If a macro can be used in an expression or the parameter passed to a macro can be an expression, use parenthesis around them.

Consider a macro that calculates the square of number.

// Not allowed !!!
#define SQUARE(p) p * p

When the macro is used as follows:

SQUARE(5 + 2)

it is expanded into:

5 + 2 * 5 + 2

The result is 17 instead of 49.

To prevent such unexpected results, always use parentheses around the entire macro and macro parameters, as in:

// OK
#define SQUARE(p) ((p) * (p))

// OK
#define SOMETHING SOMETHING_ELSE