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Enumeration types in C map to integers. The normal expectation is that each enumeration type member is distinct. However, there are some non-obvious errors that are commonly made that cause multiple enumeration type members A C enumeration defines a type with a finite set of values represented by identifiers known as enumeration constants, or enumerators. An enumerator is a constant integer expression whose value is representable as an int. Although the language allows multiple enumerators of the same type to have the same value, it is a common expectation that all enumerators of the same type have distinct values. However, defining two or more enumerators of the same type to have the same value can lead to some nonobvious errors.

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Noncompliant Code Example

In this non-compliant noncompliant code example, enumeration type members can be assigned explicit valuestwo enumerators of type Color are assigned explicit values. It may not be obvious to the programmer that yellow and indigo have been declared to be identical values (6), as are green and violet (7). Probably the least dangerous error that can result from such a definition is attempting to use the enumerators as labels of a switch statement. Because all labels in a switch statement are required to be unique, the following code violates this semantic constraint and is required to be diagnosed by a conforming compiler:

enum Color { red=4, orange, yellow, green, blue, indigo=6, violet };

It may not be obvious to the programmer (though it is fully specified in the language) that yellow and indigo have been declared to be identical values (6), as are green and violet (7).

Compliant Solution

Enumeration type declarations must either

const char* color_name(enum Color col) {
  switch (col) {
  case red: return "red";
  case orange: return "orange";
  case yellow: return "yellow";
  case green: return "green";
  case blue: return "blue";
  case indigo: return "indigo";   /* Error: duplicate label (yellow) */
  case violet: return "violet";   /* Error: duplicate label (green) */
  }
}

Compliant Solution

To prevent the error discussed of the noncompliant code example, enumeration type declarations must take one of the following forms:

• Provide provide no explicit integer assignments, as in this example:

enum Color
enum { red, orange, yellow, green, blue, indigo, violet };

• assign Assign a value to the first member only (the rest are then sequential), as in this example:

enum Color { red=4, orange, yellow, green, blue, indigo, violet };

• assign Assign a value to all members so any equivalence is explicit, as in this example:

enum Color {
  red=4, 
  orange=5, 
  yellow=6, 
  green=7, 
  blue=8, 
  indigo=6, 
  violet=7
};

It is also advisable to provide a comment explaining why multiple enumeration type members are being assigned the same value so that future maintainers don't mistakenly do not mistakenly identify this form as an error.

Of these three options, providing no explicit integer assignments is the simplest and consequently the preferred approach unless the first enumerator must have a nonzero value.

Exceptions

enum Color { red, orange, yellow, green, blue, indigo, violet=indigo };

Risk Assessment

Failing to ensure that constants within an enumeration have unique values can result in unexpected logic results.

Automated Detection

The LDRA tool suite V 7.6.0 is able to detect violations of this recommendation.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.2.2, "Enumeration specifiers"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "CCB Enumerator issues"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 9.3

Related Guidelines

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Image Added Image Added Image AddedINT08-A. Verify that all integer values are in range      04. Integers (INT)       INT10-A. Do not make assumptions about the sign of the remainder when using the % operator