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

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function-like macros and PRE01-C. Use parentheses within macros around parameter names.

Noncompliant Code Example

This CUBE() macro definition is non-compliant 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);  /* evaluatesEvaluates 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

Unexpected behavior resulting from macro expansion is not limited This compliant solution violates PRE00-C. Prefer inline or static functions to function-like macros.   The object-like macro definition in this example is non-compliant because its replacement list is not parenthesized.Consequently, this solution would be better implemented as an inline function.

Noncompliant Code Example

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

#define sum a+bEND_OF_FILE -1
/* ... */
int result = sum*4;

The value of result is a+(b*4) instead of the expected (a+b)*4.

Compliant Solution

if (getchar() END_OF_FILE) {
   /* ... */
}

In this example, the programmer has mistakenly omitted the comparison operator from the conditional statement, which should be getchar() != END_OF_FILE. (See 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 DCL00-C. Const-qualify immutable objects.

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

#define END_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 END_OF_FILE 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. In addition, 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 DCL37-C. Do not declare or define a reserved identifierParenthesizing the macro replacement list yields the expected answer.

enum { END_OF_FILE = -1 };
#define sum (a+b)
/* ... */
int result = sum*4;

...

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()

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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]

Risk Assessment

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.

References

Related Guidelines

Bibliography

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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment" \[[Summit 05|AA. C References#Summit 05]\] Question 10.1