Reusability before and after reuse: a Darwin case study

Reusability is a desirable property of a software component. Furthermore, when building a new software product by reusing an existing component, it is important that the reusability of the reused component is not destroyed in the new product. In this paper, we use common coupling, a measure of software dependency, to represent software reusability. We have studied Darwin, an open-source operating system that was built by reusing two open-source operating systems, Mach and FreeBSD. We show that, in the course of building Darwin, much of the reusability in Mach and FreeBSD was retained, and that Darwin is therefore a good example of how to reuse software. Unfortunately, the code written specifically for Darwin is less reusable than the reused code.