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D.15 Timing Events

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

Static Semantics

The following language-defined library package exists: 
package Ada.Real_Time.Timing_Events
  with Nonblocking, Global => in out synchronized is
  type Timing_Event is tagged limited private;
  type Timing_Event_Handler
       is access protected procedure (Event : in out Timing_Event)
       with Nonblocking => False;
  procedure Set_Handler (Event   : in out Timing_Event;
                         At_Time : in Time;
                         Handler : in Timing_Event_Handler);
  procedure Set_Handler (Event   : in out Timing_Event;
                         In_Time : in Time_Span;
                         Handler : in Timing_Event_Handler);
  function Current_Handler (Event : Timing_Event)
       return Timing_Event_Handler;
  procedure Cancel_Handler (Event     : in out Timing_Event;
                            Cancelled : out Boolean);
  function Time_Of_Event (Event : Timing_Event) return Time;
  ... -- not specified by the language
end Ada.Real_Time.Timing_Events;
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 nonnull 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.

Dynamic Semantics

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 as soon as possible after the completion of the call of Set_Handler.
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 part 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.

Implementation Requirements

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: 
An upper bound on the lateness of the execution of a handler. That is, the maximum time between the time specified for the event and when a handler is actually invoked assuming no other handler or task is executing during this interval.

Implementation Advice

The protected handler procedure should be executed directly by the real-time clock interrupt mechanism.
NOTE 1   Since a call of Set_Handler is not a potentially blocking operation, it can be called from within a handler.
NOTE 2   A Timing_Event_Handler can be associated with several Timing_Event objects.

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