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Macro replacement lists should be parenthesized to protect any lower-precedence operators from the surrounding expression. See also PRE00-C and PRE01-C.

Non-Compliant Code Example

This CUBE() macro definition is non-compliant noncompliant because it fails to parenthesize the replacement list.

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which is not the desired behavior.

Compliant Solution

With its replacement list parenthesized, the CUBE() macro expands correctly for this type of invocation.

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This compliant solution violates PRE00-C. Consequently, this solution would be better implemented as an inline function.

Non-Compliant Code Example

In this non-compliant noncompliant code example, EOF is defined as -1. The macro replacement list consists of a unary negation operator '-' followed by an integer literal '1'.

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In this example, the programmer has mistakenly omitted the comparison operator (see MSC02-AC. Avoid errors of omission) from the conditional statement, which should be getchar() != EOF. After macro expansion, the conditional expression is incorrectly evaluated as a binary operation: getchar()-1. This is syntactically correct, even though it is certainly not what the programmer intended. Note that this example also violates DCL00-AC. Const-qualify immutable objects.

Parenthesizing the -1 in the declaration of EOF ensures that the macro expansion is evaluated correctly.

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Once this modification is made, the non-compliant noncompliant code example no longer compiles because the macro expansion results in the conditional expression getchar() (-1), which is no longer syntactically valid. Note that there must be a space after EOF because otherwise it becomes a function-like macro (and one that is incorrectly formed, because -1 cannot be a formal parameter).

Compliant Solution

In this compliant solution, the macro definition is replaced with an enumeration constant in compliance with DCL00-AC. Const-qualify immutable objects.

enum { EOF = -1 };
/* ... */
if (getchar() != EOF) {
   /* ... */
}

Exceptions

PRE02-EX1. A macro that expands to a single identifier or function call is not affected by the precedence of any operators in the surrounding expression, so its replacement list need not be parenthesized.

#define MY_PID getpid()

Risk Assessment

Failing to parenthesize macro replacement lists can cause unexpected results.

Automated Detection

The LDRA tool suite V 7.6.0 is able to 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", "NMP Pre-processor Directions"
\[[Plum 85|AA. C References#Plum 85]\] Rule 1-1
\[[Summit 05|AA. C References#Summit 05]\] Question 10.1

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PRE01-C      Edit       PRE03-AC. Prefer typedefs to defines for encoding types