GNU Lesser General Public License

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The GNU Lesser General Public License (formerly the GNU Library General Public License) 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 and simple permissive licenses such as the BSD licenses 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 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, provided the terms allow "modification for the customer's own use and reverse engineering for debugging such modifications."

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. Essentially, it must be possible for the software to be linked with a newer version of the LGPL-covered program.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.

The LGPL is primarily intended for software libraries, although it is also used by some stand-alone applications, most notably OpenOffice.org.

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 if one wants to create a version of the code that software companies cannot use in proprietary software products. It is also necessary to ensure that the LGPL is "GPL-compatible", so that GPL-covered programs can use LGPL-covered libraries.

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 1999, Richard Stallman wrote an essay 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 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 Library 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-style license by the Vorbis project for its libraries [2].

The license uses terminology which is mainly intended for applications written in C programming language or its family. Franz Inc. published its own preamble to the license to clarify terminology in Lisp programming language context. LGPL with this preamble is sometimes referred as LLGPL. [3]

[edit] LGPL regarding inheritance

A said in the FSF FAQ, about use of LGPL for Java programs [4] :

The LGPL contains no special provisions for inheritance, because none are needed. Inheritance creates derivative works in the same way as traditional linking, and the LGPL permits this type of derivative work in the same way as it permits ordinary function calls..