Lgpl version and md5/sha check of Qt dll files



  • Hello.I want to ask a few questions about Qt's lgpl version.

    Is it possible to use Upx like software to decrease the sizes of Qt dll files which will deployed with the program(my program) files?(with Lgpl licence of Qt I mean)

    My other question, is it possible to md5/sha check(inside my program) of the dll files of Qt?I mean my program will check if Qt dll files are really the original Qt files using md5/sha.If they are not original Qt dll files,then my program will not launch.But at the same time,this means that the user will not be able to update the dll files to a newer version of Qt,because if they do the program will not launch.So I wonder if we are allowed to do this check using lgpl licence of Qt?

    I have contacted Digia about 2-3 days ago,but no answer yet.

    Kind Regards,


  • Moderators

    I think that this MD5 check might be violating LGPL when combined with Upx packing. If you do this, you definitely need to inform your users about this, and give them a way to swap the libraries if they choose to do it.



  • Thanks for your answer.

    If I use upx and also inform them and allow them to use a specific version of Qt dll files(for example 5.1) is it okay?(my program will check if md5/sha of the upx compressed dll files match ,if they don't match then it will check if md5/sha of the specific version of Qt dlls match)


  • Moderators

    I suppose it is all right, but please don't cite me in court ;P

    LGPL requires the libraries to be swappable, and in your solution they should be, so it looks good to me.



  • Thanks for your answer.

    bq. I suppose it is all right, but please don’t cite me in court ;P

    :D

    bq. LGPL requires the libraries to be swappable, and in your solution they should be, so it looks good to me.

    But if a user wants to swap a Qt library file to a newer or older version,the program will not start cause of the md5/sha check of a specific version of Qt dll files,so is it still alright ?

    Also I wonder why Digia didn'T give me an answer :D


  • Moderators

    I don't think the license talks about swapping the libraries for an older or newer versions. Library vendors are not obliged to keep binary compatibility like Qt does.



  • Thanks for your answers :)


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