PRE02-A. Macro expansion must always be parenthesized

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.

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

References

[[Summit 05]] Question 10.1
[[ISO/IEC 9899-1999]] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment"