1. Requirements

1.1 Notes

NPR 7150.2, NASA Software Engineering Requirements, does not include any notes for this requirement.

1.2 History

1.3 Applicability Across Classes

2. Rationale

To ensure safety, security, reliability, quality, maintainability, readability, and testability of the NASA code products. 

3. Guidance

NASA programs and projects have multi-year life cycle times. Often the software personnel who develop the original software work products move on to other projects. These developers are then backfilled on the team by other developers for the remainder of the development, operations, maintenance, and disposal phases of the life cycle. This personnel turnover process may occur several times during the project's life cycle. The use of uniform software coding methods, standards, and/or criteria ensures uniform coding practices, reduces errors through safe language subsets, and improves code readability. Verification that these practices have been adhered to reduces the risk of software malfunction for the project during its operations and maintenance phases.

There are five key benefits of using coding standards:

1. Understanding, maintainability, and readability of the code.

2. Consistent code quality — no matter who writes the code.

3. Software security from the start.

4. Reduced development costs.

5. Testability of the code.

Coding standards help prevent or reduce unsafe coding practices such as defect-prone coding styles, security issues from specific coding sequences, and numerous coding errors, mistakes, and misunderstandings. 

"Coding standards are the ‘materials and manufacturing standards’ for implemented software...
Human-rated certification and mission-critical software both require applying a recognized coding standard, one that is supported by automated analysis tools, for all software required to be certified for human spaceflight and mission-critical applications.  Manual verification is all but impossible.

The reduction in effort by simply adhering to a coding standard that can be tested through automation is the one certification process that truly has no other efficient verification method. ...  As security becomes an issue, security coding standards should also be applied.” 

3.1 General Coding Standard Guidance

Planning for the adoption and use of coding standards at the beginning of a software development activity sets the right tone and approach for the development team.

Software coding standards are classified by language, usage, and severity levels. 

Correlations between bugs and coding practices resulted in rules that helped prevent coding errors from occurring. These activities resulted in recommendations to develop and use coding standards.

In a team environment or group collaboration, the use of coding standards ensures uniform coding practices and reduces oversight errors and the time spent in code reviews. When NASA software development work is outsourced to a supplier (see 7.03 - Acquisition Guidance), having a set of coding standards in place helps ensure that the code meets quality requirements mandated by NASA in the NASA Software Assurance Standard, NASA STD 8739.8.

A coding standard document may be written as a general document that is independent of any project. Project-specific needs are then added as amendments to the document. Note that there is an important difference between a coding style and a coding standard. A coding style specifies how you indent lines or employ tabs and spaces to make the code easier to read by the software development team. A coding standard, which often includes a coding style, goes beyond just how to name a variable. It tells you how that variable is to be treated and when it is to be used (and not used). 

3.2 How Coding Standards are Classified

Software coding standards are classified by language, usage, and severity levels. Industry experts determine Language-specific rules and best coding practices in that particular language. 

Coding standards should address (for all the languages used):

• Code structure (includes overall project layout (files, and so on), classes, resources, and other source file types).
• Error handling (describes how objects handle errors, reporting, and logging).
• Module size (should be limited).
• Library routines, especially the following:
• Operating system routines.
• Commercial library routines (e.g., numerical analysis).
• Project-specific utility routines.
• Constants and data types (rules for defining).
• Global data use.
• Compiler-specific features are not in the language standard.

The following items may be an integral part of the coding standard to the extent their implementation affects the execution of the software. Otherwise, they are part of the coding style.

• Formatting: Includes the use of white space, indentation, and length of statement lines in code. Some standards might include, for example, common editor setup and handling for tabs versus spaces for indentation.
• Naming conventions: Specifies how developers name their methods, classes, variables, events, and parameters.
• Comments: An English description in the code that explains the logic of the code. (Quality code is usually self-documenting by default.) The use of quality commenting gives quality code better maintainability and easier understandability.

3.3 Adherence and Verification

“Code should be checked against the standards with the help of state-of-the-art static source code analyzers. ... Flight code should be checked nightly for compliance with a coding standard and subjected to rigorous analysis with state-of-the-art (static source code analysis tools). The warnings generated by each of these tools are combined with the output of mission-specific checkers that secure compliance with naming conventions, coding style, etc. Also, all warnings, if any (there should be none), from the standard C compiler, used in pedantic mode with all warnings enabled, should be provided to the software developers... (who) are required to close out all reports before a formal code review is initiated. In peer code reviews, an additional source of input is provided by designated peer code reviewers... Separately, key parts of the software design can also be checked for correctness and compliance with higher-level design requirements with the help of logic model checkers.”

Assuring the developed software's adherence to the coding standards provides the greatest benefit when followed from software development inception to completion. Coding standards are selected at the start of the software development effort. Verification activities of the software work products include reviews, such as peer reviews and inspections (see SWE-087 - Software Peer Reviews and Inspections for Requirements, Plans, Design, Code, and Test Procedures), and assessments of how the coding standards are used to develop the software work products.

Automated tools for assessing adherence to standards at appropriate reviews, or even on a batch mode run overnight, will assist the project team in adherence and verification. 

Training should be provided for the software development team to use the logic model checkers for the analysis and verification of flight software.

3.4 MISRA and CERT-C Coding Standards

“The CERT C Secure Coding Standard is composed of 89 rules and 132 recommendations for producing secure code.  It is recommended that compliance with a standard like CERT C be performed by a static analyzer, depending on program size and complexity.  A source code static analysis tool meeting ISO/IEC TS 17961 conformance is recommended.

The following quote from the author of the second edition of the CERT C Coding Standard describes what static analysis for conformance can imply.

While the application of these rules and recommendations does not guarantee the security of a software system, it does tell you ...that the software was developed to a set of industry-standard rules and recommendations developed by the leading experts in the field. ... that ...time and effort went into producing code that is free from the common coding errors that have resulted in numerous vulnerabilities ...over the past two decades ... that the software developers who produced the code have done so with a real knowledge of the types of vulnerabilities that can exist and the exploits that can be used against them, and consequently have developed the software with a real security mindset.”

Two example coding standards to improve safety, reliability, and security in software systems are MISRA, and CERT C.MISRA C has become the de facto standard for embedded C programming in safety-related industries and is also used to improve software quality even where safety is not the main consideration.

3.5 How to Interpret the "and/or" Phrase in the Requirements Statement

The "and/or" in the text of the requirement statement is meant for flexibility in tailoring the requirement to the project's needs. Inappropriate (e.g., less "critical") settings, a subset of ”methods, standards, criteria” would be sufficient and compliant. For human-rated applications, the project would want to cover methods, standards, and criteria (e.g., the use of the Klocwork® Insight  tool as part of the method,)

MISRA C (a software development standard for the C programming language developed by MISRA (Motor Industry Software Reliability Association)

 as part of the standards, avoidance of the project's restricted language constructs as criteria, etc.).

NASA-specific coding standards information and resources are available in Software Processes Across NASA (SPAN), accessible to NASA users from the SPAN tab in this Handbook.