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D.10 Synchronous Task Control

1/3
{AI05-0299-1} [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

2
The following language-defined package exists: 
3/2
{AI95-00362-01} package Ada.Synchronous_Task_Control is
  pragma Preelaborate(Synchronous_Task_Control);
4
  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);
private
     ... -- not specified by the language
end Ada.Synchronous_Task_Control;
5
The type Suspension_Object is a by-reference type.
5.a/2
Implementation Note: {AI95-00318-02} The implementation can ensure this by, for example, making the full view an explicitly limited record type.
5.1/3
  {AI05-0168-1} The following language-defined package exists: 
5.2/3
{AI05-0168-1} package Ada.Synchronous_Task_Control.EDF is
   procedure Suspend_Until_True_And_Set_Deadline
      (S  : in out Suspension_Object;
       TS : in     Ada.Real_Time.Time_Span);
end Ada.Synchronous_Task_Control.EDF;

Dynamic Semantics

6/2
{AI95-00114-01} An object of the type Suspension_Object has two visible states: True and False. Upon initialization, its value is set to False. 
6.a
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. 
7/2
{AI95-00114-01} 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.
8
Current_State returns the current state of the object.
8.a
Discussion: This state can change immediately after the operation returns. 
9/2
{AI95-00114-01} 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.
10
Program_Error is raised upon calling Suspend_Until_True if another task is already waiting on that suspension object. Suspend_Until_True is a potentially blocking operation (see 9.5.1).
10.1/3
   {AI05-0168-1} {AI05-0269-1} 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. Suspend_Until_True_And_Set_Deadline is a potentially blocking operation. 

Implementation Requirements

11
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.
NOTES
12/3
37  {AI05-0168-1} 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

12.a/2
{AI95-00362-01} Synchronous_Task_Control is now Preelaborated, so it can be used in preelaborated units. 

Extensions to Ada 2005

12.b/3
{AI05-0168-1} Child package Ada.Synchronous_Task_Control.EDF is new. 

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