Q: I have a varargs function which accepts a float parameter. Why isn't
A: In the variable-length part of variable-length argument lists, the old ``default argument promotions'' apply: arguments of type float are always promoted (widened) to type double, and types char and short int are promoted to int. Therefore, it is never correct to invoke va_arg(argp, float); instead you should always use va_arg(argp, double). Similarly, use va_arg(argp, int) to retrieve arguments which were originally char, short, or int. (For analogous reasons, the last ``fixed'' argument, as handed to va_start, should not be widenable, either.) See also questions 11.3 and 15.2.
ISO Sec. 18.104.22.168
Rationale Sec. 22.214.171.124
H&S Sec. 11.4 p. 297