D.10 Synchronous Task Control
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[This subclause describes a language-defined private semaphore (suspension
object), which can be used for
two-stage suspend operations and
as a simple building block for implementing higher-level queues.]
Static Semantics
The following language-defined
package exists:
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type Suspension_Object
is limited private;
procedure Set_True(S :
in out Suspension_Object);
procedure Set_False(S :
in out Suspension_Object);
function Current_State(S : Suspension_Object)
return Boolean;
procedure Suspend_Until_True(S :
in out Suspension_Object)
with Nonblocking => False;
private
... --
not specified by the language
end Ada.Synchronous_Task_Control;
The type Suspension_Object is a by-reference type.
Implementation Note: {
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The implementation can ensure this by, for example, making the full view
an explicitly limited record type.
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The following language-defined package exists:
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with Ada.Real_Time;
package Ada.Synchronous_Task_Control.EDF
with Nonblocking, Global =>
in out synchronized is
procedure Suspend_Until_True_And_Set_Deadline
(S :
in out Suspension_Object;
TS :
in Ada.Real_Time.Time_Span)
with Nonblocking => False;
end Ada.Synchronous_Task_Control.EDF;
Dynamic Semantics
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An object of the type Suspension_Object has two visible states: True
and False. Upon initialization, its value is set to False.
Discussion: This object is assumed to
be private to the declaring task, i.e. only that task will call Suspend_Until_True
on this object, and the count of callers is at most one. Other tasks
can, of course, change and query the state of this object.
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The operations Set_True and Set_False are atomic with respect to each
other and with respect to Suspend_Until_True; they set the state to True
and False respectively.
Current_State returns the current state of the object.
Discussion: This state can change immediately
after the operation returns.
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The procedure Suspend_Until_True blocks the calling task until the state
of the object S is True; at that point the task becomes ready and the
state of the object becomes False.
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Program_Error is raised upon calling Suspend_Until_True if another task
is already waiting on that suspension object.
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The procedure Suspend_Until_True_And_Set_Deadline blocks the calling
task until the state of the object S is True; at that point the task
becomes ready with a deadline of Ada.Real_Time.Clock + TS, and the state
of the object becomes False. Program_Error is raised upon calling Suspend_Until_True_And_Set_Deadline
if another task is already waiting on that suspension object.
Bounded (Run-Time) Errors
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It is a bounded error for two or more tasks to call Suspend_Until_True
on the same Suspension_Object concurrently. For each task, Program_Error
can be raised, the task can proceed without suspending, or the task can
suspend, potentially indefinitely. The state of the suspension object
can end up either True or False.
Implementation Requirements
The implementation is required to allow the calling
of Set_False and Set_True during any protected action, even one that
has its ceiling priority in the Interrupt_Priority range.
NOTE {
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More complex schemes, such as setting the deadline relative to when Set_True
is called, can be programmed using a protected object.
Extensions to Ada 95
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Synchronous_Task_Control is now Preelaborated, so
it can be used in preelaborated units.
Extensions to Ada 2005
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Child package Ada.Synchronous_Task_Control.EDF is
new.
Wording Changes from Ada 2012
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Correction: Clarified that Suspend_Until_True should only be called
from a single task, and what happens if that is violated.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
