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D.10 Synchronous Task Control

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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

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The following language−defined package exists:

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     package Ada.Synchronous_Task_Control is  pragma Preelaborate(Synchronous_Task_Control);

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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);
     private
          ... −− not specified by the language
     end Ada.Synchronous_Task_Control;

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The type Suspension_Object is a by−reference type.

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.

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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.

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Current_State returns the current state of the object.

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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. Suspend_Until_True is a potentially blocking operation (see 9.5.1).

Implementation Requirements

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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.