CertC-PRE01ΒΆ

Use parentheses within macros around parameter names

Required inputs: IR

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.

Recommendation Severity Likelihood Remediation Cost Priority Level
PRE01-C Medium Probable Low P12 L1
Related Guidelines
SEI CERT C++ Coding Standard VOID PRE01-CPP. Use parentheses within macros around parameter names
ISO/IEC TR 24772:2013 Operator Precedence/Order of Evaluation [JCW]
Pre-processor Directives [NMP]
MISRA C:2012 Rule 20.7 (required)
Bibliography
[ Plum 1985]
[ Summit 2005] Question 10.1
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/recommendations/preprocessor-pre/pre01-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

unsafe_macro_param

Macro parameter not enclosed in parentheses

None

False

Options