Macro: SB-EXT:ATOMIC-UPDATE

Updates PLACE atomically to the value returned by calling function designated by UPDATE-FN with ARGUMENTS and the previous value of PLACE.

PLACE may be read and UPDATE-FN evaluated and called multiple times before the update succeeds: atomicity in this context means that the value of PLACE did not change between the time it was read, and the time it was replaced with the computed value.

PLACE can be any place supported by SB-EXT:COMPARE-AND-SWAP.

Examples:

;;; Conses T to the head of FOO-LIST. (defstruct foo list) (defvar foo (make-foo)) (atomic-update (foo-list foo) #’cons t)

(let ((x (cons :count 0))) (mapc #’sb-thread:join-thread (loop repeat 1000 collect (sb-thread:make-thread (lambda () (loop repeat 1000 do (atomic-update (cdr x) #‘1+) (sleep 0.00001)))))) ;; Guaranteed to be (:COUNT . 1000000) – if you replace ;; atomic update with (INCF (CDR X)) above, the result becomes ;; unpredictable. x)

Lambda list

(sb-impl::place sb-impl::update-fn &rest sb-impl::arguments)