LispWorks User Guide and Reference Manual > 15 Multiprocessing > 15.5 Aids for implementing modification checks

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15.5.1 Example modification check

(defstruct my-cache
  (modification-count 0)
  a
  b)
 
;; modifier code
(sys:with-modification-change
 (my-cache-modification-count cache)
 (setf (my-cache-a cache) (calculate-a-value ....)
       (my-cache-b cache) (calculate-b-value ....)))
 
;; reading code
(loop
  (sys:with-modification-check-macro
   my-cache-did-not-change-p (my-cache-modification-count cache)
   (let ((a (my-cache-a cache))
	 (b (my-cache-b cache)))
     (when (my-cache-did-not-change-p)
       (return (values a b )))

Provided that all modification to the a and b slots of a my-cache object are done by the modifier code above, the return values of a and b in the reading code are guaranteed to have been set by the same setf invocation in the modifier code.


LispWorks User Guide and Reference Manual - 22 Dec 2009

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