D.14.1 Execution Time Timers

     This clause describes a language-defined package that provides a facility for calling a handler when a task has used a defined amount of CPU time.

     The following language-defined library exists:

      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).

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

      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.

      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.

      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, when the execution time of the task identified by TM.T has increased by In_Time, the timer TM is said to have expired. The second procedure Set_Handler loads the timer TM with the absolute value specified by At_Time. In this mode, when the execution time of the task identified by TM.T reaches At_Time, the timer TM is said to have expired; if the value of At_Time has already been reached when Set_Handler is called, the timer TM is said to be expired.

      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.

      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.

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

      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.

      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.

      The constant Min_Handler_Ceiling is the priority value that ensures that no ceiling violation would occur, were a handler to be executed.

      For all the subprograms defined in this package, Tasking_Error is raised if the task identified by TM.T has terminated, and Program_Error is raised if the value of TM.T is Task_Identification.Null_Task_Id.

      An exception propagated from a handler invoked as part of the expiration of a timer has no effect.

      For a call of any of the subprograms defined in this package, if the task identified by TM.T no longer exists, the execution of the program is erroneous.

      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.

      When an object of type Timer is finalized, the system resources used by the timer shall be deallocated.

      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.