GNU Lesser General Public License
The GNU Lesser General Public License (LGPL) is a free-software license published by the Free Software Foundation (FSF). The license allows developers and companies to use and integrate a software component released under the LGPL into their own (even proprietary) software without being required by the terms of a strong copyleft license to release the source code of their own components. However, any developer who modifies an LGPL-covered component is required to make their modified version available under the same LGPL license. For proprietary software, code under the LGPL is usually used in the form of a shared library, so that there is a clear separation between the proprietary and LGPL components. The LGPL is primarily used for software libraries, although it is also used by some stand-alone applications.
The LGPL was developed as a compromise between the strong copyleft of the GNU General Public License (GPL) and more permissive licenses such as the BSD licenses and the MIT License. The word "Lesser" in the title shows that the LGPL does not guarantee the end user's complete freedom in the use of software; it only guarantees the freedom of modification for components licensed under the LGPL, but not for any proprietary components.
History[edit]
The license was originally called the GNU Library General Public License and was first published in 1991, and adopted the version number 2 for parity with GPL version 2. The LGPL was revised in minor ways in the 2.1 point release, published in 1999, when it was renamed the GNU Lesser General Public License to reflect the FSF's position that not all libraries should use it. Version 3 of the LGPL was published in 2007 as a list of additional permissions applied to GPL version 3.
In addition to the term "work based on the Program" of GPL, LGPL version 2 introduced two additional clarification terms "work based on the library" and "work that uses the library".[4] LGPL version 3 partially dropped these terms.
Compatibility[edit]
One feature of the LGPL is the permission to sublicense[7] under the GPL any piece of software which is received under the LGPL (see section 3 of the LGPL version 2.1, and section 2 option b of the LGPL version 3). This feature allows for direct reuse of LGPLed code in GPLed libraries and applications.
Version 3 of the LGPL is not inherently compatible with version 2 of the GPL. However, works using the latter that have given permission to use a later version of the GPL are compatible:[8] a work released under the GPLv2 "or any later version" may be combined with code from a LGPL version 3 library, with the combined work as a whole falling under the terms of the GPLv3.[9]
FSF recommendations on library licensing[edit]
The former name GNU Library General Public License gave some the impression that the FSF recommended that all software libraries should use the LGPL and programs should use the GPL. In 1999 essay Why you shouldn't use the Lesser GPL for your next library Richard Stallman explained that while the LGPL had not been deprecated, one should not necessarily use the LGPL for all libraries, as using GPL can give advantage to free-software developers.[5]
On the other hand, the FSF sometimes advocate for even less restrictive licenses. For example, Stallman endorsed the use of BSD-style license by the Vorbis project in its libraries in 2001.[10]