Contents   Index   Search   Previous   Next

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.
5.a
Implementation Note: The implementation can ensure this by, for example, making the full view a limited record 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.
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
   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
   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
    {potentially blocking operation (Suspend_Until_True) [partial]} {blocking, potentially (Suspend_Until_True) [partial]} {Program_Error (raised by failure of run-time check)} 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.

Contents   Index   Search   Previous   Next   Legal