How does printf and co differentiate between float and double

Solution 1:

It doesn't differentiate. It's not possible to receive a float as a vararg: any float argument that you provide is first promoted to double.

6.5.2.2/6 defines "default argument promotions", and /7 states that default argument promotions are applied to "trailing arguments", that is varargs denoted by ....

how does it work for scanfs/sscanf?

The %f format for scanf requires a pointer to float. %lf requires a pointer to double, %Lf requires a pointer to long double.

copying the value to a temp and casting(is this right?)

If you provide a float argument, then the implementation creates a temporary of type double, initializes it with the float value, and passes this as the vararg. Casting by definition is explicit conversion by use of the cast operator -- you can cast if you like in order to make it exactly clear to the reader what's going on, but float f = 3; printf("%f", f); is exactly the same as float f = 3; printf("%f", (double)f);. The default argument promotion has the same meaning as the cast.