1. Requirements

3.3.2 The project shall ensure that software coding methods, standards, and/or criteria are adhered to and verified.

1.1 Notes

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

1.2 Applicability Across Classes

Classes F and G are labeled with "X (not OTS)." This means that this requirement does not apply to off-the-shelf software for these classes.

2. Rationale

NASA programs and projects have multiyear 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 development, for 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 risk of software malfunction for the project during its operations and maintenance phases.

3. Guidance

Coding standards are used to prevent or reduce unsafe coding practices. 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. Language specific rules and best coding practices are usually determined by experts in the particular language [e.g., C++, ADA] and tailored as needed by the project. Usage types and severity levels are set by the user.

Over time, correlations between bugs and coding practices resulted in a set of rules that helped to prevent coding errors from occurring.

In a team environment or group collaboration, the use of coding standards ensures uniform coding practices, reduces oversight errors, and the time spent in code reviews. When NASA software development work is outsourced to a supplier (see Topic 7.3 03 - Acquisition Guidance), having a set of coding standards in place helps to ensure that the code produced by the contractor 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 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).

One way to tell the difference between a coding style element and a coding standard element is to compare the functionality of the resulting code if either element is removed or changed. For example, if the style element is not followed in the software work product, the code should still work exactly the same way, with exactly the same behavior and safety checks. However, if a coding standard element is removed or altered, it is likely that the the code's "safety" and/or functionality is changed. That is the difference between the two. A coding standard is obviously more important than a coding style.

How Coding Standards are Classified

Software coding standards are classified by language, usage, and severity levels. Language specific rules and best coding practices are determined by industry experts in that particular language. Usage types and severity levels are set by the user.

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.
• Limiting the size of modules.
• Using library routines, especially the following:
  • Operating system routines.
  • Commercial library routines (e.g., numerical analysis).
  • Project-specific utility routines.
• Defining constants and data types.
• Using global data.
• Using compiler-specific features not in the language standard."

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The following items may be an integral part of the coding standard to the extent their implementation actually affects the outcome of 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.

Adherence and Verification

Assuring the adherence of the developed software 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 (see SWE-028) include reviews, such as peer reviews and inspections (see SWE-087), and assessments of how the coding standards are used to develop the software work products. The use of 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.

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 needs of the project. In appropriate settings (e.g., less "critical") a subset of this list (the "or" in "and/or") would be sufficient and compliant. For human-rated applications, the project would want to cover each of the items (methods, standards, criteria) in the statement (the "and" in "and/or"; e.g., the use of Klockwork tool as part of the "method," MISRA-C (software development standard for the C programming language developed by MISRA (Motor Industry Software Reliability Association).) "standards," avoidance of the project's restricted language constructs "criteria," etc).

Additional guidance related to the planning and control of software coding standards, and where they may be used, may be found in the following related requirements in this Handbook:

4. Small Projects

Smaller projects may consider using previously developed/tailored coding methods, standards and guidelines, rather than developing their own. These standard applications may be available in the software Process Asset Libraries (PAL) of other Centers, if not available at the performing Center.

5. Resources

6. Lessons Learned

The NASA Lessons Learned database contains the following lessons learned related to coding standards:

• Software Design for Maintainability. Lesson Number 0838. NASA has long recognized that software maintenance is a large cost driver in its software systems. Early planning for maintenance, which includes the use of appropriate and consistent coding standards, coding styles, and software configuration management systems, helps hold down software maintenance costs over the software life cycle. The lessons learned citation given below provides additional information on this topic of coding standards.

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• Mars Pathfinder Flight Software Development Process (1997). Lesson Number 0590. The software developer should also recognize that there is benefit to using coding standards dedicated to the project at hand. As this set of lessons learned derived from the Mars Pathfinder project shows, even though the coding standards and styles were tailored for the project, maintaining the set of derived coding standards for future upgrades and improvements will assure cost control

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