- 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);
... -- not specified by the language
- 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
- 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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