Software safety (sometimes called software system safety) is an engineering discipline that aims to ensure that software, which is used in safety-related systems (i.e. safety-related software), does not contribute to any hazards such a system might pose. There are numerous standards that govern the way how safety-related software should be developed and assured in various domains. Most of them classify software according to their criticality and propose techniques and measures that should be employed during the development and assurance:
Software for generic electronic safety-related systems: IEC 61508 (part 3 of the standard) Automotive software: ISO 26262 (part 6 of the standard) Railway software: EN 50716 Airborne software: DO-178C/ED-12C) Air traffic management software: DO-278A/ED-109A Medical devices: IEC 62304 Nuclear power plants: IEC 60880
Terminology System safety is the overarching discipline that aims to achieve safety by reducing risks in technical systems to an acceptable level. According to the widely adopted system safety standard IEC 61508, safety is “freedom from unacceptable risk of harm”. As software alone – which can be considered as pure information – cannot cause any harm by itself, the term software safety is sometimes dismissed and replaced by “software system safety” (e.g. the Joint Software Systems Safety Engineering Handbook and MIL-STD-882E use this terminology). This stresses that software can only cause harm in the context of a technical system (see NASA Software Safety Guidebook, chapter 2.1.2), that has some effect on its environment. The goal of software safety is to make sure that software does not cause or contribute to any hazards in the system where it is used and that it can be assured and demonstrated that this is the case. This is typically achieved by the assignment of a "safety level" to the software and the selection of appropriate processes for the development and assurance of the software.
Assignment of safety levels One of the first steps when creating safety-related software is to classify software according to its safety-criticality. Various standards suggest different levels, e.g. Software Levels A-E in DO-178C, SIL (Safety Integrity Level) 1-4 in IEC 61508, ASIL (Automotive Safety Integrity Level) A-D in ISO 26262. The assignment is typically done in the context of an overarching system, where the worst case consequences of software failures are investigated. For example, automotive standard ISO 26262 requires the performance of a Hazard and Risk Assessment ("HARA") on vehicle level to derive the ASIL of the software executed on a component.
Process adherence and assurance It is essential to use an adequate development and assurance process, with appropriate methods and techniques, commensurate with the safety criticality of the software. Software safety standards recommend and sometimes forbid the use of such methods and techniques, depending on the safety level. Most standards suggest a lifecycle model (e.g. EN 50716, SIL (Safety Integrity Level) 1-4 in IEC 61508 suggests – among others – a V-model) and prescribe required activities to be executed during the various phases of the software. For example, IEC 61508 requires that software is specified adequately (e.g. by using formal or semi-formal methods), that the software design should be modular and testable, that adequate programming languages are used, documented code reviews are performed and that testing should be performed an several layers to achieve an adequately high test coverage. The focus on the software development and assurance process stems from the fact that software quality (and hence safety) is heavily influenced by the software process, as suggested by IEC 25010. It is claimed that the process influences the internal software quality attributes (e.g. code quality) and these in turn influence external software quality attributes (e.g. functionality and reliability). The following activities and topics addressed in the development process contribute to safe software.
Documentation Comprehensive documentation of the complete development and assurance process is required by virtually all software safety standards. Typically, this documentation is reviewed and endorsed by third parties and therefore a prerequisite for the approval of safety-related software. The documentation ranges from various planning documents, requirements specifications, software architecture and design documentation, test cases on various abstraction levels, tool qualification reports, review evidence, verification and validation results etc. Fig C.2 in EN 50716 lists 32 documents that need to be created along the development lifecycle.
Traceability Traceability is the practice to establish relationships between different types of requirements and between requirements and design, implementation and testing artefacts. According to EN 50716, the objective “is to ensure that all requirements can be shown to have been properly met and that no untraceable material has been introduced”. By documenting and maintaining traceability, it becomes possible to follow e.g. a safety requirement into the design of a system (to verify if it considered adequately), further on into the software source code (to verify if the code fulfils the requirement), and to an appropriate test case and test execution (to verify if the safety requirement has been tested adequately).
Software implementation Safety standards can have requirements directly affecting the implementation of the software in source code, such as e.g. the selection of an appropriate programming language, the size and complexity of functions, the use of certain programming constructs and the need for coding standards. Part 3 of IEC 61508 contains the following requirements and recommendations:
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