PRE01-A. Use parentheses within macros around parameter names

Parenthesize all parameter names in macro definitions. See also PRE00-A. Prefer inline functions to macros and PRE02-A. Macro replacement lists should be parenthesized.

Non-Compliant Code Example

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

#define CUBE(I) (I * I * I)
int a = 81 / CUBE(2 + 1);

As a result, the invocation

int a = 81 / CUBE(2 + 1);

expands to

int a = 81 / (2 + 1 * 2 + 1 * 2 + 1);  /* evaluates to 11 */

which is clearly not the desired result.

Compliant Solution

Parenthesizing all parameter names in the CUBE() macro allows it to expand correctly (when invoked in this manner).

#define CUBE(I) ( (I) * (I) * (I) )
int a = 81 / CUBE(2 + 1);

Exceptions

When the parameter names are surrounded by commas in the replacement text, regardless of how complicated the actual arguments are, there is no need for parenthesizing the macro parameters. Since commas have lower precedence than any other operator, there is no chance of the actual arguments being parsed in a surprising way.

#define FOO(a, b, c) bar(a, b, c)
/* ... */
FOO(arg1, arg2, arg3);

When token pasting, stringization, and string concatenation are desired, the arguments must not be parenthesized individually.

#define JOIN(a, b) (a ## b)
#define SHOW(a) printf(#a " = %d\n", a) 

Risk Assessment

Failing to parenthesize the parameter names in a macro can result in unintended program behavior.

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

References

[[Plum 85]]
[[Summit 05]] Question 10.1
[[ISO/IEC 9899-1999]] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment"

PRE00-A. Prefer inline functions to macros      01. Preprocessor (PRE)       PRE02-A. Macro replacement lists should be parenthesized