System safety is a complex concept, which is represented by multiple attributes and which requires diverse sources of evidence to demonstrate its achievement. Safety-critical systems, which provide safety-critical services to their users, must be designed to be safe. This means that despite their complexities and despite variable environmental conditions, their operation should be demonstrably safety. A fundamental difficulty in measuring system safety arises due to the complexity of the notion – it is made up of multiple, potentially conflicting attributes, and difficult trade-offs may need to be made between these attributes. The attributes themselves are evaluated using multiple diverse sources of evidence, thus compounding the problem of measuring system safety.
Software safety has become an ever increasingly important issue in system safety due to the larger role software plays in complex cyber-physical systems. Such a system is a consisted of a number of components distributed over a predefined space. Components of a typical cyber-physical system communicate with each other and with an external world through communication gateway. The safety failures of the components of such a system result in safety hazard of the whole system. Additionally, external attackers can attack the system through sensor network and communication gateway and can manipulate software processes and data stored and exchanged in the system.
Over the last decades a good many number of safety analysis methods (i.e., FMEA, HAZOP, FTA) have been developed. Among them Fault Tree Analysis (FTA)  is a widely accepted method. It graphically shows how basic failures of components, in combination, cause a safety hazard at the system level....
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