Question

Why is it that scanf() needs the l in ‘%lf‘ when reading a double, when printf() can use ‘%f‘ regardless of whether its argument is a double or a float?

Example code:

double d; scanf('%lf', &d); printf('%f', d); 

Answer 1

Because C will promote floats to doubles for functions that take variable arguments. Pointers aren’t promoted to anything, so you should be using %lf, %lg or %le (or %la in C99) to read in doubles.