Chapter 6. Other Criteria of Open Source Software

Table of Contents

GPL Compatibility
Copyleft
Open Source vs. Sourceware

GPL Compatibility

Making a software free is not necessarily enough to make it compatible with the GNU GPL licence. The GPL makes some restrictions regarding which licences it can link against, and some perfectly good free software does not qualify to it. (examples are the Mozilla Public License, the Qt Public License, and even the original BSD licence). It is advisable that whenever possible a developer or vendor should choose a licence that is compatible with the GPL, because otherwise there may be problems integrating his code with GPLed one or using both a GPL and a non-GPL compatible library. (I am not a lawyer, so I cannot conclusively say when it is legal or not).

Mozilla is an example for a large project that started out with its custom (albeit now relatively common), non-GPL compatible licence, and recently adopted a triple licence of the Mozilla Public License, the GNU General Public License, and the GNU Lesser General Public License in order to make it compatible with the GPL and to standardise its integrability. The Qt library whose commercial vendor and originator is Troll Tech Inc., also adopted the GPL as well as its own QPL, to relieve the various legal problems that KDE (a desktop system for UNIXes which is based on it) faced when using GPL code.