The macro expansion must always be parenthesized to protect any lower-precedence operators from the surrounding expression.  See also \[[PRE00-A. Prefer inline functions to macros]\] and \[[PRE01-A. Use parentheses within macros around variable names]\].

Non-Compliant Code Example

This CUBE() macro definition is non-compliant because it fails to parenthesize the macro expansion.

#define CUBE(X) (X) * (X) * (X)
int i = 3;
int a = 81 / CUBE(i);

As a result, the invocation

int a = 81 / CUBE(i);

expands to

int a = 81 / i * i * i;

which evaluates as

int a = ((81 / i) * i) * i);  /* evaluates to 243 */

which is not the desired behavior.

Compliant Solution

By parenthesizing the macro expansion, the CUBE() macro expands correctly (when invoked in this manner).

#define CUBE(X) ((X) * (X) * (X))
int i = 3;
int a = 81 / CUBE(i);

Risk Assessment

Failing to parenthesize around a macro can result in unexpected arithmetic results.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

PRE02-A

1 (low)

1 (unlikely)

3 (low)

P3

L3

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

References

\[[Summit 05|AA. C References#Summit 05]\] Question 10.1
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment"