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#define sum a+b
/* ... */
int result = sum*4;

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

 Compliant Solution

Parenthesizing the macro yields the expected answer.

#define sum (a+b)
/* ... */
int result = sum*4;

Note that there must be a space after sum, otherwise it becomes a function-like macro.

Exceptions

A macro that expands to a single identifier or function call will not change the precedence of any operators in the surrounding expression, so it need not be parenthesized.

#define MY_PID getpid()

 Risk Assessment

Failing to parenthesize around a function-like macro can result in unexpected arithmetic results.

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