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

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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);

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int a = 81 / CUBE(i);

expands to

int a = 81 / i * i * i;

which evaluates as

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

which is not the desired behavior.

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

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

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

#define EOFEND_OF_FILE -1
/* ... */
if (getchar() EOFEND_OF_FILE) {
   /* ... */
}

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

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

#define EOFEND_OF_FILE (-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 END_OF_FILE because, otherwise, it becomes a function-like macro (and one that is incorrectly formed because -1 −1 cannot be a formal parameter).

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In this compliant solution, the macro definition is replaced with an enumeration constant in compliance with recommendation DCL00-C. Const-qualify immutable objects. In addition, since because EOF is a reserved macro defined in the <stdio.h> header, the compliant solution must also use a different indentifier in order to comply with rule DCL37-C. Do not declare or define a reserved identifier.

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

Exceptions

PRE02-C-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()

PRE02-C-EX2: A macro that expands to an array reference using the array-subscript operator [], or an expression designating a member of a structure or union object using either the member-access . or -> operators is not affected by the precedence of any operators in the surrounding expression, so its replacement list need not be parenthesized.

#define NEXT_FREE block->next_free
#define CID customer_record.account.cid
#define TOOFAR array[MAX_ARRAY_SIZE]

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Failing to parenthesize macro replacement lists can cause unexpected results.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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