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 section 9.5.1 Protected Subprograms and Protected Actions.
    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.

Go to the first, previous, next, last section, table of contents.