it becomes a de-facto standard over similar, although not

  interface-identical, proprietary alternatives

\item To encourage use of a Free Software library that already has

  interface-identical proprietary competitors that are more developed

\item To allow a greater number of users to get freedom, by encouraging

  proprietary companies to pick a Free alternative for its otherwise

  proprietary products

\end{itemize}

The LGPL's preamble sets forth the limits to which the license seeks to go in

chasing these goals. The LGPL is designed to ensure that users who happen to

acquire software linked with such libraries have full freedoms with

respect to that library. They should have the ability to upgrade to a newer

or modified Free version or to make their own modifications, even if they

cannot modify the primary software program that links to that library.

Finally, the preamble introduces two terms used throughout the license to

clarify between the different types of derivative works: ``works that use

the library,'' and ``works based on the library.''  Unlike the GPL, the LGPL must

draw some lines regarding derivative works. We do this here in this

license because we specifically seek to liberalize the rights afforded to

copyright law would otherwise prohibit if the copyright holder exercised

his full permitted controls over the work.

\section{An Application: A Work that Uses the Library}

In the effort to allow certain proprietary derivative works and prohibit

``works based on the library,'' and ``works that use the library.''  The

distinction is drawn on the bright line of binary (or runtime) derivative

works and source code derivatives. We will first consider the definition

of a ``work that uses the library,'' which is set forth in LGPLv2.1~\S5.

We noted in our discussion of GPLv2~\S3 (discussed in

Section~\ref{GPLv2s3} of this document) that binary programs when

compiled and linked with GPL'd software are derivative works of that GPL'd

software. This includes both linking that happens at compile-time (when

the binary is created) or at runtime (when the binary -- including library

and main program both -- is loaded into memory by the user). In GPL,

binary derivative works are controlled by the terms of the license (in GPLv2~\S3),

and distributors of such binary derivatives must release full

corresponding source\@.

In the case of LGPL, these are precisely the types of derivative works

we wish to permit. This scenario, defined in LGPL as ``a work that uses

the library,'' works as follows:

\newcommand{\workl}{$\mathcal{L}$}

\newcommand{\lplusi}{$\mathcal{L\!\!+\!\!I}$}

\begin{itemize}

\item A new copyright holder creates a separate and independent work,

  \worki{}, that makes interface calls (e.g., function calls) to the

  LGPL'd work, called \workl{}, whose copyright is held by some other

  party. Note that since \worki{} and \workl{} are separate and

  independent works, there is no copyright obligation on this new copyright

  holder with regard to the licensing of \worki{}, at least with regard to

  the source code.

\item The new copyright holder, for her software to be useful, realizes

  that it cannot run without combining \worki{} and \workl{}.

  Specifically, when she creates a running binary program, that running

  binary must be a derivative work, called \lplusi{}, that the user can

  run.

\item Since \lplusi{} is a derivative work of both \worki{} and \workl{},

  the license of \workl{} (the LGPL) can put restrictions on the license

\end{itemize}

We will talk about the specific restrictions LGPLv2.1 places on ``works

that use the library'' in detail in Section~\ref{lgpl-section-6}. For

now, focus on the logic related to how the LGPLv2.1 places requirements on

the license of \lplusi{}. Note, first of all, the similarity between

this explanation and that in Section~\ref{separate-and-independent},

which discussed the combination of otherwise separate and independent

works with GPL'd code. Effectively, what LGPLv2.1 does is say that when a

new work is otherwise separate and independent, but has interface

calls out to an LGPL'd library, then it is considered a ``work that

uses the library.''

In addition, the only reason that LGPLv2.1 has any control over the licensing

of a ``work that uses the library'' is for the same reason that GPL has

some say over separate and independent works. Namely, such controls exist

because the {\em binary combination\/} (\lplusi{}) that must be created to

make the separate work (\worki{}) at all useful is a derivative work of

the LGPLv2.1'd software (\workl{}).

Thus, a two-question test that will help indicate if a particular work is

a ``work that uses the library'' under LGPLv2.1 is as follows:

\item Is the source code of the new copyrighted work, \worki{}, a

  completely independent work that stands by itself, and includes no

  source code from \workl{}?

\item When the source code is compiled, does it create a derivative work

  by combining with \workl{}, either by static (compile-time) or dynamic

  (runtime) linking, to create a new binary work, \lplusi{}?

\end{enumerate}

If the answers to both questions are ``yes,'' then \worki{} is most likely

a ``work that uses the library.''  If the answer to the first question

``yes,'' but the answer to the second question is ``no,'' then most likely

\worki{} is neither a ``work that uses the library'' nor a ``work based on

the library.''  If the answer to the first question is ``no,'' but the

answer to the second question is ``yes,'' then an investigation into

whether or not \worki{} is in fact a ``work based on the library'' is

warranted.

\section{The Library, and Works Based On It}

In short, a ``work based on the library'' could be defined as any

derivative work of LGPL'd software that cannot otherwise fit the

definition of a ``work that uses the library.''  A ``work based on the

library'' extends the full width and depth of copyright derivative works,

Most typically, one creates a ``work based on the library'' by directly

modifying the source of the library. Such a work could also be created by

tightly integrating new software with the library. The lines are no doubt

fuzzy, just as they are with GPL'd works, since copyright law gives us no

litmus test for derivative works of a software program.

Thus, the test to use when considering whether something is a ``work

based on the library'' is as follows:

\begin{enumerate}

\item Is the new work, when in source form, a derivative work under

  copyright law of the LGPL'd work?

\item Is there no way in which the new work fits the definition of a

  ``work that uses the library''?

\end{enumerate}

If the answer is ``yes'' to both these questions, then you most likely

have a ``work based on the library.''  If the answer is ``no'' to the

first but ``yes'' to the second, you are in a gray area between ``work

based on the library'' and a ``work that uses the library.''