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

Noncompliant Code Example

This CUBE() macro definition is noncompliant because it fails to parenthesize the parameter names:

#define CUBE(I) (I * I * I)

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

PRE01-C-EX1: 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. Because commas have lower precedence than any other operator, there is no chance of the actual arguments being parsed in a surprising way. Comma separators, which separate arguments in a function call, also have lower precedence than other operators, although they are technically different from comma operators.

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

PRE01-C-EX2: Macro parameters cannot be individually parenthesized when concatenating tokens using the ## operator, converting macro parameters to strings using the # operator, or concatenating adjacent string literals. The following JOIN() macro concatenates both arguments to form a new token. The SHOW() macro converts the single argument into a string literal, which is then passed as a parameter to printf() and as a string and as a parameter to the %d specifier. For example, if SHOW() is invoked as SHOW(66);, the macro would be expanded to printf("66" " = %d\n", 66);.

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

See PRE05-C. Understand macro replacement when concatenating tokens or performing stringification for more information on using the ## operator to concatenate tokens.

Risk Assessment

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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When defining macros, put parentheses around all variable names. This ensures that the macro is evaluated in a predictable manner. See also PRE02.

Non-Compliant Code Example

#define PRODUCT(A,B) A * B
int a = PRODUCT(3+4, 5)

PRODUCT(3+4, 5) is converted to 3+4 * 5 by the preprocessor, which the compiler intreprets as 3+(4*5) = 23. Presumably, this does not match the intended behavior of the macro, which is (3+4) * 5 = 35.

Compliant Solution

By adding parentheses around each argument, this macro (correctly) evaluates to 35.

#define PRODUCT(A,B) ((A) * (B))
int a = PRODUCT(3+4, 5)

References

comp.lang.c FAQ list - Question 10.1