Why does scanf() need “%lf” for doubles, when printf() is okay with just “%f”?

Question

Why is it that scanf() needs the l in \"%lf\" when reading a double, when printf() can use \"%f

Answer 1

scanf needs to know the size of the data being pointed at by &d to fill it properly, whereas variadic functions promote floats to doubles (not entirely sure why), so printf is always getting a double.