PRE02-C. Macro replacement lists should be parenthesized

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

Noncompliant Code Example

This CUBE() macro definition is noncompliant 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);

This compliant solution violates PRE00-C. Prefer inline or static functions to functionlike macros. Consequently, this solution would be better implemented as an inline function.

Noncompliant Code Example

In this noncompliant code 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 (getchar() EOF) {
   /* ... */
}

In this example, the programmer has mistakenly omitted the comparison operator (see MSC02-C. 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-C. Const-qualify immutable objects.

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 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-C. 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 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]] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment"
[[ISO/IEC PDTR 24772]] "JCW Operator precedence/Order of Evaluation", "NMP Pre-processor Directions"
[[Plum 85]] Rule 1-1
[[Summit 05]] Question 10.1

      01. Preprocessor (PRE)       PRE03-C. Prefer typedefs to defines for encoding types