decf-atomic

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.

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

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Created 2010.1.21.