The strtold function converts the initial portion of the string
    pointed to by nptr to long double representation.

    First, this function decomposes the input string into three parts: an
    initial, possibly empty, sequence of white-space characters (as
    specified by the isspace function); a subject sequence resembling a
    floating-point constant or representing an infinity or NaN; and a
    final string of one or more unrecognized characters, including the
    terminating null character of the input string. Then, it attempts to
    convert the subject sequence to a floating-point number, and returns
    the result as a value of type long double.

    The expected form of the (initial portion of the) string is optional
    leading white space, an optional plus ('+') or minus sign ('-') and
    then either (i) a decimal number, or (ii) a hexadecimal number, or
    (iii) an infinity, or (iv) a NaN (not-a-number).