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     The va_start, va_arg, and va_end macros provide a portable way to
     access the arguments to a function when it takes a variable number
     of arguments. Two versions of the macros are available: the macros
     defined in STDARG.H conform to the ANSI C standard, and the macros
     defined in VARARGS.H are compatible with the UNIX System V
     definition.
 
     Both versions of the macros assume that the function takes a fixed
     number of required arguments, followed by a variable number of
     optional arguments. The required arguments are declared as
     ordinary parameters to the function and can be accessed through
     the parameter names. The optional arguments are accessed through
     the STDARG.H or VARARGS.H macros, which set a pointer to the first
     optional argument in the argument list, retrieve arguments from
     the list, and reset the pointer when argument processing is
     completed.
 
     The proposed ANSI C standard macros (defined in STDARG.H), are
     used as follows:
 
       1. All required arguments to the function are declared as
          parameters in the usual way. The va_dcl macro is not used
          with the STDARG.H macros.
 
       2. The va_start macro sets <arg_ptr> to the first optional
          argument in the list of arguments passed to the function.
          The argument <arg_ptr> must have va_list type. The argument
          <prev_param> is the name of the required parameter immediately
          preceding the first optional argument in the argument list.
          If <prev_param> is declared with the register storage class,
          the macro's behavior is undefined. The va_start macro must be
          used before va_arg is used for the first time.
 
       3. The va_arg macro does the following:
 
             ■ Retrieves a value of <type> from the location given by
               <arg_ptr>
 
             ■ Increments <arg_ptr> to point to the next argument in
               the list, using the size of <type> to determine where
               the next argument starts
 
          The va_arg macro can be used any number of times within the
          function to retrieve arguments from the list.
 
       4. After all arguments have been retrieved, va_end resets the
          pointer to NULL.
 
     Return Value
 
     The va_arg macro returns the current argument; va_start and va_end
     do not return values.
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