DTrace describes interfaces using a triplet of attributes consisting of two stability levels and a dependency class. By convention, the interface attributes are written in the following order, separated by slashes:
name-stability / data-stability / dependency-class
The name stability of an interface describes the stability level associated with its name as it appears in your D program or on the dtrace(1M) command-line. For example, the execname D variable is a Stable name: Sun guarantees that this identifier will continue to be supported in your D programs according to the rules described for Stable interfaces above.
The data stability of an interface is distinct from the stability associated with the interface name. This stability level describes Sun's commitment to maintaining the data formats used by the interface and any associated data semantics. For example, the pid D variable is a Stable interface: process IDs are a Stable concept in Solaris, and Sun guarantees that the pid variable will be of type pid_t with the semantic that it is set to the process ID corresponding to the thread that fired a given probe in accordance with the rules for Stable interfaces.
The dependency class of an interface is distinct from its name and data stability, and describes whether the interface is specific to the current operating platform or microprocessor.
DTrace and the D compiler track the stability attributes for all of the DTrace interface entities, including providers, probe descriptions, D variables, D functions, types, and program statements themselves, as we'll see shortly. Notice that all three values can vary independently. For example, the curthread D variable has Stable/Private/Common attributes: the variable name is Stable and is Common to all Solaris operating platforms, but this variable provides access to a Private data format that is an artifact of the Solaris kernel implementation. Most D variables are provided with Stable/Stable/Common attributes, as are the variables you define.