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D.10 Synchronous Task Control

1
This clause 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
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.

Dynamic Semantics

6/2
An object of the type Suspension_Object has two visible states: True and False. Upon initialization, its value is set to False.
7/2
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.
9/2
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). 

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.

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