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 GPL, 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

those who make derivative works. In GPL, we reach as far as copyright law

allows. In LGPL, we want to draw a line that allows some derivative works

copyright law would otherwise prohibit if the copyright holder exercised

his full permitted controls over the work.

In the effort to allow certain proprietary derivative works and prohibit

others, LGPL distinguishes between two classes of derivative works:

``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 LGPL \S 5.

We noted in our discussion of GPL \S 3 (discussed in

Section~\ref{GPL-Section-3} 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

  (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.

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,

in the same sense that GPL does.

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.

bright. At times, though, we have seen cases where a derivative work

appeared in some ways to be a work that used the library and in other

ways a work based on the library. We overcame this problem by

dividing the work into smaller subunits. It was soon discovered that

what we actually had were three distinct components: the original

LGPL'd work, a specific set of works that used that library, and a

specific set of works that were based on the library. Once such

distinctions are established, the licensing for each component can be

considered independently and the LGPL applied to each work as

prescribed.

In our discussion of the definition of ``works that use the library,'' we

left out a few more complex details that relate to lower-level programming

details. The fourth paragraph of LGPL's \S 5 covers these complexities,

and it has been a source of great confusion. Part of the confusion comes

because a deep understanding of how compiler programs work is nearly

mandatory to grasp the subtle nature of what \S 5, \P 4 seeks to

cover. It helps some to note that this is a border case that we cover in

the license only so that when such a border case is hit, the implications

of using LGPL continue in the expected way.

To understand this subtle point, we must recall the way that a compiler

mechanisms the compiler uses underneath to create the ``work that uses the

library.''

As it turns out, we have only once had anyone worry about this specific

idiosyncrasy, because that particular vendor wanted to ship object code

(rather than final binaries) to their customers and was worried about

this edge condition. The intent of clarifying this edge condition is

primarily to quell the worries of software engineers who understand the

level of verbatim code copying that a compiler often does, and to help

them understand that the full implications of LGPL are the same regardless

of the details of the compilation progress.

\label{lgpl-section-6}

Now that we have established a good working definition of works that

``use'' and works that ``are based on'' the library, we will consider the

rules for distributing these two different works.

The rules for distributing ``works that use the library'' are covered in

\S 6 of LGPL\@. \S 6 is much like GPL's \S 3, as it requires the release

of source when a binary version of the LGPL'd software is released. Of

course, it only requires that source code for the library itself be made

available. The work that ``uses'' the library need not be provided in

source form. However, there are also conditions in LGPL \S 6 to make sure

that a user who wishes to modify or update the library can do so.

to its fullest extent, and that means recombining it with the ``work based

on the library.''  If the full binary is linked without a shared library

mechanism, the user must have available the object code for the ``work

based on the library,'' so that the user can relink the application and

build a new binary.

The remaining options in \S 6 are very similar to the other choices

provided by GPL \S 3. There are some additional options, but time does

not permit us in this course to go into those additional options. In

almost all cases of distribution under LGPL, either \S 6(a) or \S 6(b) are

exercised.

Essentially, ``works based on the library'' must be distributed under the

same conditions as works under full GPL\@. In fact, we note that LGPL's

\S 2 is nearly identical in its terms and requirements to GPL's \S 2.

There are again subtle differences and additions, which time does not

permit us to cover in this course.

\section{And the Rest}

The remaining variations between LGPL and GPL cover the following

conditions: