Arguments

 

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.