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#define END_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() != 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.

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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).

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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

Bibliography

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