A framework for software safety in safety-critical systems

Abstract

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. Because human lives may be lost and tremendous economic costs may result if the software fails, the development of high-integrity software adopts practices that impose greater rigor on the software development processes. 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. The safety of a software component must be considered within the context of both the overall system of which it is a component and the environment in which this system operates. It is not useful to investigate the safety of a software component in isolation. 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 framework pertains to system hazard analysis, completeness of requirements, identification of software-related safety-critical requirements, safety-constraints based design, run-time issues management, and software safety-critical testing. This framework is then applied to a prototype safety-critical system viz. a software--based Railroad Crossing Control System (RCCS) to validate its utility.