Qt Frequently Asked Questions

Free Edition

  1. What is the Free Edition?

    The Free Edition is the Qt library for UNIX/X11 for development of free software. It includes the complete source code.

    You may use it free of charge as long as you adhere to the Qt Free Software License. This basically says that you must make your source code available to the users of your software, and you must give them the rights to change and redistribute your code.

  2. Can I use the Free Edition at work for running this Qt-based application I just downloaded?

    Yes. You only need the Professional Edition for developing software, not for running it.

  3. Why is Qt not distributed under the GNU General Public License (GPL)?

    This would create problems both for commercial and free software users of Qt:

    1. The GPL permits anybody to modify and redistribute a GPL'ed library. This would create a confusing situation for all users, who would have to relate to many different, perhaps incompatible, versions of Qt (we put great emphasis on avoiding the sort of version problems Tcl/Tk has). Please also see the more elaborate explanation under the question "Why can't I distribute modified versions of Qt?" below.

    2. All free software developers using a GPL'ed Qt would have to release their software under GPL. It would be illegal for them to release it under a different free license, e.g. the BSD license.

    3. A GPL'ed library would be practically unusable for commercial purposes, because the GPL requires that all software written with it must be released as free software (GPL'able).

    With Qt's licensing scheme, we try to make the best for both worlds: You can use it free of charge if you want to develop free software, or you can purchase the Professional Edition to make commercial or proprietary software. With a GPL'ed library, the only thing you are allowed to develop is GPL'able software. In this way, Qt gives you a freedom which a GPL'ed library does not.

  4. Why is Qt not distributed under the GNU Library General Public License (LGPL)?

    The LGPL is designed to "permit developers of non-free programs to use free libraries" (quote from the LGPL). In other words, if Qt were LGPL'd, companies would not have to buy the Professional Edition in order to make commercial (non-free) software, they could just use the Free Edition, free of charge. That would mean Troll Tech would not get the revenue necessary for improving and extending Qt.

    Also, item #1 under the question Why is Qt not distributed under the GNU Public License (GPL)? above applies to the LGPL also.

  5. Why is Qt not distributed under GPL/LGPL and some other license, like Perl is?

    The reasons for why we for Qt Free Edition have our own Qt Free Software License instead of using GPL or LGPL are explained in the two previous questions: Item #1 under the question Why is Qt not distributed under the GNU Public License (GPL)? above applies even in a situation with double licensing; and for LGPL, the situation is the same.

  6. Why are old versions of the Qt library not distributed under GPL or LGPL, like Ghostscript is?

    Item #1 under the question Why is Qt not distributed under the GNU Public License (GPL)? above applies to this question also.

    Other important reasons:

  7. Is software based on the Qt Free Edition really free? Does it carry Troll Tech license restrictions?

    Yes, it is really free. No, there are no special Troll Tech license restrictions on free software produced using the Free Edition. In fact, the opposite is true: The Qt Free Software License demands that the software must be free. The users must have the rights to obtain the source code, change it, and redistribute it. It is only Qt itself that may not be changed.

  8. What if Troll Tech stops releasing Qt under the Free Software License?

    We will not discontinue our strategy of releasing Qt free of charge for development of free software.

    If you do not take our word for it, think a moment about what would happen if we were to do this: Our reputation among thousands of developers world wide would be irreparably damaged in an instant. People would start building Qt clones. Our business would be seriously hurt. In short, we are not at all considering doing this.

    If you are still worried, consider this: Hypothetically, if this were to happen, the Qt Free Software License guarantees that the last free version of Qt could be used indefinitely free of charge, for running and developing free software. Furthermore, the free software community would be able to fix bugs and continue development of this library: using C++ inheritance, workarounds, encapsulation, alternative widgets etc., the license's requirement of not changing Qt source code can be honoured. Many existing Qt free software applications do this already; see KDE, for example. It is our experience that this can be an effective strategy; for the large majority of the bugs that have been reported for various version of Qt, we have been able to suggest satisfactory workarounds.

  9. Using the Free Edition, can I write software for internal use in my company/organization?

    Typically, no. If you want to use the free license within an organization, your software must be usable outside your organization (if anyone should want to use it), you must notify Troll Tech before starting to use it, you have to allow free distribution and you have to make your source code available. (Naturally, if you purchase the Professional Edition, these restrictions do not apply.)

    It is our policy that when you are using Qt for free, you should in return contribute to the free software community.

  10. Does your licensing allow Qt be put on a Linux distribution (CD/ftp archive etc.) free of charge?

    Yes. Some Linux distributions already contain Qt. Anyone can redistribute the free edition of Qt, for free or for pay, as long as they include the whole archive.

  11. Can I write software under the GNU General Public License, GNU Library General Public License or BSD licenses using the Qt Free Edition?


  12. Really? Your additional requirements seem to conflict with the GPL.

    Really yes. It is a parallel situation to that of Motif, for example. Motif is non-free software, still there exists many GPL'ed applications using it.

  13. I want to use Qt to develop free software on Windows.

    The Free Edition is not available on Windows. You have two options:

  14. Why don't you release patches for the Free Edition of Qt?

    Among our main reasons for this policy are:

    We believe that our solution is better for everybody in the long run.

  15. Can I distribute dynamically linked applications that use Qt? Can I distribute statically linked applications that use Qt?

    Yes and yes.

  16. Is there a shareware license for Qt?

    No. We used to offer one, but there was far too low demand to justify the administration related to it.

  17. I need to modify Qt.

    Are you sure? Using C++ inheritance is very often a better alternative. If you do change Qt, you may not distribute the modified toolkit or programs using it (as long as you're using the Free Edition).

    We encourage you to send patches to qt-bugs@troll.no. We will try to evaluate the patches and reply as soon as they come in. If we like what we see, we'll incorporate it in the next Qt release.

  18. Why can't I distribute modified versions of Qt? I want to distribute my application quickly, not wait for you to make a new Qt release.

    There are many good reasons why we do not permit distribution of modified versions of the Qt library:

    Please note that experience shows that both the latter two points apply even if the re-distribution were to contain some "README" file that explained that this were a non-Troll Tech, modified distribution.

    We believe both users and developers of Qt-based applications are better served with our policy in the long run.

  19. I heard someone speculate that you are going to port free Qt-based software to MS Windows and make money by selling it?

    This is absurd. If anyone thinks there's a lot of money in this, he/she can just go ahead and do it him/herself.

    But remember, most of the software is GPL'ed, so anyone could buy the windows version once and start selling/distributing it themselves. And the software could be sold statically linked, so this would not generate any additional Qt license sales for us either.

    In short, we have no such plans. Those speculations seem to be made to spread FUD (Fear, Uncertainty, and Doubt).

  20. How can I distribute my cool free widget library based on Qt?

    In several ways. Pick and choose.

    If the library is small, a widget or two, perhaps the best way is to submit it as a contribution. We will include your software on our list of Free Software Using Qt.

    If it's larger, you can distribute it, and you can include Qt along with it. But you cannot include Qt as part of your library (e.g. by making a shared library which includes Qt and your library). Your distribution must contain the file README.QT. Your users will be Qt users as well, and Qt's licensing applies.

    This applies to wrapper libraries as well, of course. If you write Ada or Perl bindings, fine. But anyone who uses your bindings will be using Qt as well, and Qt's licensing applies.

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Last modified 1997/11/03. Questions? Write to info@troll.no