The va_arg() macro expands to an expression that has the type and value of the
       next argument in the call.  The argument ap is the va_list ap initialized by
       va_start().  Each call to va_arg() modifies ap so that the next call returns
       the next argument.  The argument type is a type name specified so that the
       type of a pointer to an object that has the specified type can be obtained
       simply by adding a * to type.
 
       The first use of the va_arg() macro after that of the va_start() macro returns
       the argument after last.  Successive invocations return the values of the
       remaining arguments.
 
       If there is no next argument, or if type is not compatible with the type of
       the actual next argument (as promoted according to the default argument
       promotions), random errors will occur.
 
       If ap is passed to a function that uses va_arg(ap,type) then the value of ap
       is undefined after the return of that function.
 

The va_start(), va_arg(), and va_end() macros conform to C89. C99 defines the va_copy() macro.