Introduction to Free Software - SELF | Sharing Knowledge about ...

© FUOC • P07/M2101/02709 47 Free Software
It is possible to divide free software licences into two large families. The first
comprises licences that do not impose special conditions on the second redis-
tribution (in other words, that only specify that the software can be redistribu-
ted or modified, but that do not impose special conditions for doing so, which
allows, for example, someone receiving the program to then redistribute it
as private software): these are what we will refer to as permissive licences. The
second family, which we will call strong licences (or copyleft licences), inclu-
de those that, in the style of GNU's GPL, impose conditions in the event of
wanting to redistribute the software, aimed at ensuring compliance with the
licence's conditions following the first redistribution. Whereas the first group
emphasises the freedom of the person receiving the program to do almost
anything they want with it (in terms of the conditions for future redistribu-
tions), the second emphasises the freedom of anyone who may potentially
receive some day a work derived from the program, obliging subsequent mo-
difications and redistributions to respect the terms of the original licence.
The difference between these two types of licences has been (and remains) a
debatable issue amongst the free software community. In any case, we should
remember that they are all free licences.
3.2.2. Permissive licences
Permissive licences, also known sometimes as liberal or minimal licences, do
not impose virtually any conditions on the person receiving the software, and
yet, grant permission to use, redistribute and modify. From a certain point of
view, this approach can be seen as a guarantee of maximum freedom for the
person receiving the program. But from another, it may also be understood
as maximum neglect in respect of ensuring that once someone receives the
program, that person guarantees the same freedoms when redistributing that
program. In practice, these licences typically allow software that its author
distributes under a permissive licence to be redistributed with a private licence.
Among these licences, the BSD licence is the best known, to such an extent
that often permissive licences are referred to as BSD-type licences. The BSD
(Berkeley Software Distribution) licence stems from the publication of diffe-
rent versions of Unix produced by the University of California in Berkeley, in
the US. The only obligation it imposes is to credit the authors, while it allows
redistribution in both binary and source code formats, without enforcing eit-
her of the two in any case. It also gives permission to make any changes and
to be integrated into other programs without almost any restrictions.
Note
One of the consequences in practice of BSD-type licences has been to diffuse standards,
since developers find no obstacle to making programs compatible with a reference implementation
under this type of licence. In fact, this is one of the reasons for the extraordinary
and rapid diffusion of Internet protocols and the sockets-based programming
interface, because most commercial developers derived their realisation from the University
of Berkeley.
Note
The term copyleft when applied
to a licence, used mainly by
the Free Software Foundation
to refer to its own licences, has
similar implications to those
referred to as strong licences as
used in this text.