SuperGuard: Qualifying the C Standard Library for Use in Safety-critical Applications

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Software solutions play an ever-increasing role in safety-critical and safety-related systems, with the result that software malfunctions now represent liabilities and a real threat in terms of injury, loss of life, the interruption of essential services, or damage to the environment. As a result, international standards organizations such as the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) have published widely recognized and widely adopted standards against which software developers can certify the safety of their software. Examples include ISO 26262 (Road vehicles – Functional safety) for automotive, EN 50128 (Communication, signaling and processing systems – Software for railway control and protection systems) for rail transport, and IEC 61508 (Functional Safety of Electrical / Electronic / Programmable Electronic Safety-related Systems) for industrial applications.

The responsibility for demonstrating that application software, and the software methods, processes, and toolchains used to develop it, comply with the relevant functional safety standards lies firmly with the application developer. However, it remains a fact that significant parts of the toolchain lie outside the developer’s control. This is one of the reasons why compiler validation − an area in which Solid Sands is already a world leader − has become a key issue for developers of safety-critical systems. Virtually no compiler is bug-free, so it is extremely important to know where a compiler malfunctions so that compiler errors can be avoided.

It is also true that a significant part of the code that becomes part of the complete application is likely to be compiled with a different use-case, set of compiler options, and compilation environment from those being used by the developer. This is because part of the code that typically ends up in an application comprises pre-compiled library functions, such as those in the C Standard Library (libc) that is often supplied in binary format as part of a software development kit (SDK).

Contrary to the commonly held belief that because a library is supplied in binary format it is insensitive to any particular use-case − i.e. the code is invariant − in practice this is not the case. The inclusion of macros and type-generic templates frequently makes library components use-case sensitive. So even if the library was pre-qualified by the SDK supplier using the same compiler delivered with the SDK, the matching use-case, compiler options, and target hardware environment requirements are almost certain not to have been met, making it difficult to demonstrate functional safety standard compliance.

To overcome this limitation, Solid Sands has introduced a new library qualification tool called the SuperGuard C Library Safety Qualification Suite − a requirements-based test suite for the C Standard Library with full traceability from individual test results back to requirements derived from the ISO C language specification. SuperGuard can be used to support qualification of C Standard Library implementations for safety-critical applications both for unmodified third-party library implementations and self-developed or self- maintained implementations.


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