Use parenthesis around any macro definition that contains operators.

Non-Compliant Coding Example

In this non-compliant coding example, EOF is defined as -1. This macro definition consists of a unary negation operator '-' followed by an integer literal '1'.

#define EOF -1
/* ... */
if (c EOF) {
   /* ... */
}

In this example, the programmer has mistakenly omitted the comparison operator (see \[[MSC02-A. Avoid errors of omission]\]) from the conditional statement, which should be {{c \!= EOF}}. After macro expansion, the conditional expression is incorrectly evaluated as a binary operation: {{c-1}}. This is syntactically correct, even though it is certainly not what the programmer intended.

Parenthesizing the -1 in the declaration of EOF ensures that the macro expansion is evaluated correctly.

#define EOF (-1)

Once this modification is made, the non-compliant code example no longer compiles as the macro expansion results in the conditional expression c (-1), which is no longer syntactically valid.

Compliant Solution

The following compliant solution uses parenthesis around the macro definition and adds the (previously omitted) comparison operator.

#define EOF (-1)
/* ... */
if (c != EOF) {
   /* ... */
}

Risk Assessment

Failure to use parenthesis around macro definitions that contain operators can result in unintended program behavior.

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

References

\[[Plum 85|AA. C References#Plum 85]\] Rule 1-1
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment"