D.15 Timing Events

     This clause describes a language-defined package to allow user-defined protected procedures to be executed at a specified time without the need for a task or a delay statement.

     The following language-defined library exists:

     The type Timing_Event represents a time in the future when an event is to occur. The type Timing_Event needs finalization (see 7.6).

     An object of type Timing_Event is said to be set if it is associated with a non-null value of type Timing_Event_Handler and cleared otherwise. All Timing_Event objects are initially cleared.

     The type Timing_Event_Handler identifies a protected procedure to be executed by the implementation when the timing event occurs. Such a protected procedure is called a handler.

     The procedures Set_Handler associate the handler Handler with the event Event; if Handler is null, the event is cleared, otherwise it is set. The first procedure Set_Handler sets the execution time for the event to be At_Time. The second procedure Set_Handler sets the execution time for the event to be Real_Time.Clock + In_Time.

     A call of a procedure Set_Handler for an event that is already set replaces the handler and the time of execution; if Handler is not null, the event remains set.

     As soon as possible after the time set for the event, the handler is executed, passing the event as parameter. The handler is only executed if the timing event is in the set state at the time of execution. The initial action of the execution of the handler is to clear the event.

      If the Ceiling_Locking policy (see D.3) is in effect when a procedure Set_Handler is called, a check is made that the ceiling priority of Handler.all is Interrupt_Priority'Last. If the check fails, Program_Error is raised.

      If a procedure Set_Handler is called with zero or negative In_Time or with At_Time indicating a time in the past then the handler is executed immediately by the task executing the call of Set_Handler. The timing event Event is cleared.

      The function Current_Handler returns the handler associated with the event Event if that event is set; otherwise it returns null.

      The procedure Cancel_Handler clears the event if it is set. Cancelled is assigned True if the event was set prior to it being cleared; otherwise it is assigned False.

      The function Time_Of_Event returns the time of the event if the event is set; otherwise it returns Real_Time.Time_First.

      As the final step of the finalization of an object of type Timing_Event, the Timing_Event is cleared.

      If several timing events are set for the same time, they are executed in FIFO order of being set.

      An exception propagated from a handler invoked by a timing event has no effect.

      For a given Timing_Event object, the implementation shall perform the operations declared in this package atomically with respect to any of these operations on the same Timing_Event object. The replacement of a handler by a call of Set_Handler shall be performed atomically with respect to the execution of the handler.

      The implementation shall document the following metric:

      The protected handler procedure should be executed directly by the real-time clock interrupt mechanism.