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Macro replacement lists should be parenthesized to protect any lower-precedence operators from the surrounding expression.  See also \[[PRE00-A. Prefer inline functions to macros]\] and \[[PRE01-A. Use parentheses within macros around parameter names]\].

Non-Compliant Code Example

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

#define CUBE(X) (X) * (X) * (X)
int i = 3;
int a = 81 / CUBE(i);

As a result, the invocation

int a = 81 / CUBE(i);

expands to

int a = 81 / i * i * i;

which evaluates as

int a = ((81 / i) * i) * i);  /* evaluates to 243 */

which is not the desired behavior.

Compliant Solution

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

#define CUBE(X) ((X) * (X) * (X))
int i = 3;
int a = 81 / CUBE(i);

Non-Compliant Code Example

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

#define EOF -1
/* ... */
if (c EOF) {
   /* ... */
}

In this example, the programmer has mistakenly omitted the comparison operator (see \[[MSC02-A. Avoid errors of omission]\]) from the conditional statement, which should be {{c \!= EOF}}. After macro expansion, the conditional expression is incorrectly evaluated as a binary operation: {{c-1}}. This is syntactically correct, even though it is certainly not what the programmer intended.

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

#define EOF (-1)

Once this modification is made, the non-compliant code example no longer compiles as the macro expansion results in the conditional expression c (-1), which is no longer syntactically valid.

Compliant Solution

The following compliant solution uses parentheses around the macro replacement list and adds the (previously omitted) comparison operator.

#define EOF (-1)
/* ... */
if (c != EOF) {
   /* ... */
}

Note that there must be a space after EOF, otherwise it becomes a function-like macro (and one that is incorrectly formed since -1 cannot be a formal parameter).

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.

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"
\[[Plum 85|AA. C References#Plum 85]\] Rule 1-1
\[[Summit 05|AA. C References#Summit 05]\] Question 10.1

PRE01-A. Use parentheses within macros around parameter names      01. Preprocessor (PRE)       PRE03-A. Prefer typedefs to defines for encoding types