Feature Diagrams: A Survey and A Formal Semantics: (An extended version of this paper appears in Journal of Computer Networks, February 2007)

Feature Diagrams (FD) are a family of popular modelling languages used for engineering requirements in software product lines. FD were first introduced by Kang as part of the FODA (Feature Oriented Domain Analysis) method back in 1990. Since then, various extensions of FODA FD were devised to compensate for a purported ambiguity and lack of precision and expressiveness. However, they never received a proper formal semantics, which is the hallmark of precision and unambiguity as well as a prerequisite for efficient and safe tool automation. In this paper, we first survey FD variants. Subsequently, we generalize the various syntaxes through a generic construction called Free Feature Diagrams (FFD). Formal semantics is defined at the FFD level, which provides unambiguous definition for all the surveyed FD variants in one shot. All formalization choices found a clear answer in the original FODA FD definition, which proved that although informal and scattered throughout many pages, it suffered no ambiguity problem. Our definition has several additional advantages: it is formal, concise and generic. We thus argue that it contributes to improve the definition, understanding, comparison and reliable implementation of FD languages.