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The LGPL License

The GNU Lesser General Public License (formerly the GNU Library General Public License) or LGPL is a free software license published by the Free Software Foundation. It was designed as a compromise between the strong-copyleft GNU General Public License|GNU General Public License or GPL and permissive licenses such as the BSD license and the MIT License. The GNU Lesser General Public License was written in 1991 (and updated in 1999) by Richard Stallman, with legal advice from Eben Moglen.

The LGPL places copyleft restrictions on the program itself but does not apply these restrictions to other software that merely links with the program. There are, however, certain other restrictions on this software.

The LGPL is primarily used for Library (computer science)|software libraries, although it is also used by some stand-alone applications, most notably Mozilla and OpenOffice.org.

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, 'used by') a non-(L)GPLed program, which may be free software or proprietary software [1]. This non-(L)GPLed program can then be distributed under any chosen terms if it is not a derivative work. If it is a derivative work, then the terms must allow "modification for the customer's own use and reverse engineering for debugging such modifications." Whether a work that uses an LGPL program is a derivative work or not is a legal issue. A standalone executable that dynamically links to a library is generally accepted as not being a derivative work. It would be considered a "work that uses the library" and paragraph 5 of the LGPL applies.

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.

Essentially, it must be possible for the software to be linked with a newer version of the LGPL-covered program. The most commonly used method for doing so is to use "a suitable shared library mechanism for linking". Alternatively, a statically linked library is allowed if either source code or linkable object files are provided.

One feature of the LGPL is that one can convert any LGPLed piece of software into a GPLed piece of software (section 3 of the license). This feature is useful for direct reuse of LGPLed code in GPLed libraries and applications, or if one wants to create a version of the code that software companies cannot use in proprietary software products.

Choosing to license a library under the GPL or the LGPL

The former name of "GNU Library General Public License" gave some people 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 essay Why you shouldn't use the Lesser GPL for your next library explaining why this was not the case, and that one should not necessarily use the LGPL for libraries.

Which license is best for a given library is a matter of strategy, and it depends on the details of the situation. At present, most GNU libraries are covered by the Library GPL, and that means we are using only one of these two strategies [allowing/disallowing proprietary programs to use a library] , neglecting the other. So we are now seeking more libraries to release under the ordinary GPL.

Contrary to popular impression, however, this does not mean that the FSF deprecation|deprecates the LGPL, but merely says that it should not be used for all libraries — the same essay goes on to say:

Using the ordinary GPL is not advantageous for every library. There are reasons that can make it better to use the Lesser GPL in certain cases.

Indeed, Stallman and the FSF sometimes advocate licenses even less restrictive than the LGPL as a matter of strategy (to maximize the freedom of users). A prominent example was Stallman's endorsement of the use of a BSD license|BSD-style license by the Vorbis project for its libraries [2].