When defining macros, put parentheses around all variable names. This ensures that the macro is evaluated in a predictable manner. See also PRE02.
Non-Compliant Code Example
#define PRODUCT(A,B) A * B int a = PRODUCT(3+4, 5)
PRODUCT(3+4, 5) is converted to 3+4 * 5 by the preprocessor, which the compiler intreprets as 3+(4*5) = 23. Presumably, this does not match the intended behavior of the macro, which is (3+4) * 5 = 35.
Compliant Solution
By adding parentheses around each argument, this macro (correctly) evaluates to 35.
#define PRODUCT(A,B) ((A) * (B)) int a = PRODUCT(3+4, 5)
References
comp.lang.c FAQ list - Question 10.1