D.11 Asynchronous Task Control
[This subclause clause
introduces a language-defined package to do asynchronous suspend/resume
on tasks. It uses a conceptual held priority
value to represent
the task's held
The following language-defined
library package exists:
Hold(T : in
Continue(T : in
Is_Held(T : Ada.Task_Identification.Task_Id)
Hold operation has been applied to a task, the task becomes held
For each processor there is a conceptual idle task
, which is always
ready. The base priority of the idle task is below System.Any_Priority'First.
The held priority
is a constant of the type Integer integer
whose value is below the base priority of the idle task.
The held state should not
be confused with the blocked state as defined in 9.2
the task is still ready.
For any priority below System.Any_Priority'First,
the task dispatching policy is FIFO_Within_Priorities.
To be honest: This
applies even if a Task_Dispatching_Policy specifies the policy for all
of the priorities of the partition.
task at the held priority never runs, so it is not necessary to implement
FIFO_Within_Priorities for systems that have only one policy (such as
The Hold operation sets the state of T to held. For a held task,
the active priority is reevaluated as if the base priority of the task
were the held priority: the task's own base
priority does not constitute an inheritance source (see D.1),
and the value of the held priority is defined to be such a source instead
Ramification: For example, if T is currently
inheriting priorities from other sources (e.g. it is executing in a protected
action), its active priority does not change, and it continues to execute
until it leaves the protected action.
The Continue operation resets the state of T to not-held; its T's
active priority is then reevaluated as determined
by the task dispatching policy associated with its base priority. described
in D.1. [This time, T's base priority is taken
The Is_Held function returns True if and only if
T is in the held state.
Discussion: Note that the state of T
can be changed immediately after Is_Held returns.
As part of these operations, a check is made that
the task identified by T is not terminated.
is raised if the check fails.
Program_Error is raised
if the value of T is Null_Task_Id.
If any operation in this package
is called with a parameter T that specifies a task object that no longer
exists, the execution of the program is erroneous.
An implementation need not support Asynchronous_Task_Control
if it is infeasible to support it in the target environment.
Reason: A direct implementation of the
Asynchronous_Task_Control semantics using priorities is not necessarily
efficient enough. Thus, we envision implementations that use some other
mechanism to set the “held” state. If there is no other such
mechanism, support for Asynchronous_Task_Control might be infeasible,
because an implementation in terms of priority would require one idle
task per processor. On some systems, programs are not supposed to know
how many processors are available, so creating enough idle tasks would
42 It is a consequence of the priority
rules that held tasks cannot be dispatched on any processor in a partition
(unless they are inheriting priorities) since their priorities are defined
to be below the priority of any idle task.
43 The effect of calling Get_Priority and
Set_Priority on a Held task is the same as on any other task.
44 Calling Hold on a held task or Continue
on a non-held task has no effect.
45 The rules
affecting queuing are derived from the above rules, in addition to the
normal priority rules:
When a held task is on the ready
queue, its priority is so low as to never reach the top of the queue
as long as there are other tasks on that queue.
If a task is executing in a protected
action, inside a rendezvous, or is inheriting priorities from other sources
(e.g. when activated), it continues to execute until it is no longer
executing the corresponding construct.
If a task becomes held while waiting
(as a caller) for a rendezvous to complete, the active priority of the
accepting task is not affected.
If a task becomes held while waiting in a selective_accept
entry call is issued to one of the
open entries, the corresponding accept_alternative accept
executes. When the rendezvous completes, the active priority
of the accepting task is lowered to the held priority (unless it is still
inheriting from other sources), and the task does not execute until another
The same holds if the held task is
the only task on a protected entry queue whose barrier becomes open.
The corresponding entry body executes.
Extensions to Ada 95
now Preelaborated, so it can be used in preelaborated units.
Wording Changes from Ada 95
The description of held tasks was changed to reflect
that the calculation of active priorities depends on the dispatching
policy of the base priority. Thus, the policy of the held priority was
specified in order to avoid surprises (especially when using the EDF
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