LGPL Static Linking



  • After reading up on the LGPL license it seems that I'm probably OK releasing statically linked executables as long as I include a way for end users to change/update the Qt libraries the library uses. [1]

    I've heard talk about releasing object files so end users can recompile, but what about just releasing 2 versions of an executable-- One that is statically linked and one that is dynamically linked. That way anyone who wanted to change the libraries could simply download the dynamically linked version.

    [1] http://qt-project.org/wiki/Licensing-talk-about-mobile-platforms



  • This all comes down to the question, what is considered a "work that uses the library" and what is considered a "derivative of the library". If your program is a "work that uses the library" you may release it under a proprietary license, even if it uses a LGPL'd library. Only any modifications that you applied to the library itself would ave to be released under the LGPL. BUT: As soon as your application becomes a "derivative of the library", the complete source code of your application must be released under the LGPL, no matter what.

    The LGPL says:
    [quote]A program that contains no derivative of any portion of the Library, but is designed to work with the Library by being compiled or linked with it, is called a "work that uses the Library". Such a work, in isolation, is not a derivative work of the Library, and therefore falls outside the scope of this License.

    However, linking a "work that uses the Library" with the Library creates an executable that is a derivative of the Library (because it contains portions of the Library), rather than a "work that uses the library". The executable is therefore covered by this License. Section 6 states terms for distribution of such executables.[/quote]

    This all is a bit vague and I'm not a lawyer. But I know from my experience as a software developer that linking an application against a "shared" library (DLL or SO or DYLIB file) is generally accepted as being a "work that uses the library". At the same time, when linking the library in a "static" way, the resulting all-in-one binary (EXE) file, is considered a "derivative" of the library.

    That's why commercial application, if they make use LGPL'd libraries, pretty much always ship those as a separate DLL or SO or DYLIB file...

    See also here:
    http://qt-project.org/forums/viewthread/22609


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