MISRA C:2012 Rule 20.9

All identifiers used in the controlling expression of #if or #elif preprocessing directives shall be #define’d before evaluation

Description

Rule Definition

All identifiers used in the controlling expression of #if or #elif preprocessing directives shall be #define’d before evaluation.

Rationale

If attempt to use a macro identifier in a preprocessing directive, and you have not defined that identifier, then the preprocessor assumes that it has a value of zero. This value might not meet developer expectations.

Additional Message in Report

Identifier is not defined.

Troubleshooting

If you expect a rule violation but do not see it, refer to Coding Standard Violations Not Displayed.

Examples

Macro Identifiers

#if M == 0                   /* Non-compliant - Not defined */
#endif

#if defined (M)              /* Compliant - M is not evaluate */
#if M == 0                   /* Compliant - M is known to be defined */
#endif
#endif

#if defined (M) && (M == 0)  /* Compliant
                              * if M defined, M evaluated in ( M == 0 ) */
#endif

This example shows various uses of M in preprocessing directives. The second and third #if clauses check to see if the software defines M before evaluating M. The first #if clause does not check to see if M is defined, and because M is not defined, the statement is noncompliant.

Check Information

Group: Preprocessing Directives

Category: Required

AGC Category: Required