update-atomic

Arguments: (var place) form

This macro expands into code that does a read-modify-write operation that is atomic with respect to the value in place. That is, the value in place is read and bound to a local variable; the value of a form is computed; then the result is stored back in place as long as that value hasn't changed since the update-atomic code read and saved it. If it has changed (tested using eq), then the new value is acquired and bound to the local variable, the form value is recomputed, and another conditional store attempt is made. This loop continues until the store is successful.

The arguments are:

• var: a symbol naming the local variable that will be bound to the value in place while form is being evaluated.
• place: a form denoting a place acceptable to atomic-conditional-setf. See note below.
• form: a form that computes a value to replace the original value in place. Subforms of form are evaluated once each, in the order they appear in place, as for all setf-type macros.

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.

update-atomic could be used to implement (incf-atomic (car x)) like this:

(update-atomic (v (car x)) (1+ v))

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

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This page was not revised from the 8.1 page.
Created 2010.1.21.