D.14.1 Execution Time Timers
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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. Static Semantics
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The following language-defined library package
exists: 3/2
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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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).12/2
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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. 13/2
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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. 13.a/2
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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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.15/3
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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.15.a/2
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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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.17/2
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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.18/3
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The function Current_Handler returns the handler
associated with the timer TM if that timer is set; otherwise, it returns null.19/3
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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.20/3
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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.21/2
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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.22/2
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As part of the finalization of an object of type
Timer, the timer is cleared.23/2
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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.24/2
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An exception propagated from a handler invoked
as part of the expiration of a timer has no effect.Erroneous Execution
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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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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.26.a/2
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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When an object of type Timer is finalized, the
system resources used by the timer shall be deallocated.Implementation Permissions
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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.29/2
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A Timer_Handler can be associated with several
Timer objects.Extensions to Ada 95
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