ap

    An object pointer. You must declare and use the argument ap as
    shown in the format section.

 offset

    The number of bytes by which ap is to be incremented so that
    it points to a subsequent argument within the list (that is,
    not to the start of the argument list). Using a nonzero offset
    can initialize ap to the address of the first of the optional
    arguments that follow a number of fixed arguments.