An Approach to Modeling Software Safety

Software for safety-critical systems must deal with the hazards identified by safety analysis in order to make the system safe, risk-free and fail-safe. Software safety is a composite of many factors. Existing software quality models like McCall's and Boehm's and ISO 9126 are inadequate in addressing the software safety issues of real time safety-critical embedded systems. At present there does not exist any standard framework that comprehensively addresses the factors, criteria and metrics (FCM) approach of the quality models in respect of software safety. This paper proposes a new framework for software safety based on the McCall's software quality model that specifically identifies the criteria corresponding to software safety in safety critical applications. The criteria in the proposed software safety model pertains to system hazard analysis, completeness of requirements, identification of software-related safety- critical requirements, safety-constraints based design, runtime issues management, and software safety-critical testing. This framework is then applied to a prototype safety-critical system viz. a software-based road traffic control system (RTCS) commonly used in city traffic, to validate its utility.