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.]
Static Semantics
The following language-defined
package exists:
{
AI95-00362-01}
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.
Implementation Note: {
AI95-00318-02}
The implementation can ensure this by, for example, making the full view
an explicitly limited record type.
Dynamic Semantics
{
AI95-00114-01}
An object of the type Suspension_Object has two visible states: True
and False. Upon initialization, its value is set to False.
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.
{
AI95-00114-01}
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.
Discussion: This state can change immediately
after the operation returns.
{
AI95-00114-01}
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.
{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
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.
Extensions to Ada 95
{
AI95-00362-01}
{
extensions to Ada 95}
Synchronous_Task_Control
is now Preelaborated, so it can be used in preelaborated units.
Sponsored by Ada-Europe