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Allegro CL version 8.1
New since 8.1 release.


Arguments: place &optional (increment-form 1)

This macro acts like decf with the same arguments except that the update is atomic with respect to any other processes looking at the same place. That is, if several processes simultaneously try to decf-atomic (or incf-atomic) the same location, all operations occur sequentially in some unspecified order. As a result, the final value will be what is expected. (If you just used decf's and incf's, the first decf could read the value and while it is calculating the decrement, another decf could run, and then the initial decf could store its result effectively cancelling the action of the second decf.)

Note on the place argument: not all places legal as the first argument of setf can be used as the place argument to this macro. The list of allowable places can be found in Atomic read-modify-write primitives in smp.htm. A symbol is suitable only if it has been defined as a variable with defvar-nonbindable. If a symbol defined as a variable with defvar (or not defined at all, just used) is specified as the value of place, a compilation error will be signaled when the form is compiled.

This macro was added by a patch released in July, 2009. This macro will not be defined if that patch has not been installed. See sys:update-allegro for information on downloading patches.

See smp.htm and multiprocessing.htm for more information on this macro and on multiprocessing.

Copyright (c) 1998-2009, Franz Inc. Oakland, CA., USA. All rights reserved.
Documentation for Allegro CL version 8.1. This page is new in the 8.1 release.
Created 2007.4.30.

Allegro CL version 8.1
New since 8.1 release.