GNU Lesser General Public License

The GNU Lesser General Public License, or better known by its English name GNU Lesser General Public License (formerly The GNU Library General Public License or GNU Library General Public License), or simply by its English acronym GNU LGPL, is a software license created by the Free Software Foundation that is intended to guarantee the freedom to share and modify the software covered by it, ensuring that the software is free for all its users.

This license applies to any program or work that contains a notice posted by the copyright owner of the work stating that their work can be distributed under the terms of this "LGPL Lesser General Public License". The "Program," as used hereinafter, refers to any original program or work, and the "work based on the Program" means either the Program or any derivative work thereof under copyright law: that is, a work containing the Program or any portion of it, either in its entirety or with modifications or translations into other languages.

Activities other than copying, distribution, or modification are not covered by this license and are beyond its scope. The act of running the Program is not restricted, and the output of information from the Program is covered only if its contents constitute a work based on the Program (regardless of whether it was the result of running the Program). Whether this is true or not depends on the function of the program.

Differences from the GPL

The main difference between the GPL and the LGPL is that the latter can be linked to (in the case of a library, 'be used by') a non-GPL program, which can be free software or non-free software. In this respect, the GNU LGPL version 3 is presented as a set of permissions added to the GNU GPL.

These non-GPL or non-LGPL programs can be distributed under any chosen condition if they are not derivative works. If it is a derivative work then the terms must allow modification by the user for own use and the use of reverse engineering techniques to develop such modifications. Defining when a work that uses a LGPL program is a derivative work or not is a legal matter (see the text of the LGPL). A stand-alone executable that links to a library is generally accepted as a non-derivative work of the library. It would be considered a work using the library and paragraph 5 of the LGPL would apply.

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.

From the unofficial translation into Spanish:

A program that does not contain derived from any portion of the library, but is designed to work with the library by being compiled or linked to it is called a "work that uses the library". Such work, separately, is not a work derived from the library, and therefore falls outside the scope of this License.

Essentially it should be possible to link the software with a new version of the program covered by the LGPL. The commonly used method to achieve this is to use an appropriate dynamic or shared library mechanism. Alternatively, it is permissible to statically link an LGPL library (see static libraries) if the source code of the program is provided or the object code for linking against the LGPL library is provided.

A feature of the LGPL is that any LGPL code can be converted to GPL code (section 2 of the license). This feature is useful for direct reuse of LGPL code in GPL library and application code, or if you want to create a version of the code that cannot be used in proprietary software.

Choose between the GPL and the LGPL

The term "GNU Library General Public License" it gave the impression that the FSF wanted all libraries to use the LGPL and all programs to use the GPL. In February 1999 Richard Stallman wrote the document "Why His Next Library Shouldn't Use the Lesser GPL for Libraries" explaining why this was not the case, and that the LGPL should not be used. necessarily for libraries:

What license is the best for a certain library is a matter of strategy, and depends on the details of the situation. Currently, most of the GNU libraries are covered by the LGPL, and that means we are using only one of these two strategies, neglecting the other. So now we intend to publish more libraries under ordinary GPL.

Contrary to what many believe, this is not to say that the FSF underestimates the LGPL, but simply says that it should not be used for all libraries. In the same document it reads:

There are reasons that can make LGPL use more appropriate in certain cases. The most common case is when the features of the free library are already available for proprietary software through other alternative libraries. In that case, the library cannot give free software any particular advantage, so it is better to use the LGPL for that library.

In fact, Stallman and the FSF advocate using licenses even less restrictive than the LGPL as a strategy (to maximize users' freedom). A prominent example is Stallman's approval to use the BSD license in the Vorbis project.