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

Noncompliant Code Example

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This compliant solution violates 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 example, the programmer has mistakenly omitted the comparison operator from the conditional statement, which should be getchar() != EOF. (See MSC02-C. Avoid errors of omission.) 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.

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

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

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

Related Guidelines

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Bibliography

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ISO/IEC PDTR 24772 "JCW Operator precedence/order of evaluation", "NMP Pre-processor directions"

Bibliography