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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.

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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);  /* evaluatesEvaluates 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 than 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 below concatenates both arguments to form a new token. The SHOW() macro converts the single argument into a string literal, which is then concatenated with the adjacent string literal to form the format specification in the call to printf()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

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Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V	macro-parameter-parentheses	Fully checked
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-PRE01	Fully implemented
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.PRE01	Fully implemented
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C3410
Klocwork	Include Page Klocwork_V Klocwork_V	MISRA.DEFINE.NOPARS
LDRA tool suite	Include Page LDRA_V LDRA_V	78 S	Enhanced Enforcement
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-PRE01-a	In the definition of a function-like macro each instance of a parameter shall be enclosed in parentheses unless it is used as the operand of # or ##
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	9022	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec. PRE01-C	Checks for expanded macro parameters not enclosed in parentheses (rule partially supported)
PVS-Studio	Include Page PVS-Studio_V PVS-Studio_V	V733
RuleChecker	Include Page RuleChecker_V RuleChecker_V	macro-parameter-parentheses	Fully checked

The LDRA tool suite V 7.6.0 can detect violations of this recommendation.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "JCW Operator precedence/Order of Evaluation"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 19.1
\[[Plum 85|AA. C References#Plum 85]\]
\[[Summit 05|AA. C References#Summit 05]\] Question 10.1

Related Guidelines

Bibliography

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