You can execute multiple traversals of the argument list, each
    delimited by va_start . . . va_end. The va_end function sets ap
    equal to NULL.

    When using this function 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_end only in conjunction
    with other routines 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.