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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)
       package Ada.Synchronous_Task_Control is
(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)
An object of the type Suspension_Object has two visible states: true and false. Upon initialization, its value is set to false.
(7)
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)
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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