How to prevent scanf causing a buffer overflow in C?

I use this code:

while ( scanf("%s", buf) == 1 ){

What would be the best way to prevent possible buffer overflow so that it can be passed strings of random lengths?

I know I can limit the input string by calling for example:

while ( scanf("%20s", buf) == 1 ){

But I'd prefer to be able to process whatever the user inputs. Or can't this be done safely using scanf and I should use fgets?