D.10 Synchronous Task Control
This subclause 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
The following language-defined
package exists:
package Ada.Synchronous_Task_Control
is
pragma Preelaborate(Synchronous_Task_Control);
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.
The following language-defined
package exists:
package Ada.Synchronous_Task_Control.EDF
is
procedure Suspend_Until_True_And_Set_Deadline
(S :
in out Suspension_Object;
TS :
in Ada.Real_Time.Time_Span);
end Ada.Synchronous_Task_Control.EDF;
Dynamic Semantics
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 procedure Suspend_Until_True_And_Set_Deadline
blocks the calling task until the state of the object S is True; at that
point the task becomes ready with a deadline of Ada.Real_Time.Clock +
TS, and the state of the object becomes False. Program_Error is raised
upon calling Suspend_Until_True_And_Set_Deadline if another task is already
waiting on that suspension object. Suspend_Until_True_And_Set_Deadline
is a potentially blocking operation.
Implementation Requirements
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.
37 More complex schemes, such as setting
the deadline relative to when Set_True is called, can be programmed using
a protected object.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
