to publish Qt and your patches online for everybody. LGPL requires you to do it only for your clients (people using your app).


  • Lifetime Qt Champion

    @LeLev I think it is about Qt source code, not the code of an application. There is no requirement in LGPL2/3 to make your own source code public when linking against LGPL libs dynamically!


  • Moderators

    @Stevendragoes said in QT Commercial vs Open Source:

    Alright. So, It is possible for me to get a confirmation from the QT sales team that they will not come after me if I released a proprietary software that I developed on my own?

    No, there is no guarantee
    I had been developing with the normal Qt License for about 3 years, and then in a real d*move the Qt Company contacted me and said I had the wrong license and I would own them money.

    IIRC they did not retroactively apply it(at least)



  • @LeLev said in QT Commercial vs Open Source:

    @JonB said in QT Commercial vs Open Source:

    Where does that come from?

    column 3 row 3

    From an unlinked/unreferenced pasted screenshot? That is not what I meant....


  • Moderators

    @JonB said in QT Commercial vs Open Source:

    @LeLev said in QT Commercial vs Open Source:

    @JonB said in QT Commercial vs Open Source:

    Where does that come from?

    column 3 row 3

    From an unlinked/unreferenced pasted screenshot? That is not what I meant....

    It's linked to in my post... https://tldrlegal.com/license/gnu-lesser-general-public-license-v3-(lgpl-3)



  • @sierdzio
    Yep, I now see that. I can only say that picture/table does not come out like that in my (Chrome Firefox) browser!


  • Moderators

    @JonB probably simply "dark mode"



  • @J-Hilk
    Yep, plus the user must have manually expanded every item :) I didn't recognise it! Anyway I now see the 3 year reference, I was not aware of that, thanks.



  • @JonB +expand tabs



  • @LeLev said in QT Commercial vs Open Source:

    @JonB +expand tabs

    This is about as OT as it gets (sorry!), but what does that mean? :confused:



  • @JonB H.Hilk said, "dark mode" and i added "+ expanded tabs"



  • @LeLev
    Got it, sorry, I thought you were trying to tell me something to press in browser to auto-expand all those folded points! :)

    I will be quiet now, and allow OP's discussion to continue if needed. Sorry for interruption.



  • @All thanks for the clarifications. especially @sierdzio I will contact the QT company as well to see which license is right for me



  • Qt will always tell you that it is safest to get the commercial license. There are a few things in the LGPL (especially in back in version 2 previously used by Qt) that have never been fought in court (for any software or library). This is where some uncertainty comes from. And also Qt wants to make money, so they will always tell you to buy the commercial license.

    However, I am still unsure where the 3 years come from. This is not stated anywhere in the LGPL v3. The obligations that the LGPL states are:

    • that you keep a copy of the source code (Qt in this case) that you can provide your clients with (you have to have a copy; it is not sufficient that there is an official download page)
    • that you provide your client with everything he needs to relink your software

    The second point is easily achieved by dynamically linking to Qt (i.e. DLLs). In theory (though this has never been legally tested), you can also statically link with Qt as long as you provide (upon request) the object files of your own software (and in special cases the linker as well) so that your client could relink your object files with a different Qt version. If you comply with this the Qt company can't force you into buying the commercial license.

    I would assume that you need to provide the source code of Qt for the versions of the software you are still distributing. Though I am not a lawyer and so I don't know for sure. Keeping the source of Qt for another 3 years certainly helps legally, but as I said I can't find it in the official license itself.



  • @SimonSchroeder said in QT Commercial vs Open Source:

    Qt will always tell you that it is safest to get the commercial license.

    I would agree with this. Not that I wish to knock them, they are here to make money.

    However, I am still unsure where the 3 years come from. This is not stated anywhere in the LGPL v3.

    You will see I asked this earlier above, because I did not know about it. However, the link given --- https://tldrlegal.com/license/gnu-lesser-general-public-license-v3-(lgpl-3), go to the rightmost Must column, click on the 3rd item down, Disclose Source, it expands to show

    If you distribute this library in an executable, you must make the source available for 3 years.

    I took this to be some official LGPL document when I was referred to it by others above. I now realise it is just some guy's TL;DR. Hence why I questioned where the OP got his screenshot from. However, I also encounter in https://www.oreilly.com/library/view/understanding-open-source/0596005814/ch03.html

    This is the most favored way to make source code available. It requires no additional effort from the distributee and is not time-limited. This is the best way to comply with Section 3 for all but the largest programs.

    b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
    

    so I assume it is indeed somewhere in LGPL?

    But I must admit I am having trouble locating that quoted statement in the actual LGPL. Don't know which version/what source it was taken from....

    Meanwhile, I happenstanced across https://www.slideshare.net/BurkhardStubert/using-qt-under-lgplv3. It has 37-page slideshow of what this guy had to say. I don't know/advocate whether what he says is true, but it might be worth a read through as it's aimed specifically at Qt.


  • Lifetime Qt Champion

    @JonB said in QT Commercial vs Open Source:

    so I assume it is indeed somewhere in LGPL?

    I can't see anything like "3 years" in LGPLv3. So, no idea from where this 3 comes.



  • @jsulm
    Which is why I originally asked that at https://forum.qt.io/topic/111380/qt-commercial-vs-open-source/6 above!

    So far I have found the referenced picture plus one other source which mention the "3 years", but not much else.

    I am having trouble locating the official, full LGPL text. Not summaries, explanations or opinions! Would you be kind enough to paste the exact link for whatever the official text is?


  • Moderators

    @JonB said in QT Commercial vs Open Source:

    @jsulm
    Which is why I originally asked that at https://forum.qt.io/topic/111380/qt-commercial-vs-open-source/6 above!

    So far I have found the referenced picture plus one other source which mention the "3 years", but not much else.

    I am having trouble locating the official, full LGPL text. Not summaries, explanations or opinions! Would you be kind enough to paste the exact link for whatever the official text is?

    This is the official text: https://www.gnu.org/licenses/lgpl-3.0.en.html



  • @sierdzio
    Thank you. Yep, I had looked at that, but I thought it was way too short to be the whole text!? I recalled it was longer than that :)

    If that is the case, there is indeed no mention of "3 years", as I originally suspected :) This is the danger of using third-party, possibly out-of-date, summaries!

    STOPPRESS Ah ha! It was in LGPLv2! See https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html, search for years :) It was also longer than v3 :)


  • Moderators

    @JonB said in QT Commercial vs Open Source:

    @sierdzio
    Thank you. Yep, I had looked at that, but I thought it was way too short to be the whole text!? I recalled it was longer than that :)

    If that is the case, there is indeed no mention of "3 years", as I originally suspected :) This is the danger of using third-party, possibly out-of-date, summaries!

    That's because LGPLv3 is based on GPLv3. There, 3 years are mentioned (https://www.gnu.org/licenses/gpl-3.0.html).

    LGPL text only states that it modifies GPLv3 and is not a standalone license.



  • @sierdzio
    Your latest crossed with my post above where "three years" is indeed mentioned in LGPLv2. Are you saying that this still applies in v3, because we have to take that from GPLv3?


  • Moderators

    @JonB said in QT Commercial vs Open Source:

    @sierdzio
    Your latest crossed with my post above where "three years" is indeed mentioned in LGPLv2. Are you saying that this still applies in v3, because we have to take that from GPLv3?

    I think so.

    Just my musings on why it is there on TL;DR site, I have no idea if it is right.



  • Yes, the LGPL v3 is based on the GPL v3 and only extends on that. This means you need to take the instructions for distributing source code from the GPL. You should understand, that there are different options, though:

    a) just ship the Qt source code with your software
    b) written offer to provide the Qt source code for at least three years or your support period (whatever is longer)
    d) provide your software as download (either free or paid) and also the Qt source code as download from the same place (free of charge)

    These are picked from section 6 of the GPL v3. You see, that you have different options. Though the first might not be really feasible if your software is not at least 1GB in size (as the Qt source is quite large).


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