GNU Lesser Public License Version 2.1 (LGPL)

I. Overview

The LGPL2 is a free/open source software license written by the GNU Project, and was specifically designed for libraries. As the name LGPL  implies, it is a milder GPL-like license. The identical parts of the GPL2 and the LGPL2 are the copyleft spirit and the idea of public share, while the main difference between the two is that the LGPL2 includes some more flexible clauses. The LGPL2 defined “link” and “combine” as different terms, and determined whether programs should include the GPL/copyleft sort of provisions based on this difference. Whenever combined with a LGPLed free/open source software, the newly combined programs must also be licensed under the LGPL2. Nonetheless, if the program is merely linked to a LGPLed library, they are not obligated to use the LGPL2. The reason for Free Software Foundation (FSF) to make the LGPL2 so differently from the GPL2 is to encourage the widest possible use of free/open source software. Since libraries are designated to provide external programs with opportunities to access certain data, a GPL2 limitation must actually decrease the popularity of these free/open source libraries. In a nutshell, the LGPL2 was a compromise made to promote free/open source libraries as well as to expand its market share and user community.

II. Current Status

Blended with the copyleft spirit while avoiding certain severity, the LGPL2 has always been very popular. It is well-accepted and is chosen by many library programs.   

III. Rights and Duties

  1. Rights

    (1) The licensee may copy and distribute verbatim copies of the Library’s complete source code as he/she receives it, provided that he/she conspicuously publishes on each copy an appropriate copyright notice and disclaimer of warranty, and distribute a copy of the LGPL2 along with the Library.

    (2) The licensee may charge a fee for cost of transferring a copy; alternatively, he/she may choose to offer warranty protection in exchange for a higher fee.

    (3) The licensee may opt to apply the terms of the ordinary GPL instead of the LGPL2 to a given copy of the Library, by replacing all the notices (may be on the GPL2 or a later version). This process is neither reversible nor revocable.

    (4) The licensee may also combine or link other programs with the Library to produce a derivative work containing portions of the Library.

    (5) The licensee may place the LGPL2 library facilities together with other library facilities not covered by this license. No structural combination, however, is permitted.

  2. Duties

    (1) The licensee must cause the files modified to carry prominent notices stating that the files were changed and the date of any change.

    (2) The licensee must cause the modified work to be licensed under the terms of this license, and at no charge to all third parties.

    (3) While modifying the library, the licensee must make a good faith effort to ensure that, the facility operates in any event, and performs whatever part of its purpose remains meaningful.

    (4) Duty to provide source code: the licensee may copy and distribute the Library or derivative of it, provided that he/she accompanies it with the complete corresponding machine-readable source code.

    (5) Both object code and source code must be distributed with derivative works.

    (6) The licensee must give prominent notice with each copy of the derivative work that the LGPL2 Library is used in it and that the LGPL2 covers the Library and its use. Under the LGPL2, the licensee must include the copyright notice for the Library, as well as a reference directing the user to the copy of this license.

    (7) Establish a suitable shared library mechanism for linking with the Library in order to keep the completeness and to remain conveniently linked with application programs.

    (8) Accompany the derivative work with a written offer, valid for at least three years, to give the users object code and source code, for a charge no more than the cost of performing this distribution of the source code.

    (9) The licensee cannot use the LGPL2 Library together with other proprietary libraries of contradictory license restrictions.

    (10) If object code is distributed by offering access to copy from a designated place, then offer equivalent access to copy the source code from the same place to ensure that users are able to copy both files.

    (11) When distributing the combined library under section 7 of the LGPL2, give prominent notice with the combined library of the fact that part of it is a work based on the Library, and explaining where to find the accompanying uncombined form of the same work.

    (12) The licensee may not impose any further restrictions on the recipients’ exercise of the rights granted herein. The licensee is not responsible for any violation of this license by third parties.

    (13) No reason may excuse the licensee from complying this license. If the licensee cannot distribute so as to satisfy multiple objective obligations, then as a consequence he/she may not distribute the LGPL2 library at all.

IV. Other Important Features

  1. The LGPL2 is designed specifically for the promotion of libraries and was a compromise made to promote free/open source libraries as well as to expand its market share and user community. As a result, “in principal” it only applies to libraries and other library-like programs.

  2. The main point is to establish a criterion so as to differentiate “based-on libraries” and “combined-with libraries.” The rules and exceptions are also applied according to this criterion.  

  3. Only a library kind of derivative work may be continued licensed under the LGPL2; otherwise, they must be converted into the GPL.

  4. A LGPLed library may later on be converted and licensed under the GPL. The licensee may also opt to apply the terms of the ordinary GPL instead of the LGPL2 to proportion of a given copy of the Library. Note that this process is neither reversible nor revocable.

  5. Exercise of patent right is limited. Patent disputes, court orders or any settlements cannot contradict with the requirement of this license. If the licensee cannot satisfy multiple objective obligations, then as a consequence he/she may not distribute the LGPL2 library at the first place.

  6. The original copyright holder who places the Library under this license may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded.