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D.14.1 Execution Time Timers

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{AI95-00307-01} {AI05-0299-1} This subclause describes a language-defined package that provides a facility for calling a handler when a task has used a defined amount of CPU time.

Static Semantics

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{AI95-00307-01} The following language-defined library package exists: 
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with System;
package Ada.Execution_Time.Timers is
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   type Timer (T : not null access constant
                       Ada.Task_Identification.Task_Id) is
      tagged limited private;
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   type Timer_Handler is
      access protected procedure (TM : in out Timer);
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   Min_Handler_Ceiling : constant System.Any_Priority :=
   implementation-defined;
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   procedure Set_Handler (TM      : in out Timer;
                          In_Time : in Time_Span;
                          Handler : in Timer_Handler);
   procedure Set_Handler (TM      : in out Timer;
                          At_Time : in CPU_Time;
                          Handler : in Timer_Handler);
   function Current_Handler (TM : Timer) return Timer_Handler;
   procedure Cancel_Handler (TM        : in out Timer;
                             Cancelled :    out Boolean);
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   function Time_Remaining (TM : Timer) return Time_Span;
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   Timer_Resource_Error : exception;
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private
   ... -- not specified by the language
end Ada.Execution_Time.Timers;
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 {AI95-00307-01} The type Timer represents an execution-time event for a single task and is capable of detecting execution-time overruns. The access discriminant T identifies the task concerned. The type Timer needs finalization (see 7.6).
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 {AI95-00307-01} An object of type Timer is said to be set if it is associated with a nonnull value of type Timer_Handler and cleared otherwise. All Timer objects are initially cleared.
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 {AI95-00307-01} The type Timer_Handler identifies a protected procedure to be executed by the implementation when the timer expires. Such a protected procedure is called a handler.
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Discussion: Type Timer is tagged. This makes it possible to share a handler between several events. In simple cases, 'Access can be used to compare the parameter with a specific timer object (this works because a tagged type is a by-reference type). In more complex cases, a type extension of type Timer can be declared; a double type conversion can be used to access the extension data. An example of how this can be done can be found for the similar type Timing_Event, see D.15

Dynamic Semantics

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 {AI95-00307-01} When a Timer object is created, or upon the first call of a Set_Handler procedure with the timer as parameter, the resources required to operate an execution-time timer based on the associated execution-time clock are allocated and initialized. If this operation would exceed the available resources, Timer_Resource_Error is raised.
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 {AI95-00307-01} {AI05-0264-1} The procedures Set_Handler associate the handler Handler with the timer TM: if Handler is null, the timer is cleared; otherwise, it is set. The first procedure Set_Handler loads the timer TM with an interval specified by the Time_Span parameter. In this mode, the timer TM expires when the execution time of the task identified by TM.T.all has increased by In_Time; if In_Time is less than or equal to zero, the timer expires immediately. The second procedure Set_Handler loads the timer TM with the absolute value specified by At_Time. In this mode, the timer TM expires when the execution time of the task identified by TM.T.all reaches At_Time; if the value of At_Time has already been reached when Set_Handler is called, the timer expires immediately.
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Implementation Note: Since an access-to-constant can designate a variable, the Task_Id value designated by the discriminant of a Timer object can be changed after the object is created. Thus, an implementation cannot use the value of the Task_Id other than where this Standard specifies. For instance, the Task_Id should be read when the timer is set, but it should not be used when the timer expires (as it may designate a different task at that point). 
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 {AI95-00307-01} A call of a procedure Set_Handler for a timer that is already set replaces the handler and the (absolute or relative) execution time; if Handler is not null, the timer remains set.
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 {AI95-00307-01} When a timer expires, the associated handler is executed, passing the timer as parameter. The initial action of the execution of the handler is to clear the event.
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 {AI95-00307-01} {AI05-0264-1} The function Current_Handler returns the handler associated with the timer TM if that timer is set; otherwise, it returns null.
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 {AI95-00307-01} {AI05-0264-1} The procedure Cancel_Handler clears the timer if it is set. Cancelled is assigned True if the timer was set prior to it being cleared; otherwise, it is assigned False.
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 {AI95-00307-01} {AI05-0264-1} The function Time_Remaining returns the execution time interval that remains until the timer TM would expire, if that timer is set; otherwise, it returns Time_Span_Zero.
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 {AI95-00307-01} The constant Min_Handler_Ceiling is the minimum ceiling priority required for a protected object with a handler to ensure that no ceiling violation will occur when that handler is invoked.
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 {AI95-00307-01} As part of the finalization of an object of type Timer, the timer is cleared.
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 {AI95-00307-01} For all the subprograms defined in this package, Tasking_Error is raised if the task identified by TM.T.all has terminated, and Program_Error is raised if the value of TM.T.all is Task_Identification.Null_Task_Id.
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 {AI95-00307-01} An exception propagated from a handler invoked as part of the expiration of a timer has no effect.

Erroneous Execution

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 {AI95-00307-01} For a call of any of the subprograms defined in this package, if the task identified by TM.T.all no longer exists, the execution of the program is erroneous.

Implementation Requirements

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 {AI95-00307-01} For a given Timer object, the implementation shall perform the operations declared in this package atomically with respect to any of these operations on the same Timer object. The replacement of a handler by a call of Set_Handler shall be performed atomically with respect to the execution of the handler.
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Reason: This prevents various race conditions. In particular it ensures that if an event occurs when Set_Handler is changing the handler then either the new or old handler is executed in response to the appropriate event. It is never possible for a new handler to be executed in response to an old event 
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 {AI95-00307-01} When an object of type Timer is finalized, the system resources used by the timer shall be deallocated.

Implementation Permissions

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 {AI95-00307-01} {AI05-0264-1} Implementations may limit the number of timers that can be defined for each task. If this limit is exceeded, then Timer_Resource_Error is raised.
NOTES
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45  {AI95-00307-01} A Timer_Handler can be associated with several Timer objects.

Extensions to Ada 95

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{AI95-00307-01} The package Execution_Time.Timers is new. 

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