Q: How can %f be used for both float and double arguments in printf? Aren't they different types?
A: In the variable-length part of a variable-length argument list, the ``default argument promotions'' apply: types char and short int are promoted to int, and float is promoted to double. (These are the same promotions that apply to function calls without a prototype in scope, also known as ``old style'' function calls; see question 11.3.) Therefore, printf's %f format always sees a double. (Similarly, %c always sees an int, as does %hd.) See also questions 12.9 and 12.13.
ISO Sec. 18.104.22.168
H&S Sec. 6.3.5 p. 177, Sec. 9.4 pp. 272-3