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 enumberation type members to have the same value.

Non-Compliant Code Example

In this non-compliant code example, numeration type members can be assigned explicit values:

enum {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

• provide no explicit integer assignments, for example:

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

• assign a value to the first member only (the rest are then sequential), for example:

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

• assign a value to all members, so any equivalence is explicit, for example:

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

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

Risk Assessment

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

Examples of vulnerabilities resulting from the violation of this recommendation can be found on the CERT website.

References

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