D.10 Synchronous Task Control
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.
The following language-defined package exists:
package Ada.Synchronous_Task_Control is
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;
The type Suspension_Object is a by-reference type.
An object of the type Suspension_Object has two visible states: true and false. Upon initialization, its value is set to false.
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.
Current_State returns the current state of the object.
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.
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).
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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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual