The va_arg function interprets the object at the address
    specified by the list incrementor according to type. If there
    is no corresponding argument, the behavior is undefined.

    When using va_arg to write portable applications, include the
    <stdarg.h> header file (defined by the ANSI C standard), not the
    <varargs.h> header file, and use va_arg only in conjunction with
    other functions and macros defined in <stdarg.h>.

    For an example of argument-list processing using the <stdarg.h>
    functions and definitions, see the "Character, String, and
    Argument-List Functions" chapter of the HP C RTL Reference
    Manual.