Predicates are expressions that are enclosed in slashes / /. Predicates are evaluated at probe firing time to determine whether the associated actions should be executed. Predicates are the primary conditional construct used for building more complex control flow in a D program. You can omit the predicate section of the probe clause entirely for any probe. If the predicate section is omitted, the actions are always executed when the probe fires.
Predicate expressions can use any of the previously described D operators. Predicate expressions refer to D data objects such as variables and constants. The predicate expression must evaluate to a value of integer or pointer type. As with all D expressions, a zero value is interpreted as false and any non-zero value is interpreted as true.