Question

For example:

#define FOO(x)     (printf(x))

and

#define FOO(x)     {printf(x)}

It seems that both are viable for preprocessing, but which is better?

Answer 1

If you’re treating the macro as an expression, use the () form.

If you’re treating it as a command (and never as an expression) then use the {} form. Or rather, use the do{}while(0) form as that has fewer substitution hazards when used near keywords like if:

#define FOO(x) do {    \
    printf(x);         \
} while(0)