Scieneer Common Lisp 1.3.9 online documentation

kernel:set-symbol-value-conditional symbol old new[Function]

Atomically compare symbol's value to old and if eq then store new in symbol's value cell. The original symbol value is returned. If the symbol has a binding in the current thread then the symbol dynamic value cell is accessed, otherwise the symbol global value cell is accessed. See also: kernel:set-symbol-value-conditional and kernel:set-symbol-global-value-conditional.