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NASA's 10 Rules for Safety-Critical Code: A Pragmatic Approach to Software Reliability
0:00:03 Introduction: NASA's approach to writing safety-critical code is discussed, emphasizing the need for strict, verifiable coding guidelines.
0:00:10 Problem with Existing Guidelines: Most existing coding guidelines are overly long (over 100 rules), often based on personal preferences, and lack comprehensive tool-based compliance checks.
0:02:01 Proposed Solution: A small, well-chosen, and verifiable set of coding rules (limited to 10) can significantly improve software reliability and analyzability.
0:03:33 Language Choice (C): The rules primarily target C due to its extensive tool support (source code analyzers, model extractors, debuggers, etc.) and history in critical systems.
0:04:54 Rule 1 (Simple Control Flow): Restrict code to simple control flow constructs. Avoid goto, setjmp, longjmp, and recursion (direct or indirect) to enhance verifiability and code clarity. This allows for easy verification that all code exectutions that should be bounded are in fact bounded.
0:09:09 Rule 2 (Fixed Loop Bounds): All loops must have a statically provable upper bound on iterations to prevent runaway code.
0:10:36 Rule 3 (No Dynamic Memory After Initialization): Avoid dynamic memory allocation (e.g., malloc, garbage collection) after initialization to eliminate memory-related errors and ensure predictable memory usage. All needed memory should use pre-allocation or stack memory.
0:14:06 Rule 4 (Function Length Limit): Functions should be short enough to fit on a single printed page (typically around 60 lines) to maintain understandability and verifiability.
0:15:51 Rule 5 (Assertion Density): Maintain an average of at least two assertions per function to check for anomaly conditions. Assertions must be side-effect-free and trigger explicit recovery actions (e.g., returning an error).
0:19:29 Rule 6 (Smallest Scope): Declare data objects at the smallest possible scope to support data hiding, prevent corruption, and simplify fault diagnosis.
0:21:12 Rule 7 (Check Return Values and Parameters): The return value of non-void functions must be checked by the calling function, and parameter validity must be checked within each function. Explicitly cast to void if ignoring a return value is intentional.
0:24:31 Rule 8 (Limited Preprocessor Use): Restrict preprocessor use to header file inclusion and simple macro definitions. Avoid complex features like token pasting, variable argument lists, and recursive macros to maintain code clarity and avoid testing nightmares.
0:27:54 Rule 9 (Restricted Pointer Use): Limit pointer dereferencing to one level. Avoid hiding pointer operations in macros or typedef declarations. Function pointers are not permitted.
0:30:11 Rule 10 (Compiler Warnings and Static Analysis): Compile code with all warnings enabled at the most pedantic setting, and ensure zero warnings. Use static source code analyzers daily, and address any warnings by rewriting the code.
0:33:16 Conclusion: These rules, while strict, are designed to enhance code clarity, analyzability, and safety, particularly in safety-critical applications. They act as "seat belts" for software development, becoming second nature over time.