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Most GNU software, including some libraries, is covered by the ordinary GNU General Public License.
This license, the GNU Lesser General Public License, applies to certain designated libraries, and is
quite different from the ordinary General Public License. We use this license for certain libraries in
order to permit linking those libraries into non-free programs.
When a program is linked with a library, whether statically or using a shared library, the combination of
the two is legally speaking a combined work, a derivative of the original library. The ordinary General
Public License therefore permits such linking only if the entire combination fits its criteria of freedom.
The Lesser General Public License permits more lax criteria for linking other code with the library.
We call this license the “Lesser” General Public License because it does Less to protect the user’s
freedom than the ordinary General Public License. It also provides other free software developers Less
of an advantage over competing non-free programs. These disadvantages are the reason we use the
ordinary General Public License for many libraries. However, the Lesser license provides advantages
in certain special circumstances.
For example, on rare occasions, there may be a special need to encourage the widest possible use of
a certain library, so that it becomes a de-facto standard. To achieve this, non-free programs must be
allowed to use the library. A more frequent case is that a free library does the same job as widely used
non-free libraries. In this case, there is little to gain by limiting the free library to free software only, so
we use the Lesser General Public License.
In other cases, permission to use a particular library in non-free programs enables a greater number of
people to use a large body of free software. For example, permission to use the GNU C Library in
non-free programs enables many more people to use the whole GNU operating system, as well as its
variant, the GNU/Linux operating system.
Although the Lesser General Public License is Less protective of the users’ freedom, it does ensure
that the user of a program that is linked with the Library has the freedom and the wherewithal to run
that program using a modified version of the Library.
The precise terms and conditions for copying, distribution and modification follow. Pay close attention
to the difference between a “work based on the library” and a “work that uses the library”. The former
contains code derived from the library, whereas the latter must be combined with the library in order to
run.