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Do I have to pay money?



  • Hello, can I make a program using qt and post it on github. the program will be free and open source. Do I have to pay money or do I have to do something else for the company that created qt (I have a trial version of qt)?

    [Russian: https://forum.qt.io/topic/122087/должен-ли-я-платить-деньги ~kshegunov]


  • Lifetime Qt Champion

    @Alinwer said in Do I have to pay money?:

    Do I have to pay money

    No. You do not have to pay money if you develop open source app. You just have to make sure you do not violate LGPLv3 license.


  • Qt Champions 2017

    @jsulm said in Do I have to pay money?:

    You just have to make sure you do not violate LGPLv3 license.

    Isn't this automatically fulfilled for OSS?
    Just sayin' ...



  • @jsulm Thanks for the answer, how long is the trial version available?
    And why LGPLv3 and not just GPLv3?



  • @Alinwer
    There is no point using a trial version of you are going to go Open Source. Just download and start using the Open Source version.

    It's LGPL, and not GPL, because that's what its license is. Not much point asking why. Besides, LGPL is more permissive than GPL, so it's not a problem.



  • @JonB Actually, Qt is available both under LGPL and GPL. You can pick which license you want to use. Note that if you pick the GPL your own software also needs to be under the GPL (maybe there are a few corner cases for other open source licenses). With the LGPL you can pick (almost) any other open source license.

    If someone just uses your github repo, I would guess you are done. However, if you start distributing compiled versions of your software you should consider the requirements for making Qt's source code available as well (maybe host it on github as well under your account). I am not a legal expert, though, and never had to consider use of Qt in open source projects before.



  • @SimonSchroeder
    Why would someone choosing Qt Open Source elect GPL over LGPL? Obviously, assuming not using those few Qt components which require GPL not LGPL.



  • @JonB said in Do I have to pay money?:

    Why would someone choosing Qt Open Source elect GPL over LGPL?

    If your own software is already GPL then it doesn't matter. What I said is just that Qt is available under GPL and not just LGPL.


  • Moderators

    @kshegunov said in Do I have to pay money?:

    @jsulm said in Do I have to pay money?:

    You just have to make sure you do not violate LGPLv3 license.

    Isn't this automatically fulfilled for OSS?
    Just sayin' ...

    No. LGPL clearly states a few requirements: your need to ship a copy of the license, you need to inform your users that LGPL Qt is used etc.

    For OSS software this is very easy to fulfill, of course - but still has to be done.


  • Moderators

    @JonB said in Do I have to pay money?:

    @SimonSchroeder
    Why would someone choosing Qt Open Source elect GPL over LGPL? Obviously, assuming not using those few Qt components which require GPL not LGPL.

    Some people like & want GPL. For example if they want to make it hard or impossible to be used in commercial apps. Or they want to make sure all contributions will also be open.


  • Qt Champions 2017

    @sierdzio said in Do I have to pay money?:

    No. LGPL clearly states a few requirements: your need to ship a copy of the license, you need to inform your users that LGPL Qt is used etc.
    For OSS software this is very easy to fulfill, of course - but still has to be done.

    Fair enough!

    @sierdzio said in Do I have to pay money?:

    Or they want to make sure all contributions will also be open.

    I'm pretty sure this also applies to the LGPL.


  • Lifetime Qt Champion

    @kshegunov said in Do I have to pay money?:

    I'm pretty sure this also applies to the LGPL.

    You have to provide the modifications you did to the LGPL dependencies you are using but you are not required to upstream them.


  • Moderators

    @sierdzio said in Do I have to pay money?:

    @JonB said in Do I have to pay money?:

    @SimonSchroeder
    Why would someone choosing Qt Open Source elect GPL over LGPL? Obviously, assuming not using those few Qt components which require GPL not LGPL.

    Some people like & want GPL. For example if they want to make it hard or impossible to be used in commercial apps.

    To add another perspective: We're used to thinking in terms of pricing and obligations for the developers, but the free software movement thinks in terms of freedoms for the end-user. Specifically, freedom to run, study, share, and modify software.

    From that perspective, the GPL guarantees greater freedoms for the end-user compared to the LGPL (L = "Lesser"). If I release my library under the GPL, I'm guaranteeing that all apps that use my library can be studied & modified by the end-user. However, if I release my library under the LGPL, an app that uses my library might be proprietary and can't be studied & modified by the end user.


  • Qt Champions 2017

    @SGaist said in Do I have to pay money?:

    You have to provide the modifications you did to the LGPL dependencies you are using but you are not required to upstream them.

    Hm, okay. I was wrong then.


  • Lifetime Qt Champion

    @kshegunov said in Do I have to pay money?:

    @SGaist said in Do I have to pay money?:

    You have to provide the modifications you did to the LGPL dependencies you are using but you are not required to upstream them.

    Hm, okay. I was wrong then.

    Not completely, upstreaming is the nice thing to do so everybody can benefit from your changes. However, it can happen that some features might not align with a project goal hence they might not be suitable for inclusion.
    You might also be using a library which is working fine but is currently not maintained, etc.


  • Qt Champions 2017

    @SGaist, sure, but I was under the false impression that you're obligated to upstream the changes. Perhaps I was mixing it up with Qt's CLA.



  • Reading this thread I was thinking about the situation like this:

    For whatever reason I want to use GPL version of Qt (for instance because WebAssembly isn't available under LGPL) in my application (which in a sense is "proprietary"). Now, I also use proprietary library which is optional dependency (if not used the program would be usable but could have some usability issues in some specific areas). AFAIK my application needs to be also GPL (or could I use another license???). But from what I've read in the FAQ GPL application must not link to the proprietary library.

    So what are my options?


  • Moderators

    @Trigve said in Do I have to pay money?:

    AFAIK my application needs to be also GPL

    Correct. If your application links to a GPL library, then your application must also be released under the GPL license.

    from what I've read in the FAQ GPL application must not link to the proprietary library.

    Correct. A GPL application can link to libraries whose licenses that are compatible with GPL (see https://www.gnu.org/licenses/license-list.html#GPLCompatibleLicenses for a list of compatible licenses). However, proprietary licenses are not compatible with GPL, so proprietary libraries cannot be used in a GPL application.

    Now, I also use proprietary library which is optional dependency (if not used the program would be usable but could have some usability issues in some specific areas)

    If removing the library creates some usability issues, then I'm not convinced that it's truly optional... See @Chris-Kawa's link below about plugins.

    So what are my options?

    Some options include:

    • Get a commercial license for all the libraries you want to use. This way, you obtain the rights for your application to link to all of those libraries. Note: Using this option, you cannot release your application under a free software license.
    • Convince the owners of the proprietary library to release their library under a GPL-compatible license too. This way, your GPL application can link to it.
    • Replace the proprietary library with a different one which is GPL-compatible. This way, your GPL application can link to it.


  • @JKSH
    Programs can "interact" with each other in a myriad of ways. For example, they might use sockets or files to exchange information. You can mix GPL with commercial/proprietary in this way, because everyone does it.

    So... why can't I/the OP provide a non-linked, on-demand dynamic library for his optional dependency features, which he does LoadLibrary/GetProcAddress-type interface to allow that kind of "interaction"?


  • Moderators

    @JonB said in Do I have to pay money?:

    So... why can't I/the OP provide a non-linked, on-demand dynamic library for his optional dependency features,

    Because dynamic linking is still linking. FSF considers it "cheating" and treats plugins, shared memory etc. to fall under a single combined program definition and thus also require such plugins to be GPL compatible. See source.

    Keep in mind that GPL is aggressively viral and intentionally so and thus trying to "workaround" it in any way is strictly against its idea.


  • Moderators

    @JonB said in Do I have to pay money?:

    @JKSH
    Programs can "interact" with each other in a myriad of ways. For example, they might use sockets or files to exchange information. You can mix GPL with commercial/proprietary in this way

    If the 2 components are truly independent, then sure you can "mix" them this way.

    However, @Trigve's description does not sound like independent components so @Chris-Kawa's point applies.



  • @JKSH said in Do I have to pay money?:

    If the 2 components are truly independent, then sure you can "mix" them this way.

    However, @Trigve's description does not sound like independent components so @Chris-Kawa's point applies.

    In my case the optional dependency is library used for viewing some documents, which isn't necessary altogether, because I could render the document as .pdf and use system viewer (in my case the "proprietary dependency" contains also an executable which encapsulate the library so in this case I could use the .exe instead of library and should be OK, AFAIK).



  • @Trigve said in Do I have to pay money?:

    because I could render the document as .pdf and use system viewer

    Indeed! The sort of thing I had in mind. So perhaps you should indeed do that!


  • Moderators

    @Trigve said in Do I have to pay money?:

    In my case the optional dependency is library used for viewing some documents, which isn't necessary altogether, because I could render the document as .pdf and use system viewer

    Does this mean you've found a way to resolve the "usability issues" you mentioned before, without depending on the proprietary library/application? If so, then I don't see a problem -- You can certainly distribute a GPL app which produces PDF files and launches the system viewer (via the Qt PDF module and QDesktopServices class, I presume?)



  • @JKSH said in Do I have to pay money?:

    Does this mean you've found a way to resolve the "usability issues" you mentioned before, without depending on the proprietary library/application?

    Usability issue is that you either use proprietary viewer with more functionality or else convert to .pdf and use system viewer. What I had in mind was that application could either use or not proprietary library (if one has a license), which should be resolved at build time. But in this case the application cannot be GPL.


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