If a constant value is given for a given identifier, do not diminish the modifiability of the code in which it is used by assuming its value in expressions. Just giving the constant a name is not enough to ensure modifiability; you must be careful always to use the name, and remember that the value could change. This is related to DCL06-A. Use meaningful symbolic constants to represent literal values in program logic.
Non-Compliant Coding Example
This non-compliant coding example defeats the purpose of defining BUFSIZ as a constant by assuming its value in the following expression:
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The assumption is that "everyone knows that BUFSIZ equals 512," and right-shifting nine bits is the same (for positive numbers) as dividing by 512. However, if BUFSIZ changes to 1024 on some systems, modifications are difficult and error-prone.
Compliant Solution
This compliant solution uses the identifier assigned the constant value in the expression.
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Most modern C compilers will optimize this code appropriately.
References
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\[[Plum 85|AA. C References#Plum 85]\] Rule 1-5 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment" |