Library Qualification

Library qualification is the systematic process of establishing confidence that pre-existing software components (libraries) are suitable for use in safety-critical systems. This process demonstrates that libraries behave as specified, are free from unacceptable defects, and meet the required safety integrity levels.

Library qualification is especially relevant in industries like automotive (ISO 26262), aerospace (DO-178C), medical devices (IEC 62304), and railway (EN 50128), where standards mandate rigorous evaluation of all software components integrated into safety-critical applications. 

Software libraries contain code that is directly integrated into the final application running on the target hardware. Any defects, vulnerabilities, or unexpected behaviors within library code can directly compromise the safety and reliability of the entire system. Unlike development tools that are used during the build process but don't appear in the final product, libraries become embedded components of the safety-critical system itself. Library qualification provides documented evidence that each library behaves as specified across all required operating conditions, including both normal operation and fault scenarios, thereby reducing the risk of systematic failures in the final system.

Modern automotive systems incorporate vast amounts of pre-existing software, such as the C++ Standard Template Library, mathematical function libraries, communication protocol stacks, and operating system components. While using these libraries can accelerate development and reduce costs, their trustworthiness for safety-critical applications is not automatically guaranteed. If a library contains undetected errors, race conditions, or vulnerabilities, these flaws could propagate into the final safety-critical system, potentially leading to hazardous situations such as unintended vehicle acceleration, brake failures, or steering malfunctions.

Library qualification acts as a safeguard, ensuring that these reusable software elements meet stringent safety requirements and are traceable to specific Automotive Safety Integrity Level (ASIL) ratings before being integrated into vital automotive functions.

While both library qualification and tool qualification aim to establish confidence in software used during safety-critical development, they address fundamentally different risks:

• Tool Qualification focuses on software tools (such as compilers, static analyzers, or code generators) used to develop, build, or verify the application. Tools do not reside in the final product deployed on target hardware. Tool qualification ensures that these development tools do not introduce errors into the final application.

• Library Qualification focuses on software components (such as mathematical libraries, middleware, or protocol stacks) that become embedded parts of the final application running on the target system. Libraries are directly integrated into the executable code and their behavior directly affects system safety.

The key distinction is that tools support the development process while libraries become integral parts of the safety-critical system itself, requiring different qualification approaches and evidence.

ISO 26262, the automotive functional safety standard, places significant emphasis on the proper management and qualification of all software components, including pre-existing libraries. Part 8, Clause 12 specifically addresses "Qualification of software components," outlining comprehensive requirements for demonstrating the trustworthiness and suitability of software elements not developed according to the standard.

The standard requires a systematic approach to evaluate library suitability, involving:

• Assessment of the library's development process and quality management

• Evaluation of testing history and code coverage metrics

• Analysis of the library's potential impact on system safety

• Verification that the library meets required ASIL (Automotive Safety Integrity Level) ratings

• Documentation of all qualification evidence for audit purposes

The goal is to ensure that even pre-existing software components meet the necessary ASIL requirements (A, B, C, or D) for the specific automotive application, with ASIL D representing the highest safety criticality level.

While ISO 26262 provides specific guidance for the automotive industry, the concept of qualifying pre-existing software components is fundamental across all safety-critical domains. Major international standards include specific requirements for library qualification:

• IEC 61508 (General Functional Safety): Establishes the foundation for functional safety management, including requirements for pre-existing software elements used in industrial safety systems

• DO-178C (Aerospace Software): Requires rigorous qualification of software components used in airborne systems, with emphasis on code coverage and traceability

• IEC 62304 (Medical Device Software): Mandates qualification of software libraries used in medical devices, particularly those affecting patient safety

• EN 50128 (Railway Software): Specifies qualification requirements for software components in railway control and protection systems

This cross-industry emphasis underscores the critical importance of robust library qualification in achieving overall system safety. While the specific requirements and terminology may vary between standards, the fundamental principles remain consistent: all software components integrated into safety-critical systems must be systematically evaluated, tested, and documented to demonstrate their suitability and safety.