Looking at the LGPL license

The Lesser General Public License (LGPL) is a software license that is based on the GPL, but is more permissive.

I have seen the GPL software license described as a cancer before,  and this analogy does indeed hold, even though it misses the point.  If you want to borrow any code released as “GPL” for use in your own source code, any of your own code that the GPL is mixed with will also need to become GPL if ever you plan on distributing software.  However, this rule of the GPL is actually a very good thing, especially for hard working open source developers, because it ensures that their work can never be exploited by companies wishing to make a buck, unless those companies give their contributions to the code back under the same license, effectively becoming part of the same open source development effort.

The LGPL is mostly the same as the GPL except that it releases some of these restrictions.  I am not a law expert, but I have come to the following understanding.  In particular, it allows for code to be combined in the same project with code from any other license including proprietary licenses, under certain conditions.  If the whole thing, including the LGPL code as well as the other code, is distributed together, then it becomes known as a ‘combined work’ under the terminology of the LGPL.  The distribution must abide by the following (a complete list exists in the text of the license under ‘Combined Works’):

  • There must be some sort of clear separation between the LGPL code and the other code.  In particular, it must be possible for the recipient to modify the LGPL code or even completely replace it with other code, such as a modified version or later version of the libary.  Therefore, if it is a compiled program, then the LGPL code must either be dynamically linked (like, a separate DLL or shared file) such that it could easily be substituted for a similar library and still be interoperable; or, if it is statically linked, the minimum required source files and/or object files must be provided, to allow recompiling with an alternative library.  The non-LGPL portion may not contain any part of the LGPL code except for very simple header files.
  • It must be clearly pointed out which part of the code is LGPL covered, including its original copyright notice and the text of the LGPL (including the GPL on which it is based).
  • If the combined software displays copyright notices during the course of running, then the copyright notice for the LGPL covered portion must also appear here, along with a link to the LGPL and GPL.
  • In some cases, you would need to provide installation information detailing how to use a modified version of the LGPLd code in the combined application.

The above is just a list of stuff that you have to do if you are going to redistribute some LGPL code as part of your non-LGPL application.  However, the text of the LGPL doesn’t really explain the benefits, or what opportunities it opens up to you.  If you are distributing a combined work which consists of both someone else’s LGPL code, and other code that is under a different license, you benefit from:

  • You don’t need to release source code for the rest of your application (ie, the non-LGPL part).  The only exception to this being as described above – if it’s all statically linked then you’d need to provide just enough code (and/or object files) to be able re-link it with an alternative or modified version of the LGPL code.  If you are linking dynamically and interacting over a normal API, you don’t need to worry about that.
  • You don’t need to release the rest of your application under the GPL.  You can use any license you want, including more restrictive proprietary licenses, provided that when you do distribute it you follow the rules.
  • Unlike GPL version 3, which prohibits using the code if you are implementing copy protection or DRM software, you can use LGPL version 3 licensed code in an application which includes copy protection or DRM.

So, LGPL is a fairly permissive license in comparison to the GPL, which cannot be ‘combined’ with other proprietary licenses in this way.  So, what can’t you do with LGPL?  Well, apart from relaxed rules such as those already discussed, LGPL still contains all of the other restrictions of the GPL.  I won’t go into all of the detail of the GPL here, but here are some of the restrictions that you still retain when using the LGPL, that you may not retain with an even more permissive license (such as a BSD or X11 license):

  • Anybody wishing to modify and redistribute LGPL code must also make the source code of the LGPL code, including their modifications, available and licensed under the LGPL.  Therefore, the open source community and the original author can still benefit if a commercial enterprise contributes improvements to the LGPL code.
  • All existing copyright notices in the LGPL code, as well as the GPL and LGPL licenses themselves, must be prominently displayed in any redistribution.  In addition to this, any application using the LGPL code must display such copyright notices and refer to the license whereever they display their own copyright notices.

Something else of note is that whether you are building software that relies on either GPL or LGPL code, if you distribute that software without any of the GPL or LGPL included (ie, the user needs to download and install it themselves),  then it’s not considered a ‘combined work’ and you don’t need to worry about complying with any terms.  However, if you include even so much as a header file from the GPL or LGPL code in your software, then you’ll be bound by the license terms of the GPL or LGPL.

2 Replies to “Looking at the LGPL license”

  1. Hi,
    my name is awaludin
    I had a question about LGPL

    I would like to ask you some questions about LGPL, I’ve used a library
    written on PHP with license LGPL, I want to make software and I’m not
    license it using LGPL or GPL,

    The libraries is class form
    I had modified the libraries and make a class to access the library,
    and I access the library through that class, but I’m not make it
    inheritence from it, I was just make a object definitions

    for example
    Library’s file is only one
    that is libraries.php licensed under LGPL v 2.1


    and I made another file to access the Access_Lib
    for example
    $c = new Database($d0);


    and the above file (Program.php) access several files too ie:
    database.php and view.php

    As I know that when we used LGPL lincensed libraries, we can make our
    software not licenced using LGPL or GPL.
    I had few questions about that

    1. Had I violated the LGPL License
    because I wanted to license my program into something else including
    two files that associated with the libraries and several files that
    depend on libraries (ie:database.php, view.php), on the above example
    and the libraries.php remains LGPL?

    2. Do file Access_lib.php and Program.php must be licensed LGPL too
    because of I had modificated libraries.php?

    I took a portion of LGPL, it’s said that

    “2. You may modify your copy or copies of the Library or any portion
    of it, thus forming a work based on the Library, and copy and
    distribute such modifications or work under the terms of Section 1
    above, provided that you also meet all of these conditions:
    a) The modified work must itself be a software library.

    b) You must cause the files modified to carry prominent notices
    stating that you changed the files and the date of any change.

    c) You must cause the whole of the work to be licensed at no
    charge to all third parties under the terms of this License.

    These requirements apply to the modified work as a whole. If
    identifiable sections of that work are not derived from the Library,
    and can be reasonably considered independent and separate works in
    themselves, then this License, and its terms, do not apply to those
    sections when you distribute them as separate works. But when you
    distribute the same sections as part of a whole which is a work based
    on the Library, the distribution of the whole must be on the terms of
    this License, whose permissions for other licensees extend to the
    entire whole, and thus to each and every part regardless of who wrote

    3 what is means “whole of the work” on
    “c) You must cause the whole of the work to be licensed at no
    charge to all third parties under the terms of this License.”
    does it mean modified copy of libraries.php or whole files that access
    the libraries.php like Access_Lib and others, or what exactly it’s

    4. what means “derived” here, does any file that use libraries.php is
    derived work?
    5. which files that must be licensed LGPL, is it only libraries.php?

    6. does the explanation about LGPL v 2.1 at
    about these statement

    “If you modify your copy or copies of the library or any portion of
    it, you may distribute the resulting library provided you do so under
    the GNU Lesser General Public License. However, programs that link to
    the library may be licensed under terms of your choice, so long as the
    library itself can be changed. Any translation of the GNU Lesser
    General Public License must be accompanied by the GNU Lesser General
    Public License”

    is correct and suit what I’ve done in the first question?

    I’m sorry for my bad english,
    Thank for the incoming explanations, I hope you could help me,



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