Annex D
(normative)
Real-Time Systems
1 This Annex specifies additional characteristics of Ada implementations
intended for real-time systems software. To conform to this Annex, an
implementation shall also conform to the Systems Programming Annex.
Metrics
2 The metrics are documentation requirements; an implementation shall
document the values of the language-defined metrics for at least one
configuration [of hardware or an underlying system] supported by the
implementation, and shall document the details of that configuration.
2.a/2 This paragraph was deleted.
2.a.1/2 Documentation Requirement: The details of the configuration used
to generate the values of all metrics.
2.b Reason: The actual values of the metrics are likely to depend on
hardware configuration details that are variable and generally
outside the control of a compiler vendor.
3 The metrics do not necessarily yield a simple number. [For some, a range
is more suitable, for others a formula dependent on some parameter is
appropriate, and for others, it may be more suitable to break the metric into
several cases.] Unless specified otherwise, the metrics in this annex are
expressed in processor clock cycles. For metrics that require documentation of
an upper bound, if there is no upper bound, the implementation shall report
that the metric is unbounded.
3.a Discussion: There are several good reasons to specify metrics in
seconds; there are however equally good reasons to specify them in
processor clock cycles. In defining the metrics, we have tried to
strike a balance on a case-by-case basis.
3.b It has been suggested that all metrics should be given names, so
that "data-sheets" could be formulated and published by vendors.
However the paragraph number can serve that purpose.
NOTES
4 1 The specification of the metrics makes a distinction between upper
bounds and simple execution times. Where something is just specified
as "the execution time of" a piece of code, this leaves one the
freedom to choose a nonpathological case. This kind of metric is of
the form "there exists a program such that the value of the metric is
V". Conversely, the meaning of upper bounds is "there is no program
such that the value of the metric is greater than V". This kind of
metric can only be partially tested, by finding the value of V for one
or more test programs.
5 2 The metrics do not cover the whole language; they are limited to
features that are specified in Annex C, "Systems Programming" and in
this Annex. The metrics are intended to provide guidance to potential
users as to whether a particular implementation of such a feature is
going to be adequate for a particular real-time application. As such,
the metrics are aimed at known implementation choices that can result
in significant performance differences.
6 3 The purpose of the metrics is not necessarily to provide
fine-grained quantitative results or to serve as a comparison between
different implementations on the same or different platforms. Instead,
their goal is rather qualitative; to define a standard set of
approximate values that can be measured and used to estimate the
general suitability of an implementation, or to evaluate the
comparative utility of certain features of an implementation for a
particular real-time application.
Extensions to Ada 83
6.a This Annex is new to Ada 95.
D.1 Task Priorities
1/3 {AI05-0299-1} [This subclause specifies the priority model for real-time
systems. In addition, the methods for specifying priorities are defined.]
Paragraphs 2 through 6 were moved to Annex J, "Obsolescent Features".
Static Semantics
6.1/3 {AI05-0229-1} For a task type (including the anonymous type of a
single_task_declaration), protected type (including the anonymous type of a
single_protected_declaration), or subprogram, the following language-defined
representation aspects may be specified:
6.2/3 Priority The aspect Priority is an expression, which shall be of type
Integer.
6.a/3 Aspect Description for Priority: Priority of a task object or
type, or priority of a protected object or type; the priority is
not in the interrupt range.
6.3/3 Interrupt_Priority
The aspect Interrupt_Priority is an expression, which shall be
of type Integer.
6.b/3 Aspect Description for Interrupt_Priority: Priority of a task
object or type, or priority of a protected object or type; the
priority is in the interrupt range.
Legality Rules
7/3 This paragraph was deleted.{AI05-0229-1}
8/3 {AI05-0229-1} If the Priority aspect is specified for a subprogram, the
expression shall be static, and its value shall be in the range of
System.Priority.
8.a Reason: This value is needed before it gets elaborated, when the
environment task starts executing.
8.1/3 {AI05-0229-1} At most one of the Priority and Interrupt_Priority aspects
may be specified for a given entity.
8.b/3 Ramification: This includes specifying via pragmas (see J.15.11).
Note that 13.1 prevents multiple specifications of a single
representation aspect by any means.
8.2/3 {AI05-0229-1} Neither of the Priority or Interrupt_Priority aspects
shall be specified for a synchronized interface type.
Static Semantics
9 The following declarations exist in package System:
10 subtype Any_Priority is Integer range implementation-defined;
subtype Priority is Any_Priority
range Any_Priority'First .. implementation-defined;
subtype Interrupt_Priority is Any_Priority
range Priority'Last+1 .. Any_Priority'Last;
11 Default_Priority : constant Priority := (Priority'First + Priority'Last)/2;
11.a Implementation defined: The declarations of Any_Priority and
Priority.
12 The full range of priority values supported by an implementation is
specified by the subtype Any_Priority. The subrange of priority values that
are high enough to require the blocking of one or more interrupts is specified
by the subtype Interrupt_Priority. [The subrange of priority values below
System.Interrupt_Priority'First is specified by the subtype System.Priority.]
13/3 This paragraph was deleted.{AI05-0229-1}
Dynamic Semantics
14/3 {AI05-0229-1} The Priority aspect has no effect if it is specified for a
subprogram other than the main subprogram; the Priority value is not
associated with any task.
15 A task priority is an integer value that indicates a degree of urgency and
is the basis for resolving competing demands of tasks for resources. Unless
otherwise specified, whenever tasks compete for processors or other
implementation-defined resources, the resources are allocated to the task with
the highest priority value. The base priority of a task is the priority with
which it was created, or to which it was later set by
Dynamic_Priorities.Set_Priority (see D.5). At all times, a task also has an
active priority, which generally reflects its base priority as well as any
priority it inherits from other sources. Priority inheritance is the process
by which the priority of a task or other entity (e.g. a protected object; see
D.3) is used in the evaluation of another task's active priority.
15.a Implementation defined: Implementation-defined execution resources.
16/3 {AI05-0229-1} The effect of specifying a Priority or Interrupt_Priority
aspect for a protected type or single_protected_declaration is discussed in
D.3.
17/4 {AI05-0229-1} {AI12-0081-1} The expression specified for the Priority or
Interrupt_Priority aspect of a task type is evaluated each time an object of
the task type is created (see 9.1). For the Priority aspect, the value of the
expression is converted to the subtype Priority; for the Interrupt_Priority
aspect, this value is converted to the subtype Any_Priority. The priority
value is then associated with the task object.
18/3 {AI05-0229-1} Likewise, the priority value is associated with the
environment task if the aspect is specified for the main subprogram.
19/3 {AI05-0229-1} The initial value of a task's base priority is specified by
default or by means of a Priority or Interrupt_Priority aspect. [After a task
is created, its base priority can be changed only by a call to
Dynamic_Priorities.Set_Priority (see D.5).] The initial base priority of a
task in the absence of an aspect is the base priority of the task that creates
it at the time of creation (see 9.1). If the aspect Priority is not specified
for the main subprogram, the initial base priority of the environment task is
System.Default_Priority. [The task's active priority is used when the task
competes for processors. Similarly, the task's active priority is used to
determine the task's position in any queue when Priority_Queuing is specified
(see D.4).]
20/2 {AI95-00357-01} At any time, the active priority of a task is the maximum
of all the priorities the task is inheriting at that instant. For a task that
is not held (see D.11), its base priority is a source of priority inheritance
unless otherwise specified for a particular task dispatching policy. Other
sources of priority inheritance are specified under the following conditions:
20.a Discussion: Other parts of the annex, e.g. D.11, define other
sources of priority inheritance.
21/1 * {8652/0072} {AI95-00092-01} During activation, a task being activated
inherits the active priority that its activator (see 9.2) had at the
time the activation was initiated.
22/1 * {8652/0072} {AI95-00092-01} During rendezvous, the task accepting the
entry call inherits the priority of the entry call (see 9.5.3 and
D.4).
23 * During a protected action on a protected object, a task inherits the
ceiling priority of the protected object (see 9.5 and D.3).
24 In all of these cases, the priority ceases to be inherited as soon as the
condition calling for the inheritance no longer exists.
Implementation Requirements
25 The range of System.Interrupt_Priority shall include at least one value.
26 The range of System.Priority shall include at least 30 values.
NOTES
27 4 The priority expression can include references to discriminants of
the enclosing type.
28 5 It is a consequence of the active priority rules that at the point
when a task stops inheriting a priority from another source, its
active priority is re-evaluated. This is in addition to other
instances described in this Annex for such re-evaluation.
29/3 6 {AI05-0248-1} An implementation may provide a nonstandard mode in
which tasks inherit priorities under conditions other than those
specified above.
29.a/3 Ramification: {AI05-0229-1} The use of a Priority or
Interrupt_Priority aspect does not require the package System to
be named in a with_clause for the enclosing compilation_unit.
Extensions to Ada 83
29.b The priority of a task is per-object and not per-type.
29.c Priorities need not be static anymore (except for the main
subprogram).
Wording Changes from Ada 83
29.d The description of the Priority pragma has been moved to this
annex.
Wording Changes from Ada 95
29.e/2 {8652/0072} {AI95-00092-01} Corrigendum: Clarified that dynamic
priority changes are not transitive - that is, they don't apply to
tasks that are being activated by or in rendezvous with the task
that had its priority changed.
29.f/2 {AI95-00357-01} Generalized the definition of priority inheritance
to take into account the differences between the existing and new
dispatching policies.
Extensions to Ada 2005
29.g/3 {AI05-0229-1} Aspects Priority and Interrupt_Priority are new;
pragmas Priority and Interrupt_Priority are now obsolescent.
Wording Changes from Ada 2012
29.h/4 {AI12-0081-1} Corrigendum: Clarified when the Priority and
Interrupt_Priority aspect expressions are evaluated.
D.2 Priority Scheduling
1/3 {AI95-00321-01} {AI05-0299-1} [This subclause describes the rules that
determine which task is selected for execution when more than one task is
ready (see 9).]
Wording Changes from Ada 95
1.a/3 {AI95-00321-01} {AI05-0299-1} This introduction is simplified in
order to reflect the rearrangement and expansion of this
subclause.
D.2.1 The Task Dispatching Model
1/2 {AI95-00321-01} [The task dispatching model specifies task scheduling,
based on conceptual priority-ordered ready queues.]
Static Semantics
1.1/2 {AI95-00355-01} The following language-defined library package exists:
1.2/3 {AI05-0166-1} package Ada.Dispatching is
pragma Preelaborate(Dispatching);
1.3/3 {AI05-0166-1} procedure Yield;
1.4/3 {AI05-0166-1} Dispatching_Policy_Error : exception;
end Ada.Dispatching;
1.5/2 Dispatching serves as the parent of other language-defined library units
concerned with task dispatching.
Dynamic Semantics
2/2 {AI95-00321-01} A task can become a running task only if it is ready (see
9) and the execution resources required by that task are available. Processors
are allocated to tasks based on each task's active priority.
3 It is implementation defined whether, on a multiprocessor, a task that is
waiting for access to a protected object keeps its processor busy.
3.a Implementation defined: Whether, on a multiprocessor, a task that
is waiting for access to a protected object keeps its processor
busy.
4/2 {AI95-00321-01} Task dispatching is the process by which one ready task is
selected for execution on a processor. This selection is done at certain
points during the execution of a task called task dispatching points. A task
reaches a task dispatching point whenever it becomes blocked, and when it
terminates. [Other task dispatching points are defined throughout this Annex
for specific policies.]
4.a Ramification: On multiprocessor systems, more than one task can be
chosen, at the same time, for execution on more than one
processor, as explained below.
5/2 {AI95-00321-01} Task dispatching policies are specified in terms of
conceptual ready queues and task states. A ready queue is an ordered list of
ready tasks. The first position in a queue is called the head of the queue,
and the last position is called the tail of the queue. A task is ready if it
is in a ready queue, or if it is running. Each processor has one ready queue
for each priority value. At any instant, each ready queue of a processor
contains exactly the set of tasks of that priority that are ready for
execution on that processor, but are not running on any processor; that is,
those tasks that are ready, are not running on any processor, and can be
executed using that processor and other available resources. A task can be on
the ready queues of more than one processor.
5.a Discussion: The core language defines a ready task as one that is
not blocked. Here we refine this definition and talk about ready
queues.
6/2 {AI95-00321-01} Each processor also has one running task, which is the
task currently being executed by that processor. Whenever a task running on a
processor reaches a task dispatching point it goes back to one or more ready
queues; a task (possibly the same task) is then selected to run on that
processor. The task selected is the one at the head of the highest priority
nonempty ready queue; this task is then removed from all ready queues to which
it belongs.
6.a Discussion: There is always at least one task to run, if we count
the idle task.
7/3 {AI95-00321-01} {AI05-0166-1} A call of Yield is a task dispatching point.
Yield is a potentially blocking operation (see 9.5.1).
7.a/2 This paragraph was deleted.
8/2 This paragraph was deleted.{AI95-00321-01}
8.a/2 This paragraph was deleted.
Implementation Permissions
9/2 {AI95-00321-01} An implementation is allowed to define additional
resources as execution resources, and to define the corresponding allocation
policies for them. Such resources may have an implementation-defined effect on
task dispatching.
9.a/2 Implementation defined: The effect of implementation-defined
execution resources on task dispatching.
10 An implementation may place implementation-defined restrictions on tasks
whose active priority is in the Interrupt_Priority range.
10.a/3 Ramification: {AI05-0229-1} For example, on some operating
systems, it might be necessary to disallow them altogether. This
permission applies to tasks whose priority is set to interrupt
level for any reason: via an aspect, via a call to
Dynamic_Priorities.Set_Priority, or via priority inheritance.
10.1/2 {AI95-00321-01} [For optimization purposes,] an implementation may
alter the points at which task dispatching occurs, in an
implementation-defined manner. However, a delay_statement always corresponds
to at least one task dispatching point.
NOTES
11/3 7 {AI05-0299-1} Clause 9 specifies under which circumstances a task
becomes ready. The ready state is affected by the rules for task
activation and termination, delay statements, and entry calls. When a
task is not ready, it is said to be blocked.
12 8 An example of a possible implementation-defined execution resource
is a page of physical memory, which needs to be loaded with a
particular page of virtual memory before a task can continue execution.
13 9 The ready queues are purely conceptual; there is no requirement
that such lists physically exist in an implementation.
14 10 While a task is running, it is not on any ready queue. Any time
the task that is running on a processor is added to a ready queue, a
new running task is selected for that processor.
15 11 In a multiprocessor system, a task can be on the ready queues of
more than one processor. At the extreme, if several processors share
the same set of ready tasks, the contents of their ready queues is
identical, and so they can be viewed as sharing one ready queue, and
can be implemented that way. [Thus, the dispatching model covers
multiprocessors where dispatching is implemented using a single ready
queue, as well as those with separate dispatching domains.]
16 12 The priority of a task is determined by rules specified in this
subclause, and under D.1, "Task Priorities", D.3, "
Priority Ceiling Locking", and D.5, "Dynamic Priorities".
17/2 13 {AI95-00321-01} The setting of a task's base priority as a result
of a call to Set_Priority does not always take effect immediately when
Set_Priority is called. The effect of setting the task's base priority
is deferred while the affected task performs a protected action.
Wording Changes from Ada 95
17.a/3 {AI95-00321-01} {AI05-0005-1} This description is simplified to
describe only the parts of the dispatching model common to all
policies. In particular, rules about preemption are moved
elsewhere. This makes it easier to add other policies (which might
not involve preemption).
Incompatibilities With Ada 2005
17.b/3 {AI05-0166-1} Procedure Yield is added to Dispatching. If
Dispatching is referenced in a use_clause, and an entity E with a
defining_identifier of Yield is defined in a package that is also
referenced in a use_clause, the entity E may no longer be
use-visible, resulting in errors. This should be rare and is
easily fixed if it does occur.
17.c/4 {AI05-0166-1} {AI12-0005-1} Package Dispatching was a Pure
package, but now is Preelaborated with the addition of Yield. This
is incompatible as Dispatching can no longer be depended upon from
a Pure package. This should happen rarely in practice as the only
contents was the exception Dispatching_Policy_Error and none of
the child packages that could raise that exception are pure.
D.2.2 Task Dispatching Pragmas
1/3 {AI95-00355-01} {AI05-0299-1} [This subclause allows a single task
dispatching policy to be defined for all priorities, or the range of
priorities to be split into subranges that are assigned individual dispatching
policies.]
Syntax
2 The form of a pragma Task_Dispatching_Policy is as follows:
3 pragma Task_Dispatching_Policy(policy_identifier);
3.1/2 {AI95-00355-01} The form of a pragma Priority_Specific_Dispatching is
as follows:
3.2/2 pragma Priority_Specific_Dispatching (
policy_identifier, first_priority_expression,
last_priority_expression);
Name Resolution Rules
3.3/2 {AI95-00355-01} The expected type for first_priority_expression and
last_priority_expression is Integer.
Legality Rules
4/2 {AI95-00321-01} {AI95-00355-01} The policy_identifier used in a pragma
Task_Dispatching_Policy shall be the name of a task dispatching policy.
4.a/2 This paragraph was deleted.
4.1/2 {AI95-00355-01} The policy_identifier used in a pragma
Priority_Specific_Dispatching shall be the name of a task dispatching policy.
4.2/2 {AI95-00355-01} Both first_priority_expression and last_priority_-
expression shall be static expressions in the range of System.Any_Priority;
last_priority_expression shall have a value greater than or equal to
first_priority_expression.
Static Semantics
4.3/2 {AI95-00355-01} Pragma Task_Dispatching_Policy specifies the single task
dispatching policy.
4.4/2 {AI95-00355-01} Pragma Priority_Specific_Dispatching specifies the task
dispatching policy for the specified range of priorities. Tasks with base
priorities within the range of priorities specified in a
Priority_Specific_Dispatching pragma have their active priorities determined
according to the specified dispatching policy. Tasks with active priorities
within the range of priorities specified in a Priority_Specific_Dispatching
pragma are dispatched according to the specified dispatching policy.
4.b/2 Reason: {AI95-00355-01} Each ready queue is managed by exactly one
policy. Anything else would be chaos. The ready queue is
determined by the active priority. However, how the active
priority is calculated is determined by the policy; in order to
break out of this circle, we have to say that the active priority
is calculated by the method determined by the policy of the base
priority.
4.5/3 {AI95-00355-01} {AI05-0262-1} If a partition contains one or more
Priority_Specific_Dispatching pragmas, the dispatching policy for priorities
not covered by any Priority_Specific_Dispatching pragmas is
FIFO_Within_Priorities.
Post-Compilation Rules
5/2 {AI95-00355-01} A Task_Dispatching_Policy pragma is a configuration
pragma. A Priority_Specific_Dispatching pragma is a configuration pragma.
5.1/2 {AI95-00355-01} The priority ranges specified in more than one
Priority_Specific_Dispatching pragma within the same partition shall not be
overlapping.
5.2/2 {AI95-00355-01} If a partition contains one or more
Priority_Specific_Dispatching pragmas it shall not contain a
Task_Dispatching_Policy pragma.
6/2 This paragraph was deleted.{AI95-00333-01}
Dynamic Semantics
7/2 {AI95-00355-01} [A task dispatching policy specifies the details of task
dispatching that are not covered by the basic task dispatching model. These
rules govern when tasks are inserted into and deleted from the ready queues.]
A single task dispatching policy is specified by a Task_Dispatching_Policy
pragma. Pragma Priority_Specific_Dispatching assigns distinct dispatching
policies to subranges of System.Any_Priority.
7.1/2 {AI95-00355-01} If neither pragma applies to any of the program units
comprising a partition, the task dispatching policy for that partition is
unspecified.
7.2/3 {AI95-00355-01} {AI05-0262-1} If a partition contains one or more
Priority_Specific_Dispatching pragmas, a task dispatching point occurs for the
currently running task of a processor whenever there is a nonempty ready queue
for that processor with a higher priority than the priority of the running
task.
7.a/3 Discussion: {AI05-0005-1} If we have priority specific dispatching
then we want preemption across the entire range of priorities.
That prevents higher priority tasks from being blocked by lower
priority tasks that have a different policy. On the other hand, if
we have a single policy for the entire partition, we want the
characteristics of that policy to apply for preemption;
specifically, we might not require any preemption. Note that
policy Non_Preemptive_FIFO_Within_Priorities is not allowed in a
priority specific dispatching pragma.
7.3/2 {AI95-00355-01} A task that has its base priority changed may move from
one dispatching policy to another. It is immediately subject to the new
dispatching policy.
7.b/2 Ramification: Once subject to the new dispatching policy, it may
be immediately preempted or dispatched, according the rules of the
new policy.
Paragraphs 7 through 13 were moved to D.2.3.
Implementation Requirements
14.1/2 {AI95-00333-01} {AI95-00355-01} An implementation shall allow, for a
single partition, both the locking policy (see D.3) to be specified as
Ceiling_Locking and also one or more Priority_Specific_Dispatching pragmas to
be given.
Documentation Requirements
Paragraphs 14 through 16 were moved to D.2.3.
17.a/2 This paragraph was deleted.
Implementation Permissions
18/2 {AI95-00256-01} Implementations are allowed to define other task
dispatching policies, but need not support more than one task dispatching
policy per partition.
19/2 {AI95-00355-01} An implementation need not support pragma
Priority_Specific_Dispatching if it is infeasible to support it in the target
environment.
19.a/2 Implementation defined: Implementation defined task dispatching
policies.
NOTES
Paragraphs 19 through 21 were deleted.
Extensions to Ada 95
22.a/2 {AI95-00333-01} Amendment Correction: It is no longer required to
specify Ceiling_Locking with the language-defined task dispatching
policies; we only require that implementations allow them to be
used together.
22.b/3 {AI95-00355-01} {AI05-0005-1} Pragma Priority_Specific_Dispatching
is new; it allows the specification of different policies for
different priorities.
Wording Changes from Ada 95
22.c/2 {AI95-00256-01} Clarified that an implementation need support only
one task dispatching policy (of any kind, language-defined or
otherwise) per partition.
22.d/3 {AI95-00321-01} {AI05-0005-1} This description is simplified to
describe only the rules for the Task_Dispatching_Policy pragma
that are common to all policies. In particular, rules about
preemption are moved elsewhere. This makes it easier to add other
policies (which might not involve preemption).
D.2.3 Preemptive Dispatching
1/3 {AI95-00321-01} {AI05-0299-1} [This subclause defines a preemptive task
dispatching policy.]
Static Semantics
2/2 {AI95-00355-01} The policy_identifier FIFO_Within_Priorities is a task
dispatching policy.
Dynamic Semantics
3/2 {AI95-00321-01} When FIFO_Within_Priorities is in effect, modifications to
the ready queues occur only as follows:
4/2 * {AI95-00321-01} When a blocked task becomes ready, it is added at the
tail of the ready queue for its active priority.
5/2 * When the active priority of a ready task that is not running changes,
or the setting of its base priority takes effect, the task is removed
from the ready queue for its old active priority and is added at the
tail of the ready queue for its new active priority, except in the
case where the active priority is lowered due to the loss of inherited
priority, in which case the task is added at the head of the ready
queue for its new active priority.
6/2 * When the setting of the base priority of a running task takes effect,
the task is added to the tail of the ready queue for its active
priority.
7/2 * When a task executes a delay_statement that does not result in
blocking, it is added to the tail of the ready queue for its active
priority.
7.a/2 Ramification: If the delay does result in blocking, the task moves
to the "delay queue", not to the ready queue.
8/2 {AI95-00321-01} Each of the events specified above is a task dispatching
point (see D.2.1).
9/2 {AI95-00321-01} A task dispatching point occurs for the currently running
task of a processor whenever there is a nonempty ready queue for that
processor with a higher priority than the priority of the running task. The
currently running task is said to be preempted and it is added at the head of
the ready queue for its active priority.
Implementation Requirements
10/2 {AI95-00333-01} An implementation shall allow, for a single partition,
both the task dispatching policy to be specified as FIFO_Within_Priorities and
also the locking policy (see D.3) to be specified as Ceiling_Locking.
10.a/2 Reason: This is the preferred combination of the
FIFO_Within_Priorities policy with a locking policy, and we want
that combination to be portable.
Documentation Requirements
11/2 {AI95-00321-01} Priority inversion is the duration for which a task
remains at the head of the highest priority nonempty ready queue while the
processor executes a lower priority task. The implementation shall document:
12/2 * The maximum priority inversion a user task can experience due to
activity of the implementation (on behalf of lower priority tasks),
and
12.a/2 Documentation Requirement: The maximum priority inversion a user
task can experience from the implementation.
13/2 * whether execution of a task can be preempted by the implementation
processing of delay expirations for lower priority tasks, and if so,
for how long.
13.a/2 Documentation Requirement: The amount of time that a task can be
preempted for processing on behalf of lower-priority tasks.
NOTES
14/2 14 {AI95-00321-01} If the active priority of a running task is
lowered due to loss of inherited priority (as it is on completion of a
protected operation) and there is a ready task of the same active
priority that is not running, the running task continues to run
(provided that there is no higher priority task).
15/2 15 {AI95-00321-01} Setting the base priority of a ready task causes
the task to move to the tail of the queue for its active priority,
regardless of whether the active priority of the task actually changes.
Wording Changes from Ada 95
15.a/2 {AI95-00321-01} This subclause is new; it mainly consists of text
that was found in D.2.1 and D.2.2 in Ada 95. This was separated
out so the definition of additional policies was easier.
15.b/2 {AI95-00333-01} We require that implementations allow this policy
and Ceiling_Locking together.
15.c/2 {AI95-00355-01} We explicitly defined FIFO_Within_Priorities to be
a task dispatching policy.
D.2.4 Non-Preemptive Dispatching
1/3 {AI95-00298-01} {AI05-0299-1} [This subclause defines a non-preemptive
task dispatching policy.]
Static Semantics
2/2 {AI95-00298-01} {AI95-00355-01} The policy_identifier
Non_Preemptive_FIFO_Within_Priorities is a task dispatching policy.
2.1/3 {AI05-0166-1} The following language-defined library package exists:
2.2/3 package Ada.Dispatching.Non_Preemptive is
pragma Preelaborate(Non_Preemptive);
procedure Yield_To_Higher;
procedure Yield_To_Same_Or_Higher renames Yield;
end Ada.Dispatching.Non_Preemptive;
2.3/3 {AI05-0166-1} {AI05-0264-1} A call of Yield_To_Higher is a task
dispatching point for this policy. If the task at the head of the highest
priority ready queue has a higher active priority than the calling task, then
the calling task is preempted.
2.a/3 Ramification: For language-defined policies other than
Non_Preemptive_FIFO_Within_Priorities, a higher priority task
should never be on a ready queue while a lower priority task is
executed. Thus, for such policies, Yield_To_Higher does nothing.
2.b/3 Yield_To_Higher is not a potentially blocking operation; it can be
used during a protected operation. That is allowed, as under the
predefined Ceiling_Locking policy any task with a higher priority
than the protected operation cannot call the operation (that would
violate the locking policy). An implementation-defined locking
policy may need to define the semantics of Yield_To_Higher
differently.
Legality Rules
3/2 {AI95-00355-01} Non_Preemptive_FIFO_Within_Priorities shall not be
specified as the policy_identifier of pragma Priority_Specific_Dispatching
(see D.2.2).
3.a/2 Reason: The non-preemptive nature of this policy could cause the
policies of higher priority tasks to malfunction, missing
deadlines and having unlimited priority inversion. That would
render the use of such policies impotent and misleading. As such,
this policy only makes sense for a complete system.
Dynamic Semantics
4/2 {AI95-00298-01} When Non_Preemptive_FIFO_Within_Priorities is in effect,
modifications to the ready queues occur only as follows:
5/2 * {AI95-00298-01} When a blocked task becomes ready, it is added at the
tail of the ready queue for its active priority.
6/2 * When the active priority of a ready task that is not running changes,
or the setting of its base priority takes effect, the task is removed
from the ready queue for its old active priority and is added at the
tail of the ready queue for its new active priority.
7/2 * When the setting of the base priority of a running task takes effect,
the task is added to the tail of the ready queue for its active
priority.
8/2 * When a task executes a delay_statement that does not result in
blocking, it is added to the tail of the ready queue for its active
priority.
8.a/2 Ramification: If the delay does result in blocking, the task moves
to the "delay queue", not to the ready queue.
9/3 {AI05-0166-1} For this policy, blocking or termination of a task, a
delay_statement, a call to Yield_To_Higher, and a call to
Yield_To_Same_Or_Higher or Yield are the only task dispatching points (see
D.2.1).
9.a/3 Ramification: {AI05-0166-1} A delay_statement is always a task
dispatching point even if it is not blocking. Similarly, a call to
Yield_To_Higher is never blocking, but it is a task dispatching
point In each of these cases, they can cause the current task to
stop running (it is still ready). Otherwise, the running task
continues to run until it is blocked.
Implementation Requirements
10/2 {AI95-00333-01} An implementation shall allow, for a single partition,
both the task dispatching policy to be specified as
Non_Preemptive_FIFO_Within_Priorities and also the locking policy (see D.3) to
be specified as Ceiling_Locking.
10.a/2 Reason: This is the preferred combination of the
Non_Preemptive_FIFO_Within_Priorities policy with a locking
policy, and we want that combination to be portable.
Implementation Permissions
11/3 {AI95-00298-01} {AI05-0229-1} {AI05-0269-1} Since implementations are
allowed to round all ceiling priorities in subrange System.Priority to
System.Priority'Last (see D.3), an implementation may allow a task of a
partition using the Non_Premptive_FIFO_Within_Priorities policy to execute
within a protected object without raising its active priority provided the
associated protected unit does not contain any subprograms with aspects
Interrupt_Handler or Attach_Handler specified, nor does the unit have aspect
Interrupt_Priority specified. When the locking policy (see D.3) is
Ceiling_Locking, an implementation taking advantage of this permission shall
ensure that a call to Yield_to_Higher that occurs within a protected action
uses the ceiling priority of the protected object (rather than the active
priority of the task) when determining whether to preempt the task.
11.a.1/3 Reason: {AI05-0269-1} We explicitly require that the ceiling
priority be used in calls to Yield_to_Higher in order to prevent a
risk of priority inversion and consequent loss of mutual exclusion
when Yield_to_Higher is used in a protected object. This
requirement might lessen the value of the permission (as the
current Ceiling_Priority will have to be maintained in the TCB),
but loss of mutual exclusion cannot be tolerated. The primary
benefit of the permission (eliminating the need for preemption at
the end of a protected action) is still available. As noted above,
an implementation-defined locking policy will need to specify the
semantics of Yield_to_Higher, including this case.
Extensions to Ada 95
11.a/2 {AI95-00298-01} {AI95-00355-01} Policy
Non_Preemptive_FIFO_Within_Priorities is new.
Extensions to Ada 2005
11.b/3 {AI05-0166-1} Package Dispatching.Non_Preemptive is new.
D.2.5 Round Robin Dispatching
1/3 {AI95-00355-01} {AI05-0299-1} [This subclause defines the task dispatching
policy Round_Robin_Within_Priorities and the package Round_Robin.]
Static Semantics
2/2 {AI95-00355-01} The policy_identifier Round_Robin_Within_Priorities is a
task dispatching policy.
3/2 {AI95-00355-01} The following language-defined library package exists:
4/2 with System;
with Ada.Real_Time;
package Ada.Dispatching.Round_Robin is
Default_Quantum : constant Ada.Real_Time.Time_Span :=
implementation-defined;
procedure Set_Quantum (Pri : in System.Priority;
Quantum : in Ada.Real_Time.Time_Span);
procedure Set_Quantum (Low, High : in System.Priority;
Quantum : in Ada.Real_Time.Time_Span);
function Actual_Quantum (Pri : System.Priority)
return Ada.Real_Time.Time_Span;
function Is_Round_Robin (Pri : System.Priority) return Boolean;
end Ada.Dispatching.Round_Robin;
4.a.1/2 Implementation defined: The value of Default_Quantum in
Dispatching.Round_Robin.
5/2 {AI95-00355-01} When task dispatching policy Round_Robin_Within_Priorities
is the single policy in effect for a partition, each task with priority in the
range of System.Interrupt_Priority is dispatched according to policy
FIFO_Within_Priorities.
Dynamic Semantics
6/2 {AI95-00355-01} The procedures Set_Quantum set the required Quantum value
for a single priority level Pri or a range of priority levels Low .. High. If
no quantum is set for a Round Robin priority level, Default_Quantum is used.
7/2 {AI95-00355-01} The function Actual_Quantum returns the actual quantum
used by the implementation for the priority level Pri.
8/3 {AI95-00355-01} {AI05-0264-1} The function Is_Round_Robin returns True if
priority Pri is covered by task dispatching policy
Round_Robin_Within_Priorities; otherwise, it returns False.
9/2 {AI95-00355-01} A call of Actual_Quantum or Set_Quantum raises exception
Dispatching.Dispatching_Policy_Error if a predefined policy other than
Round_Robin_Within_Priorities applies to the specified priority or any of the
priorities in the specified range.
10/2 {AI95-00355-01} For Round_Robin_Within_Priorities, the dispatching rules
for FIFO_Within_Priorities apply with the following additional rules:
11/2 * When a task is added or moved to the tail of the ready queue for its
base priority, it has an execution time budget equal to the quantum
for that priority level. This will also occur when a blocked task
becomes executable again.
12/2 * When a task is preempted (by a higher priority task) and is added to
the head of the ready queue for its priority level, it retains its
remaining budget.
13/2 * While a task is executing, its budget is decreased by the amount of
execution time it uses. The accuracy of this accounting is the same as
that for execution time clocks (see D.14).
13.a/2 Ramification: Note that this happens even when the task is
executing at a higher, inherited priority, and even if that higher
priority is dispatched by a different policy than round robin.
14/2 * When a task has exhausted its budget and is without an inherited
priority (and is not executing within a protected operation), it is
moved to the tail of the ready queue for its priority level. This is a
task dispatching point.
14.a/2 Ramification: In this case, it will be given a budget as described
in the first bullet.
14.b/2 The rules for FIFO_Within_Priority (to which these bullets are
added) say that a task that has its base priority set to a Round
Robin priority is moved to the tail of the ready queue for its new
priority level, and then will be given a budget as described in
the first bullet. That happens whether or not the task's original
base priority was a Round Robin priority.
Implementation Requirements
15/2 {AI95-00333-01} {AI95-00355-01} An implementation shall allow, for a
single partition, both the task dispatching policy to be specified as
Round_Robin_Within_Priorities and also the locking policy (see D.3) to be
specified as Ceiling_Locking.
15.a/2 Reason: This is the preferred combination of the
Round_Robin_Within_Priorities policy with a locking policy, and we
want that combination to be portable.
Documentation Requirements
16/2 {AI95-00355-01} An implementation shall document the quantum values
supported.
16.a.1/2 Documentation Requirement: The quantum values supported for round
robin dispatching.
17/2 {AI95-00355-01} An implementation shall document the accuracy with which
it detects the exhaustion of the budget of a task.
17.a.1/2 Documentation Requirement: The accuracy of the detection of the
exhaustion of the budget of a task for round robin dispatching.
NOTES
18/2 16 {AI95-00355-01} Due to implementation constraints, the quantum
value returned by Actual_Quantum might not be identical to that set
with Set_Quantum.
19/2 17 {AI95-00355-01} A task that executes continuously with an
inherited priority will not be subject to round robin dispatching.
Extensions to Ada 95
19.a/2 {AI95-00355-01} Policy Round_Robin_Within_Priorities and package
Dispatching.Round_Robin are new.
D.2.6 Earliest Deadline First Dispatching
1/2 {AI95-00357-01} The deadline of a task is an indication of the urgency of
the task; it represents a point on an ideal physical time line. The deadline
might affect how resources are allocated to the task.
2/3 {AI95-00357-01} {AI05-0229-1} {AI05-0299-1} This subclause defines a
package for representing the deadline of a task and a dispatching policy that
defines Earliest Deadline First (EDF) dispatching. An aspect is defined to
assign an initial deadline to a task.
2.a/3 Discussion: {AI05-0229-1} This aspect is the only way of assigning
an initial deadline to a task so that its activation can be
controlled by EDF scheduling. This is similar to the way aspect
Priority is used to give an initial priority to a task.
Language Design Principles
2.b/3 {AI95-00357-01} {AI05-0299-1} To predict the behavior of a
multi-tasking program it is necessary to control access to the
processor which is preemptive, and shared objects which are
usually non-preemptive and embodied in protected objects. Two
common dispatching policies for the processor are fixed priority
and EDF. The most effective control over shared objects is via
preemption levels. With a pure priority scheme a single notion of
priority is used for processor dispatching and preemption levels.
With EDF and similar schemes priority is used for preemption
levels (only), with another measure used for dispatching. T.P.
Baker showed (Real-Time Systems, March 1991, vol. 3, num. 1,
Stack-Based Scheduling of Realtime Processes) that for EDF a newly
released task should only preempt the currently running task if it
has an earlier deadline and a higher preemption level than any
currently "locked" protected object. The rules of this subclause
implement this scheme including the case where the newly released
task should execute before some existing tasks but not preempt the
currently executing task.
Paragraphs 3 through 6 were moved to Annex J, "Obsolescent Features".
Static Semantics
7/2 {AI95-00357-01} The policy_identifier EDF_Across_Priorities is a task
dispatching policy.
8/2 {AI95-00357-01} The following language-defined library package exists:
9/2 with Ada.Real_Time;
with Ada.Task_Identification;
package Ada.Dispatching.EDF is
subtype Deadline is Ada.Real_Time.Time;
Default_Deadline : constant Deadline :=
Ada.Real_Time.Time_Last;
procedure Set_Deadline (D : in Deadline;
T : in Ada.Task_Identification.Task_Id :=
Ada.Task_Identification.Current_Task);
procedure Delay_Until_And_Set_Deadline (
Delay_Until_Time : in Ada.Real_Time.Time;
Deadline_Offset : in Ada.Real_Time.Time_Span);
function Get_Deadline (T : Ada.Task_Identification.Task_Id :=
Ada.Task_Identification.Current_Task) return Deadline;
end Ada.Dispatching.EDF;
9.1/3 {AI05-0229-1} For a task type (including the anonymous type of a
single_task_declaration) or subprogram, the following language-defined
representation aspect may be specified:
9.2/3 Relative_Deadline
The aspect Relative_Deadline is an expression, which shall be
of type Real_Time.Time_Span.
9.a/3 Aspect Description for Relative_Deadline: Task parameter used in
Earliest Deadline First Dispatching.
Legality Rules
9.3/3 {AI05-0229-1} The Relative_Deadline aspect shall not be specified on a
task interface type.
Post-Compilation Rules
10/2 {AI95-00357-01} If the EDF_Across_Priorities policy is specified for a
partition, then the Ceiling_Locking policy (see D.3) shall also be specified
for the partition.
11/2 {AI95-00357-01} If the EDF_Across_Priorities policy appears in a
Priority_Specific_Dispatching pragma (see D.2.2) in a partition, then the
Ceiling_Locking policy (see D.3) shall also be specified for the partition.
11.a/2 Reason: Unlike the other language-defined dispatching policies,
the semantic description of EDF_Across_Priorities assumes
Ceiling_Locking (and a ceiling priority) in order to make the
mapping between deadlines and priorities work. Thus, we require
both policies to be specified if EDF is used in the partition.
Dynamic Semantics
12/3 {AI95-00357-01} {AI05-0229-1} The Relative_Deadline aspect has no effect
if it is specified for a subprogram other than the main subprogram.
13/3 {AI95-00357-01} {AI05-0229-1} The initial absolute deadline of a task for
which aspect Relative_Deadline is specified is the value of Real_Time.Clock +
the expression that is the value of the aspect, where this entire expression,
including the call of Real_Time.Clock, is evaluated between task creation and
the start of its activation. If the aspect Relative_Deadline is not specified,
then the initial absolute deadline of a task is the value of Default_Deadline.
The environment task is also given an initial deadline by this rule, using the
value of the Relative_Deadline aspect of the main subprogram (if any).
13.a/2 Proof: The environment task is a normal task by 10.2, so of course
this rule applies to it.
14/2 {AI95-00357-01} The procedure Set_Deadline changes the absolute deadline
of the task to D. The function Get_Deadline returns the absolute deadline of
the task.
15/2 {AI95-00357-01} The procedure Delay_Until_And_Set_Deadline delays the
calling task until time Delay_Until_Time. When the task becomes runnable again
it will have deadline Delay_Until_Time + Deadline_Offset.
16/2 {AI95-00357-01} On a system with a single processor, the setting of the
deadline of a task to the new value occurs immediately at the first point that
is outside the execution of a protected action. If the task is currently on a
ready queue it is removed and re-entered on to the ready queue determined by
the rules defined below.
17/2 {AI95-00357-01} When EDF_Across_Priorities is specified for priority
range Low..High all ready queues in this range are ordered by deadline. The
task at the head of a queue is the one with the earliest deadline.
18/2 {AI95-00357-01} A task dispatching point occurs for the currently running
task T to which policy EDF_Across_Priorities applies:
19/2 * when a change to the deadline of T occurs;
20/2 * there is a task on the ready queue for the active priority of T with
a deadline earlier than the deadline of T; or
21/2 * there is a nonempty ready queue for that processor with a higher
priority than the active priority of the running task.
22/2 In these cases, the currently running task is said to be preempted and is
returned to the ready queue for its active priority.
23/2 {AI95-00357-01} For a task T to which policy EDF_Across_Priorities
applies, the base priority is not a source of priority inheritance; the active
priority when first activated or while it is blocked is defined as the maximum
of the following:
24/2 * the lowest priority in the range specified as EDF_Across_Priorities
that includes the base priority of T;
25/2 * the priorities, if any, currently inherited by T;
26/3 * {AI05-0055-1} the highest priority P, if any, less than the base
priority of T such that one or more tasks are executing within a
protected object with ceiling priority P and task T has an earlier
deadline than all such tasks; and furthermore T has an earlier
deadline than all other tasks on ready queues with priorities in the
given EDF_Across_Priorities range that are strictly less than P.
26.a/2 Ramification: The active priority of T might be lower than its
base priority.
27/2 {AI95-00357-01} When a task T is first activated or becomes unblocked, it
is added to the ready queue corresponding to this active priority. Until it
becomes blocked again, the active priority of T remains no less than this
value; it will exceed this value only while it is inheriting a higher priority.
27.a/2 Discussion: These rules ensure that a task executing in a
protected object is preempted only by a task with a shorter
deadline and a higher base priority. This matches the traditional
preemption level description without the need to define a new kind
of protected object locking.
28/2 {AI95-00357-01} When the setting of the base priority of a ready task
takes effect and the new priority is in a range specified as
EDF_Across_Priorities, the task is added to the ready queue corresponding to
its new active priority, as determined above.
29/2 {AI95-00357-01} For all the operations defined in Dispatching.EDF,
Tasking_Error is raised if the task identified by T has terminated.
Program_Error is raised if the value of T is Null_Task_Id.
Bounded (Run-Time) Errors
30/2 {AI95-00357-01} If EDF_Across_Priorities is specified for priority range
Low..High, it is a bounded error to declare a protected object with ceiling
priority Low or to assign the value Low to attribute 'Priority. In either case
either Program_Error is raised or the ceiling of the protected object is
assigned the value Low+1.
Erroneous Execution
31/2 {AI95-00357-01} If a value of Task_Id is passed as a parameter to any of
the subprograms of this package and the corresponding task object no longer
exists, the execution of the program is erroneous.
Documentation Requirements
32/2 {AI95-00357-01} On a multiprocessor, the implementation shall document
any conditions that cause the completion of the setting of the deadline of a
task to be delayed later than what is specified for a single processor.
32.a.1/2 Documentation Requirement: Any conditions that cause the
completion of the setting of the deadline of a task to be delayed
for a multiprocessor.
NOTES
33/3 18 {AI95-00357-01} {AI05-0264-1} If two adjacent priority ranges,
A..B and B+1..C are specified to have policy EDF_Across_Priorities,
then this is not equivalent to this policy being specified for the
single range, A..C.
34/2 19 {AI95-00357-01} The above rules implement the preemption-level
protocol (also called Stack Resource Policy protocol) for resource
sharing under EDF dispatching. The preemption-level for a task is
denoted by its base priority. The definition of a ceiling
preemption-level for a protected object follows the existing rules for
ceiling locking.
34.a/2 Implementation Note: {AI95-00357-01} An implementation may support
additional dispatching policies by replacing absolute deadline
with an alternative measure of urgency.
Extensions to Ada 95
34.b/2 {AI95-00357-01} Policy EDF_Across_Priorities and package
Dispatching.EDF are new.
Extensions to Ada 2005
34.c/3 {AI05-0229-1} Aspect Relative_Deadline is new; pragma
Relative_Deadline is now obsolescent.
Wording Changes from Ada 2005
34.d/3 {AI05-0055-1} Correction: Corrected definition of active priority
to avoid deadline inversion in an unusual case.
D.3 Priority Ceiling Locking
1/3 {AI05-0299-1} [This subclause specifies the interactions between priority
task scheduling and protected object ceilings. This interaction is based on
the concept of the ceiling priority of a protected object.]
Syntax
2 The form of a pragma Locking_Policy is as follows:
3 pragma Locking_Policy(policy_identifier);
Legality Rules
4 The policy_identifier shall either be Ceiling_Locking or an
implementation-defined identifier.
4.a Implementation defined: Implementation-defined policy_identifiers
allowed in a pragma Locking_Policy.
Post-Compilation Rules
5 A Locking_Policy pragma is a configuration pragma.
Dynamic Semantics
6/2 {8652/0073} {AI95-00091-01} {AI95-00327-01} [A locking policy specifies
the details of protected object locking. All protected objects have a
priority. The locking policy specifies the meaning of the priority of a
protected object, and the relationships between these priorities and task
priorities. In addition, the policy specifies the state of a task when it
executes a protected action, and how its active priority is affected by the
locking.] The locking policy is specified by a Locking_Policy pragma. For
implementation-defined locking policies, the meaning of the priority of a
protected object is implementation defined. If no Locking_Policy pragma
applies to any of the program units comprising a partition, the locking policy
for that partition, as well as the meaning of the priority of a protected
object, are implementation defined.
6.a/2 Implementation defined: The locking policy if no Locking_Policy
pragma applies to any unit of a partition.
6.1/3 {AI95-00327-01} {AI05-0229-1} The expression specified for the Priority
or Interrupt_Priority aspect (see D.1) is evaluated as part of the creation of
the corresponding protected object and converted to the subtype
System.Any_Priority or System.Interrupt_Priority, respectively. The value of
the expression is the initial priority of the corresponding protected object.
If no Priority or Interrupt_Priority aspect is specified for a protected
object, the initial priority is specified by the locking policy.
7 There is one predefined locking policy, Ceiling_Locking; this policy is
defined as follows:
8/3 * {AI95-00327-01} {AI05-0229-1} Every protected object has a ceiling
priority, which is determined by either a Priority or
Interrupt_Priority aspect as defined in D.1, or by assignment to the
Priority attribute as described in D.5.2. The ceiling priority of a
protected object (or ceiling, for short) is an upper bound on the
active priority a task can have when it calls protected operations of
that protected object.
9/2 * {AI95-00327-01} The initial ceiling priority of a protected object is
equal to the initial priority for that object.
10/4 * {AI95-00327-01} {AI05-0229-1} {AI12-0051-1} If an Interrupt_Handler
or Attach_Handler aspect (see C.3.1) is specified for a protected
subprogram of a protected type that does not have either the Priority
or Interrupt_Priority aspect specified, the initial priority of
protected objects of that type is implementation defined, but in the
range of the subtype System.Interrupt_Priority.
10.a Implementation defined: Default ceiling priorities.
11/3 * {AI95-00327-01} {AI05-0229-1} If neither aspect Priority nor
Interrupt_Priority is specified for a protected type, and no protected
subprogram of the type has aspect Interrupt_Handler or Attach_Handler
specified, then the initial priority of the corresponding protected
object is System.Priority'Last.
12 * While a task executes a protected action, it inherits the ceiling
priority of the corresponding protected object.
13 * When a task calls a protected operation, a check is made that its
active priority is not higher than the ceiling of the corresponding
protected object; Program_Error is raised if this check fails.
Bounded (Run-Time) Errors
13.1/2 {AI95-00327-01} Following any change of priority, it is a bounded error
for the active priority of any task with a call queued on an entry of a
protected object to be higher than the ceiling priority of the protected
object. In this case one of the following applies:
13.2/2 * at any time prior to executing the entry body Program_Error is
raised in the calling task;
13.3/2 * when the entry is open the entry body is executed at the ceiling
priority of the protected object;
13.4/2 * when the entry is open the entry body is executed at the ceiling
priority of the protected object and then Program_Error is raised in
the calling task; or
13.5/2 * when the entry is open the entry body is executed at the ceiling
priority of the protected object that was in effect when the entry
call was queued.
13.a.1/2 Ramification: Note that the error is "blamed" on the task that did
the entry call, not the task that changed the priority of the task
or protected object. This seems to make sense for the case of
changing the priority of a task blocked on a call, since if the
Set_Priority had happened a little bit sooner, before the task
queued a call, the entry-calling task would get the error.
Similarly, there is no reason not to raise the priority of a task
that is executing in an abortable_part, so long as its priority is
lowered before it gets to the end and needs to cancel the call.
The priority might need to be lowered to allow it to remove the
call from the entry queue, in order to avoid violating the
ceiling. This seems relatively harmless, since there is an error,
and the task is about to start raising an exception anyway.
Implementation Permissions
14 The implementation is allowed to round all ceilings in a certain subrange
of System.Priority or System.Interrupt_Priority up to the top of that
subrange, uniformly.
14.a Discussion: For example, an implementation might use Priority'Last
for all ceilings in Priority, and Interrupt_Priority'Last for all
ceilings in Interrupt_Priority. This would be equivalent to having
two ceiling priorities for protected objects, "nonpreemptible" and
"noninterruptible", and is an allowed behavior.
14.b Note that the implementation cannot choose a subrange that crosses
the boundary between normal and interrupt priorities.
15/2 {AI95-00256-01} Implementations are allowed to define other locking
policies, but need not support more than one locking policy per partition.
16 [Since implementations are allowed to place restrictions on code that runs
at an interrupt-level active priority (see C.3.1 and D.2.1), the
implementation may implement a language feature in terms of a protected object
with an implementation-defined ceiling, but the ceiling shall be no less than
Priority'Last.]
16.a Implementation defined: The ceiling of any protected object used
internally by the implementation.
16.b Proof: This permission follows from the fact that the
implementation can place restrictions on interrupt handlers and on
any other code that runs at an interrupt-level active priority.
16.c The implementation might protect a storage pool with a protected
object whose ceiling is Priority'Last, which would cause
allocators to fail when evaluated at interrupt priority. Note that
the ceiling of such an object has to be at least Priority'Last,
since there is no permission for allocators to fail when evaluated
at a noninterrupt priority.
Implementation Advice
17 The implementation should use names that end with "_Locking" for
implementation-defined locking policies.
17.a/2 Implementation Advice: Names that end with "_Locking" should be
used for implementation-defined locking policies.
NOTES
18 20 While a task executes in a protected action, it can be preempted
only by tasks whose active priorities are higher than the ceiling
priority of the protected object.
19 21 If a protected object has a ceiling priority in the range of
Interrupt_Priority, certain interrupts are blocked while protected
actions of that object execute. In the extreme, if the ceiling is
Interrupt_Priority'Last, all blockable interrupts are blocked during
that time.
20 22 The ceiling priority of a protected object has to be in the
Interrupt_Priority range if one of its procedures is to be used as an
interrupt handler (see C.3).
21 23 When specifying the ceiling of a protected object, one should
choose a value that is at least as high as the highest active priority
at which tasks can be executing when they call protected operations of
that object. In determining this value the following factors, which
can affect active priority, should be considered: the effect of
Set_Priority, nested protected operations, entry calls, task
activation, and other implementation-defined factors.
22 24 Attaching a protected procedure whose ceiling is below the
interrupt hardware priority to an interrupt causes the execution of
the program to be erroneous (see C.3.1).
23 25 On a single processor implementation, the ceiling priority rules
guarantee that there is no possibility of deadlock involving only
protected subprograms (excluding the case where a protected operation
calls another protected operation on the same protected object).
Extensions to Ada 95
23.a/2 {AI95-00327-01} All protected objects now have a priority, which
is the value of the Priority attribute of D.5.2. How this value is
interpreted depends on the locking policy; for instance, the
ceiling priority is derived from this value when the locking
policy is Ceiling_Locking.
Wording Changes from Ada 95
23.b/2 {8652/0073} {AI95-00091-01} Corrigendum: Corrected the wording to
reflect that pragma Locking_Policy cannot be inside of a program
unit.
23.c/2 {AI95-00256-01} Clarified that an implementation need support only
one locking policy (of any kind, language-defined or otherwise)
per partition.
23.d/2 {AI95-00327-01} The bounded error for the priority of a task being
higher than the ceiling of an object it is currently in was moved
here from D.5, so that it applies no matter how the situation
arises.
Wording Changes from Ada 2005
23.e/3 {AI05-0229-1} Revised to use aspects Priority and
Interrupt_Priority as pragmas Priority and Interrupt_Priority are
now obsolescent.
Wording Changes from Ada 2012
23.f/4 {AI12-0051-1} Corrigendum: Clarified that the Priority aspect can
be used to set the initial ceiling priority of a protected object
that contains an interrupt handler.
D.4 Entry Queuing Policies
1/3 {8652/0074} {AI95-00068-01} {AI05-0299-1} [ This subclause specifies a
mechanism for a user to choose an entry queuing policy. It also defines two
such policies. Other policies are implementation defined.]
1.a Implementation defined: Implementation-defined queuing policies.
Syntax
2 The form of a pragma Queuing_Policy is as follows:
3 pragma Queuing_Policy(policy_identifier);
Legality Rules
4 The policy_identifier shall be either FIFO_Queuing, Priority_Queuing or an
implementation-defined identifier.
Post-Compilation Rules
5 A Queuing_Policy pragma is a configuration pragma.
Dynamic Semantics
6 [A queuing policy governs the order in which tasks are queued for entry
service, and the order in which different entry queues are considered for
service.] The queuing policy is specified by a Queuing_Policy pragma.
6.a Ramification: The queuing policy includes entry queuing order, the
choice among open alternatives of a selective_accept, and the
choice among queued entry calls of a protected object when more
than one entry_barrier condition is True.
7/2 {AI95-00355-01} Two queuing policies, FIFO_Queuing and Priority_Queuing,
are language defined. If no Queuing_Policy pragma applies to any of the
program units comprising the partition, the queuing policy for that partition
is FIFO_Queuing. The rules for this policy are specified in 9.5.3 and 9.7.1.
8 The Priority_Queuing policy is defined as follows:
9 * The calls to an entry [(including a member of an entry family)] are
queued in an order consistent with the priorities of the calls. The
priority of an entry call is initialized from the active priority of
the calling task at the time the call is made, but can change later.
Within the same priority, the order is consistent with the calling (or
requeuing, or priority setting) time (that is, a FIFO order).
10/1 * {8652/0075} {AI95-00205-01} After a call is first queued, changes to
the active priority of a task do not affect the priority of the call,
unless the base priority of the task is set while the task is blocked
on an entry call.
11 * When the base priority of a task is set (see D.5), if the task is
blocked on an entry call, and the call is queued, the priority of the
call is updated to the new active priority of the calling task. This
causes the call to be removed from and then reinserted in the queue at
the new active priority.
11.a Reason: A task is blocked on an entry call if the entry call is
simple, conditional, or timed. If the call came from the
triggering_statement of an asynchronous_select, or a requeue
thereof, then the task is not blocked on that call; such calls do
not have their priority updated. Thus, there can exist many queued
calls from a given task (caused by many nested ATC's), but a task
can be blocked on only one call at a time.
11.b A previous version of Ada 9X required queue reordering in the
asynchronous_select case as well. If the call corresponds to a "
synchronous" entry call, then the task is blocked while queued,
and it makes good sense to move it up in the queue if its priority
is raised.
11.c However, if the entry call is "asynchronous," that is, it is due
to an asynchronous_select whose triggering_statement is an entry
call, then the task is not waiting for this entry call, so the
placement of the entry call on the queue is irrelevant to the rate
at which the task proceeds.
11.d Furthermore, when an entry is used for asynchronous_selects, it is
almost certain to be a "broadcast" entry or have only one caller
at a time. For example, if the entry is used to notify tasks of a
mode switch, then all tasks on the entry queue would be signaled
when the mode changes. Similarly, if it is indicating some
interrupting event such as a control-C, all tasks sensitive to the
interrupt will want to be informed that the event occurred. Hence,
the order on such a queue is essentially irrelevant.
11.e Given the above, it seems an unnecessary semantic and
implementation complexity to specify that asynchronous queued
calls are moved in response to dynamic priority changes.
Furthermore, it is somewhat inconsistent, since the call was
originally queued based on the active priority of the task, but
dynamic priority changes are changing the base priority of the
task, and only indirectly the active priority. We say explicitly
that asynchronous queued calls are not affected by normal changes
in active priority during the execution of an abortable_part.
Saying that, if a change in the base priority affects the active
priority, then we do want the calls reordered, would be
inconsistent. It would also require the implementation to maintain
a readily accessible list of all queued calls which would not
otherwise be necessary.
11.f Several rules were removed or simplified when we changed the rules
so that calls due to asynchronous_selects are never moved due to
intervening changes in active priority, be they due to protected
actions, some other priority inheritance, or changes in the base
priority.
12 * When more than one condition of an entry_barrier of a protected object
becomes True, and more than one of the respective queues is nonempty,
the call with the highest priority is selected. If more than one such
call has the same priority, the call that is queued on the entry whose
declaration is first in textual order in the protected_definition is
selected. For members of the same entry family, the one with the lower
family index is selected.
13 * If the expiration time of two or more open delay_alternatives is the
same and no other accept_alternatives are open, the
sequence_of_statements of the delay_alternative that is first in
textual order in the selective_accept is executed.
14 * When more than one alternative of a selective_accept is open and has
queued calls, an alternative whose queue has the highest-priority call
at its head is selected. If two or more open alternatives have
equal-priority queued calls, then a call on the entry in the
accept_alternative that is first in textual order in the
selective_accept is selected.
Implementation Permissions
15/2 {AI95-00256-01} Implementations are allowed to define other queuing
policies, but need not support more than one queuing policy per partition.
15.a.1/2 Discussion: {8652/0116} {AI95-00069-01} {AI95-00256-01} This rule
is really redundant, as 10.1.5 allows an implementation to limit
the use of configuration pragmas to an empty environment. In that
case, there would be no way to have multiple policies in a
partition.
15.1/2 {AI95-00188-02} Implementations are allowed to defer the reordering of
entry queues following a change of base priority of a task blocked on the
entry call if it is not practical to reorder the queue immediately.
15.a.2/2 Reason: Priority change is immediate, but the effect of the change
on entry queues can be deferred. That is necessary in order to
implement priority changes on top of a non-Ada kernel.
15.a.3/2 Discussion: The reordering should occur as soon as the blocked
task can itself perform the reinsertion into the entry queue.
Implementation Advice
16 The implementation should use names that end with "_Queuing" for
implementation-defined queuing policies.
16.a/2 Implementation Advice: Names that end with "_Queuing" should be
used for implementation-defined queuing policies.
Wording Changes from Ada 95
16.b/2 {8652/0074} {AI95-00068-01} Corrigendum: Corrected the number of
queuing policies defined.
16.c/2 {8652/0075} {AI95-00205-01} Corrigendum: Corrected so that a call
of Set_Priority in an abortable part does not change the priority
of the triggering entry call.
16.d/2 {AI95-00188-02} Added a permission to defer queue reordering when
the base priority of a task is changed. This is a counterpart to
stronger requirements on the implementation of priority change.
16.e/2 {AI95-00256-01} Clarified that an implementation need support only
one queuing policy (of any kind, language-defined or otherwise)
per partition.
16.f/2 {AI95-00355-01} Fixed wording to make clear that pragma never
appears inside of a unit; rather it "applies to" the unit.
D.5 Dynamic Priorities
1/3 {AI95-00327-01} {AI05-0299-1} [This subclause describes how the priority
of an entity can be modified or queried at run time.]
Wording Changes from Ada 95
1.a/3 {AI95-00327-01} {AI05-0299-1} This subclause is turned into two
subclauses. This subclause introduction is new.
D.5.1 Dynamic Priorities for Tasks
1/3 {AI05-0299-1} [This subclause describes how the base priority of a task
can be modified or queried at run time.]
Static Semantics
2 The following language-defined library package exists:
3/2 {AI95-00362-01} with System;
with Ada.Task_Identification; -- See C.7.1
package Ada.Dynamic_Priorities is
pragma Preelaborate(Dynamic_Priorities);
4 procedure Set_Priority(Priority : in System.Any_Priority;
T : in Ada.Task_Identification.Task_Id :=
Ada.Task_Identification.Current_Task);
5 function Get_Priority (T : Ada.Task_Identification.Task_Id :=
Ada.Task_Identification.Current_Task)
return System.Any_Priority;
6 end Ada.Dynamic_Priorities;
Dynamic Semantics
7 The procedure Set_Priority sets the base priority of the specified task to
the specified Priority value. Set_Priority has no effect if the task is
terminated.
8 The function Get_Priority returns T's current base priority. Tasking_Error
is raised if the task is terminated.
8.a Reason: There is no harm in setting the priority of a terminated
task. A previous version of Ada 9X made this a run-time error.
However, there is little difference between setting the priority
of a terminated task, and setting the priority of a task that is
about to terminate very soon; neither case should be an error.
Furthermore, the run-time check is not necessarily feasible to
implement on all systems, since priority changes might be deferred
due to inter-processor communication overhead, so the error might
not be detected until after Set_Priority has returned.
8.b However, we wish to allow implementations to avoid storing "
extra" information about terminated tasks. Therefore, we make
Get_Priority of a terminated task raise an exception; the
implementation need not continue to store the priority of a task
that has terminated.
9 Program_Error is raised by Set_Priority and Get_Priority if T is equal to
Null_Task_Id.
10/2 {AI95-00188-02} On a system with a single processor, the setting of the
base priority of a task T to the new value occurs immediately at the first
point when T is outside the execution of a protected action.
10.a/2 Implementation Note: {AI95-00188-02} The priority change is
immediate if the target task is on a delay queue or a ready queue
outside of a protected action. However, consider when Set_Priority
is called by a task T1 to set the priority of T2, if T2 is
blocked, waiting on an entry call queued on a protected object,
the entry queue needs to be reordered. Since T1 might have a
priority that is higher than the ceiling of the protected object,
T1 cannot, in general, do the reordering. One way to implement
this is to wake T2 up and have T2 do the work. This is similar to
the disentangling of queues that needs to happen when a
high-priority task aborts a lower-priority task, which might have
a call queued on a protected object with a low ceiling. We have an
Implementation Permission in D.4 to allow this implementation. We
could have required an immediate priority change if on a ready
queue during a protected action, but that would have required
extra checks for ceiling violations to meet
Bounded (Run-Time) Error requirements of D.3 and potentially could
cause a protected action to be abandoned in the middle (by raising
Program_Error). That seems bad.
10.b Reason: A previous version of Ada 9X made it a run-time error for
a high-priority task to set the priority of a lower-priority task
that has a queued call on a protected object with a low ceiling.
This was changed because:
10.c * The check was not feasible to implement on all systems, since
priority changes might be deferred due to inter-processor
communication overhead. The calling task would continue to
execute without finding out whether the operation succeeded or
not.
10.d * The run-time check would tend to cause intermittent system
failures - how is the caller supposed to know whether the
other task happens to have a queued call at any given time?
Consider for example an interrupt that needs to trigger a
priority change in some task. The interrupt handler could not
safely call Set_Priority without knowing exactly what the
other task is doing, or without severely restricting the
ceilings used in the system. If the interrupt handler wants to
hand the job off to a third task whose job is to call
Set_Priority, this won't help, because one would normally want
the third task to have high priority.
Paragraph 11 was deleted.
Erroneous Execution
12 If any subprogram 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.
12.a Ramification: Note that this rule overrides the above rule saying
that Program_Error is raised on Get_Priority of a terminated task.
If the task object still exists, and the task is terminated,
Get_Priority raises Program_Error. However, if the task object no
longer exists, calling Get_Priority causes erroneous execution.
Documentation Requirements
12.1/2 {AI95-00188-02} On a multiprocessor, the implementation shall document
any conditions that cause the completion of the setting of the priority of a
task to be delayed later than what is specified for a single processor.
12.a.1/2 Documentation Requirement: Any conditions that cause the
completion of the setting of the priority of a task to be delayed
for a multiprocessor.
Metrics
13 The implementation shall document the following metric:
14 * The execution time of a call to Set_Priority, for the nonpreempting
case, in processor clock cycles. This is measured for a call that
modifies the priority of a ready task that is not running (which
cannot be the calling one), where the new base priority of the
affected task is lower than the active priority of the calling task,
and the affected task is not on any entry queue and is not executing a
protected operation.
14.a/2 Documentation Requirement: The metrics for Set_Priority.
NOTES
15/2 26 {AI95-00321-01} Setting a task's base priority affects task
dispatching. First, it can change the task's active priority. Second,
under the FIFO_Within_Priorities policy it always causes the task to
move to the tail of the ready queue corresponding to its active
priority, even if the new base priority is unchanged.
16 27 Under the priority queuing policy, setting a task's base priority
has an effect on a queued entry call if the task is blocked waiting
for the call. That is, setting the base priority of a task causes the
priority of a queued entry call from that task to be updated and the
call to be removed and then reinserted in the entry queue at the new
priority (see D.4), unless the call originated from the
triggering_statement of an asynchronous_select.
17 28 The effect of two or more Set_Priority calls executed in parallel
on the same task is defined as executing these calls in some serial
order.
17.a Proof: This follows from the general reentrancy requirements
stated near the beginning of Annex A, "
Predefined Language Environment".
18/3 29 {AI05-0092-1} The rule for when Tasking_Error is raised for
Set_Priority or Get_Priority is different from the rule for when
Tasking_Error is raised on an entry call (see 9.5.3). In particular,
querying the priority of a completed or an abnormal task is allowed,
so long as the task is not yet terminated, and setting the priority of
a task is allowed for any task state (including for terminated tasks).
19 30 Changing the priorities of a set of tasks can be performed by a
series of calls to Set_Priority for each task separately. For this to
work reliably, it should be done within a protected operation that has
high enough ceiling priority to guarantee that the operation completes
without being preempted by any of the affected tasks.
Extensions to Ada 95
19.a/2 {AI95-00188-02} Amendment Correction: Priority changes are now
required to be done immediately so long as the target task is not
on an entry queue.
19.b/2 {AI95-00362-01} Dynamic_Priorities is now Preelaborated, so it can
be used in preelaborated units.
Wording Changes from Ada 95
19.c/3 {AI95-00327-01} {AI05-0299-1} This Ada 95 subclause was moved down
a level. The paragraph numbers are the same as those for D.5 in
Ada 95.
19.d/2 {AI95-00321-01} There is no "standard" policy anymore, so that
phrase was replaced by the name of a specific policy in the notes.
19.e/2 {AI95-00327-01} The bounded error for the priority of a task being
higher than the ceiling of an object it is currently in was moved
to D.3, so that it applies no matter how the situation arises.
D.5.2 Dynamic Priorities for Protected Objects
1/3 {AI95-00327-01} {AI05-0299-1} This subclause specifies how the priority of
a protected object can be modified or queried at run time.
Static Semantics
2/2 {AI95-00327-01} The following attribute is defined for a prefix P that
denotes a protected object:
3/2 P'Priority {AI95-00327-01} Denotes a non-aliased component of the
protected object P. This component is of type
System.Any_Priority and its value is the priority of P.
P'Priority denotes a variable if and only if P denotes a
variable. A reference to this attribute shall appear only
within the body of P.
4/2 {AI95-00327-01} The initial value of this attribute is the initial value
of the priority of the protected object[, and can be changed by an
assignment].
Dynamic Semantics
5/3 {AI95-00327-01} {AI05-0264-1} If the locking policy Ceiling_Locking (see
D.3) is in effect, then the ceiling priority of a protected object P is set to
the value of P'Priority at the end of each protected action of P.
6/3 {AI95-00445-01} {AI05-0229-1} If the locking policy Ceiling_Locking is in
effect, then for a protected object P with either an Attach_Handler or
Interrupt_Handler aspect specified for one of its procedures, a check is made
that the value to be assigned to P'Priority is in the range
System.Interrupt_Priority. If the check fails, Program_Error is raised.
Metrics
7/2 {AI95-00327-01} The implementation shall document the following metric:
8/2 * The difference in execution time of calls to the following procedures
in protected object P:
9/2 protected P is
procedure Do_Not_Set_Ceiling (Pr : System.Any_Priority);
procedure Set_Ceiling (Pr : System.Any_Priority);
end P;
10/2 protected body P is
procedure Do_Not_Set_Ceiling (Pr : System.Any_Priority) is
begin
null;
end;
procedure Set_Ceiling (Pr : System.Any_Priority) is
begin
P'Priority := Pr;
end;
end P;
10.a/2 Documentation Requirement: The metrics for setting the priority of
a protected object.
NOTES
11/2 31 {AI95-00327-01} Since P'Priority is a normal variable, the value
following an assignment to the attribute immediately reflects the new
value even though its impact on the ceiling priority of P is postponed
until completion of the protected action in which it is executed.
Extensions to Ada 95
11.a/2 {AI95-00327-01} {AI95-00445-01} The ability to dynamically change
and query the priority of a protected object is new.
D.6 Preemptive Abort
1/3 {AI05-0299-1} [This subclause specifies requirements on the immediacy with
which an aborted construct is completed.]
Dynamic Semantics
2 On a system with a single processor, an aborted construct is completed
immediately at the first point that is outside the execution of an
abort-deferred operation.
Documentation Requirements
3 On a multiprocessor, the implementation shall document any conditions that
cause the completion of an aborted construct to be delayed later than what is
specified for a single processor.
3.a/2 This paragraph was deleted.
3.b/2 Documentation Requirement: On a multiprocessor, any conditions
that cause the completion of an aborted construct to be delayed
later than what is specified for a single processor.
Metrics
4 The implementation shall document the following metrics:
5 * The execution time, in processor clock cycles, that it takes for an
abort_statement to cause the completion of the aborted task. This is
measured in a situation where a task T2 preempts task T1 and aborts
T1. T1 does not have any finalization code. T2 shall verify that T1
has terminated, by means of the Terminated attribute.
6 * On a multiprocessor, an upper bound in seconds, on the time that the
completion of an aborted task can be delayed beyond the point that it
is required for a single processor.
7/2 * {AI95-00114-01} An upper bound on the execution time of an
asynchronous_select, in processor clock cycles. This is measured
between a point immediately before a task T1 executes a protected
operation Pr.Set that makes the condition of an entry_barrier Pr.Wait
True, and the point where task T2 resumes execution immediately after
an entry call to Pr.Wait in an asynchronous_select. T1 preempts T2
while T2 is executing the abortable part, and then blocks itself so
that T2 can execute. The execution time of T1 is measured separately,
and subtracted.
8 * An upper bound on the execution time of an asynchronous_select, in the
case that no asynchronous transfer of control takes place. This is
measured between a point immediately before a task executes the
asynchronous_select with a nonnull abortable part, and the point where
the task continues execution immediately after it. The execution time
of the abortable part is subtracted.
8.a/2 Documentation Requirement: The metrics for aborts.
Implementation Advice
9 Even though the abort_statement is included in the list of potentially
blocking operations (see 9.5.1), it is recommended that this statement be
implemented in a way that never requires the task executing the
abort_statement to block.
9.a/2 Implementation Advice: The abort_statement should not require the
task executing the statement to block.
10 On a multi-processor, the delay associated with aborting a task on another
processor should be bounded; the implementation should use periodic polling,
if necessary, to achieve this.
10.a/2 Implementation Advice: On a multi-processor, the delay associated
with aborting a task on another processor should be bounded.
NOTES
11 32 Abortion does not change the active or base priority of the
aborted task.
12 33 Abortion cannot be more immediate than is allowed by the rules for
deferral of abortion during finalization and in protected actions.
D.7 Tasking Restrictions
1/3 {AI05-0299-1} [This subclause defines restrictions that can be used with a
pragma Restrictions (see 13.12) to facilitate the construction of highly
efficient tasking run-time systems.]
Static Semantics
2 The following restriction_identifiers are language defined:
3/3 {AI05-0013-1} {AI05-0216-1} No_Task_Hierarchy
No task depends on a master other than the library-level
master.
3.a/3 Ramification: {AI05-0216-1} This is equivalent to saying "no task
depends on a master other than the master that is the execution of
the body of the environment task of the partition", but it is much
easier to understand. This is a post-compilation check, which can
be checked at compile-time.
3.b/3 {AI05-0013-1} This disallows any function returning an object with
a task part or coextension, even if called at the library level,
as such a task would temporarily depend on a nested master (the
master of the return statement), which is disallowed by this
restriction.
4/3 {8652/0042} {AI95-00130-01} {AI95-00360-01} {AI05-0013-1}
No_Nested_Finalization
Objects of a type that needs finalization (see 7.6) are
declared only at library level. If an access type does not
have library-level accessibility, then there are no
allocators of the type where the type determined by the
subtype_mark of the subtype_indication or
qualified_expression needs finalization.
4.a/1 This paragraph was deleted.{8652/0042} {AI95-00130-01}
4.b/3 Ramification: {AI05-0013-1} The second sentence prevents the
declaration of objects of access types which would require nested
finalization. It also prevents the declarations of coextensions
that need finalization in a nested scope. The latter cannot be
done by preventing the declaration of the objects, as it is not
necessarily known if the coextension type needs finalization (it
could be a limited view).
5/3 {AI05-0211-1} No_Abort_Statements
There are no abort_statements, and there is no use of a name
denoting Task_Identification.Abort_Task.
6 No_Terminate_Alternatives
There are no selective_accepts with terminate_alternatives.
7 No_Task_Allocators
There are no allocators for task types or types containing
task subcomponents.
7.1/3 {AI05-0224-1} In the case of an initialized allocator of an
access type whose designated type is class-wide and limited, a
check is made that the specific type of the allocated object
has no task subcomponents. Program_Error is raised if this
check fails.
8 No_Implicit_Heap_Allocations
There are no operations that implicitly require heap storage
allocation to be performed by the implementation. The
operations that implicitly require heap storage allocation are
implementation defined.
8.a Implementation defined: Any operations that implicitly require
heap storage allocation.
9/2 {AI95-00327-01} No_Dynamic_Priorities
There are no semantic dependences on the package
Dynamic_Priorities, and no occurrences of the attribute
Priority.
10/3 {AI95-00305-01} {AI95-00394-01} {AI05-0013-1} {AI05-0211-1}
No_Dynamic_Attachment
There is no use of a name denoting any of the operations
defined in package Interrupts (Is_Reserved, Is_Attached,
Current_Handler, Attach_Handler, Exchange_Handler,
Detach_Handler, and Reference).
10.a/3 Ramification: {AI05-0013-1} This includes 'Access and 'Address of
any of these operations, as well as inherited versions of these
operations.
10.1/4 {AI12-0055-1} No_Dynamic_CPU_Assignment
No task has the CPU aspect specified to be a non-static
expression. Each task (including the environment task) that
has the CPU aspect specified as Not_A_Specific_CPU will be
assigned to a particular implementation-defined CPU. The same
is true for the environment task when the CPU aspect is not
specified. [Any other task without a CPU aspect will activate
and execute on the same processor as its activating task.]
10.b/4 Proof: The processor of a task without a CPU aspect is defined in
D.16, and this restriction guarantees that the activator always
has a CPU assigned.
10.c/4 Reason: This restriction prevents any migration of tasks.
10.d/4 Ramification: If no CPU aspects are specified, then the program
will run on a single CPU, as all of the tasks will be activated
directly or indirectly by the environment task, and the rules
require the same CPU to be used as the activating task.
10.d.1/4 Implementation defined: When restriction No_Dynamic_CPU_Assignment
applies to a partition, the processor on which a task with a CPU
value of a Not_A_Specific_CPU will execute.
10.2/3 {AI95-00305-01} {AI05-0013-1} No_Local_Protected_Objects
Protected objects are declared only at library level.
10.3/3 {AI95-00297-01} {AI05-0013-1} No_Local_Timing_Events
Timing_Events are declared only at library level.
10.4/2 {AI95-00305-01} No_Protected_Type_Allocators
There are no allocators for protected types or types
containing protected type subcomponents.
10.5/3 {AI05-0224-1} In the case of an initialized allocator of an
access type whose designated type is class-wide and limited, a
check is made that the specific type of the allocated object
has no protected subcomponents. Program_Error is raised if
this check fails.
10.6/3 {AI95-00305-01} {AI05-0211-1} No_Relative_Delay
There are no delay_relative_statements, and there is no use of
a name that denotes the Timing_Events.Set_Handler subprogram
that has a Time_Span parameter.
10.7/3 {AI95-00305-01} No_Requeue_Statements
There are no requeue_statements.
10.8/3 {AI95-00305-01} No_Select_Statements
There are no select_statements.
10.9/3 {AI95-00394-01} {AI05-0211-1} No_Specific_Termination_Handlers
There is no use of a name denoting the Set_Specific_Handler
and Specific_Handler subprograms in Task_Termination.
10.10/4 {AI12-0117-1} No_Tasks_Unassigned_To_CPU
The CPU aspect is specified for the environment task. No CPU
aspect is specified to be statically equal to
Not_A_Specific_CPU. If aspect CPU is specified (dynamically)
to the value Not_A_Specific_CPU, then Program_Error is raised.
If Set_CPU or Delay_Until_And_Set_CPU are called with the CPU
parameter equal to Not_A_Specific_CPU, then Program_Error is
raised.
10.e/4 Ramification: If this restriction is used in a context for which
restriction No_Dynamic_CPU_Assignment is in effect, then no
runtime check is needed when specifying the CPU aspect. If the
restriction is used with the Ravenscar profile, no runtime checks
are needed.
10.11/3 {AI95-00305-01} {AI05-0013-1} Simple_Barriers
The Boolean expression in each entry barrier is either a
static expression or a name that statically denotes a
component of the enclosing protected object.
11 The following restriction_parameter_identifiers are language defined:
12 Max_Select_Alternatives
Specifies the maximum number of alternatives in a
selective_accept.
13 Max_Task_Entries
Specifies the maximum number of entries per task. The bounds
of every entry family of a task unit shall be static, or shall
be defined by a discriminant of a subtype whose corresponding
bound is static. [A value of zero indicates that no rendezvous
are possible.]
14 Max_Protected_Entries
Specifies the maximum number of entries per protected type.
The bounds of every entry family of a protected unit shall be
static, or shall be defined by a discriminant of a subtype
whose corresponding bound is static.
Dynamic Semantics
15/2 {8652/0076} {AI95-00067-01} {AI95-00305-01} The following
restriction_identifier is language defined:
15.1/2 {AI95-00305-01} {AI95-00394-01} No_Task_Termination
All tasks are nonterminating. It is implementation-defined
what happens if a task attempts to terminate. If there is a
fall-back handler (see C.7.3) set for the partition it should
be called when the first task attempts to terminate.
15.a.1/2 Implementation defined: When restriction No_Task_Termination
applies to a partition, what happens when a task terminates.
16 The following restriction_parameter_identifiers are language defined:
17/1 {8652/0076} {AI95-00067-01} Max_Storage_At_Blocking
Specifies the maximum portion [(in storage elements)] of a
task's Storage_Size that can be retained by a blocked task. If
an implementation chooses to detect a violation of this
restriction, Storage_Error should be raised; otherwise, the
behavior is implementation defined.
17.a.1/2 Implementation defined: The behavior when restriction
Max_Storage_At_Blocking is violated.
18/1 {8652/0076} {AI95-00067-01} Max_Asynchronous_Select_Nesting
Specifies the maximum dynamic nesting level of
asynchronous_selects. A value of zero prevents the use of any
asynchronous_select and, if a program contains an asynchronous_-
select, it is illegal. If an implementation chooses to detect
a violation of this restriction for values other than zero,
Storage_Error should be raised; otherwise, the behavior is
implementation defined.
18.a.1/2 Implementation defined: The behavior when restriction
Max_Asynchronous_Select_Nesting is violated.
19/1 {8652/0076} {AI95-00067-01} Max_Tasks
Specifies the maximum number of task creations that may be
executed over the lifetime of a partition, not counting the
creation of the environment task. A value of zero prevents any
task creation and, if a program contains a task creation, it
is illegal. If an implementation chooses to detect a violation
of this restriction, Storage_Error should be raised;
otherwise, the behavior is implementation defined.
19.a Ramification: Note that this is not a limit on the number of tasks
active at a given time; it is a limit on the total number of task
creations that occur.
19.b Implementation Note: We envision an implementation approach that
places TCBs or pointers to them in a fixed-size table, and never
reuses table elements.
19.b.1/2 Implementation defined: The behavior when restriction Max_Tasks is
violated.
19.1/2 {AI95-00305-01} Max_Entry_Queue_Length
Max_Entry_Queue_Length defines the maximum number of calls
that are queued on an entry. Violation of this restriction
results in the raising of Program_Error at the point of the
call or requeue.
19.2/3 {AI05-0189-1} No_Standard_Allocators_After_Elaboration
Specifies that an allocator using a standard storage pool (see
13.11) shall not occur within a parameterless library
subprogram, nor within the handled_sequence_of_statements of a
task body. For the purposes of this rule, an allocator of a
type derived from a formal access type does not use a standard
storage pool.
19.3/3 {AI05-0189-1} {AI05-0262-1} At run time, Storage_Error is
raised if an allocator using a standard storage pool is
evaluated after the elaboration of the library_items of the
partition has completed.
20 It is implementation defined whether the use of pragma Restrictions
results in a reduction in executable program size, storage requirements, or
execution time. If possible, the implementation should provide quantitative
descriptions of such effects for each restriction.
20.a/2 Implementation defined: Whether the use of pragma Restrictions
results in a reduction in program code or data size or execution
time.
Implementation Advice
21 When feasible, the implementation should take advantage of the specified
restrictions to produce a more efficient implementation.
21.a/2 Implementation Advice: When feasible, specified restrictions
should be used to produce a more efficient implementation.
NOTES
22 34 The above Storage_Checks can be suppressed with pragma Suppress.
Incompatibilities With Ada 95
22.a/2 {AI95-00360-01} Amendment Correction: The No_Nested_Finalization
is now defined in terms of types that need finalization. These
types include a variety of language-defined types that might be
implemented with a controlled type. If the restriction
No_Nested_Finalization (see D.7) applies to the partition, and one
of these language-defined types does not have a controlled part,
it will not be allowed in local objects in Ada 2005 whereas it
would be allowed in original Ada 95. Such code is not portable, as
other Ada compilers may have had a controlled part, and thus would
be illegal under the restriction.
Extensions to Ada 95
22.b/2 {AI95-00297-01} {AI95-00305-01} {AI95-00394-01} Restrictions
No_Dynamic_Attachment, No_Local_Protected_Objects,
No_Protected_Type_Allocators, No_Local_Timing_Events,
No_Relative_Delay, No_Requeue_Statement, No_Select_Statements,
No_Specific_Termination_Handlers, No_Task_Termination,
Max_Entry_Queue_Length, and Simple_Barriers are newly added to
Ada.
Wording Changes from Ada 95
22.c/2 {8652/0042} {AI95-00130-01} Corrigendum: Clarified that
No_Nested_Finalization covered task and protected parts as well.
22.d/2 {8652/0076} {AI95-00067-01} Corrigendum: Changed the description
of Max_Tasks and Max_Asynchronous_Select_Nested to eliminate
conflicts with the High Integrity Annex (see H.4).
22.e/2 {AI95-00327-01} Added using of the new Priority attribute to the
restriction No_Dynamic_Priorities.
22.f/2 {AI95-00394-01} Restriction No_Asynchronous_Control is now
obsolescent.
Incompatibilities With Ada 2005
22.g/3 {AI05-0013-1} Correction: Changed so that coextensions of types
that require nested finalization are also prohibited; this is done
by prohibiting allocators rather than objects of specific access
types. It seems unlikely that any program depending on this
restriction would violate it in this blatant manner, so it is
expected that very few programs will be affected by this change.
22.h/3 {AI05-0211-1} Correction: The restriction No_Relative_Delay was
changed to include the Timing_Events routine that uses a relative
delay. This means that a program that uses that routine and this
restriction will now be rejected. However, such a program violates
the spirit and intent of the restriction and as such the program
should never have been allowed. Moreover, it is unlikely that any
program depending on this restriction would violate it in such an
obvious manner, so it is expected that very few programs will be
affected by this change.
22.i/3 {AI05-0211-1} Correction: A number of restrictions were changed
from "no calls" on some subprogram to "no use of a name that
denotes" that subprogram. This closes a hole where renames, uses
as the prefix of 'Access, and the like, would not be rejected by
the restriction, possibly allowing backdoor access to the
prohibited subprogram. A program that uses one of these
restrictions and using such backdoor access will now be rejected;
however, it is extremely unlikely that any program that relies on
these restrictions would also use an end-run around the
restriction, so it is expected that very few programs will be
affected by this change.
Extensions to Ada 2005
22.j/3 {AI05-0189-1} Restriction No_Standard_Allocators_After_Elaboration
is newly added to Ada.
Wording Changes from Ada 2005
22.k/3 {AI05-0013-1} {AI05-0216-1} Correction: Improved the wording of
various restrictions to make it clearer that they prohibit things
that would otherwise be legal, and to word them as definitions,
not Legality Rules;.
22.l/3 {AI05-0192-1} Correction: Added wording to explain how
No_Task_Allocators and No_Protected_Type_Allocators are checked
for class-wide types. This might be an extension if the compiler
assumed the worst in the past (it is now a runtime check).
Extensions to Ada 2012
22.m/4 {AI12-0055-1} Corrigendum: Restriction No_Dynamic_CPU_Assignment
is newly added to Ada, for use as part of the Ravenscar profile
(see D.13).
22.n/4 {AI12-0117-1} Corrigendum: Restriction No_Tasks_Unassigned_To_CPU
is newly added to Ada; it ensures that no task is running on an
implementation-defined CPU so that task scheduling can be
analyzed.
D.8 Monotonic Time
1/3 {AI05-0299-1} [This subclause specifies a high-resolution, monotonic clock
package.]
Static Semantics
2 The following language-defined library package exists:
3 package Ada.Real_Time is
4 type Time is private;
Time_First : constant Time;
Time_Last : constant Time;
Time_Unit : constant := implementation-defined-real-number;
5 type Time_Span is private;
Time_Span_First : constant Time_Span;
Time_Span_Last : constant Time_Span;
Time_Span_Zero : constant Time_Span;
Time_Span_Unit : constant Time_Span;
6 Tick : constant Time_Span;
function Clock return Time;
7 function "+" (Left : Time; Right : Time_Span) return Time;
function "+" (Left : Time_Span; Right : Time) return Time;
function "-" (Left : Time; Right : Time_Span) return Time;
function "-" (Left : Time; Right : Time) return Time_Span;
8 function "<" (Left, Right : Time) return Boolean;
function "<="(Left, Right : Time) return Boolean;
function ">" (Left, Right : Time) return Boolean;
function ">="(Left, Right : Time) return Boolean;
9 function "+" (Left, Right : Time_Span) return Time_Span;
function "-" (Left, Right : Time_Span) return Time_Span;
function "-" (Right : Time_Span) return Time_Span;
function "*" (Left : Time_Span; Right : Integer) return Time_Span;
function "*" (Left : Integer; Right : Time_Span) return Time_Span;
function "/" (Left, Right : Time_Span) return Integer;
function "/" (Left : Time_Span; Right : Integer) return Time_Span;
10 function "abs"(Right : Time_Span) return Time_Span;
11/1 This paragraph was deleted.
12 function "<" (Left, Right : Time_Span) return Boolean;
function "<="(Left, Right : Time_Span) return Boolean;
function ">" (Left, Right : Time_Span) return Boolean;
function ">="(Left, Right : Time_Span) return Boolean;
13 function To_Duration (TS : Time_Span) return Duration;
function To_Time_Span (D : Duration) return Time_Span;
14/2 {AI95-00386-01} function Nanoseconds
(NS : Integer) return Time_Span;
function Microseconds (US : Integer) return Time_Span;
function Milliseconds (MS : Integer) return Time_Span;
function Seconds (S : Integer) return Time_Span;
function Minutes (M : Integer) return Time_Span;
15 type Seconds_Count is range implementation-defined;
16 procedure Split
(T : in Time; SC : out Seconds_Count; TS : out Time_Span);
function Time_Of(SC : Seconds_Count; TS : Time_Span) return Time;
17 private
... -- not specified by the language
end Ada.Real_Time;
17.a/2 This paragraph was deleted.
18 In this Annex, real time is defined to be the physical time as observed in
the external environment. The type Time is a time type as defined by 9.6;
[values of this type may be used in a delay_until_statement.] Values of this
type represent segments of an ideal time line. The set of values of the type
Time corresponds one-to-one with an implementation-defined range of
mathematical integers.
18.a Discussion: Informally, real time is defined to be the
International Atomic Time (TAI) which is monotonic and
nondecreasing. We use it here for the purpose of discussing rate
of change and monotonic behavior only. It does not imply anything
about the absolute value of Real_Time.Clock, or about
Real_Time.Time being synchronized with TAI. It is also used for
real time in the metrics, for comparison purposes.
18.b Implementation Note: The specification of TAI as "real time" does
not preclude the use of a simulated TAI clock for simulated
execution environments.
19 The Time value I represents the half-open real time interval that starts
with E+I*Time_Unit and is limited by E+(I+1)*Time_Unit, where Time_Unit is an
implementation-defined real number and E is an unspecified origin point, the
epoch, that is the same for all values of the type Time. It is not specified
by the language whether the time values are synchronized with any standard
time reference. [For example, E can correspond to the time of system
initialization or it can correspond to the epoch of some time standard.]
19.a Discussion: E itself does not have to be a proper time value.
19.b This half-open interval I consists of all real numbers R such that
E+I*Time_Unit <= R < E+(I+1)*Time_Unit.
20 Values of the type Time_Span represent length of real time duration. The
set of values of this type corresponds one-to-one with an
implementation-defined range of mathematical integers. The Time_Span value
corresponding to the integer I represents the real-time duration I*Time_Unit.
20.a Reason: The purpose of this type is similar to Standard.Duration;
the idea is to have a type with a higher resolution.
20.b Discussion: We looked at many possible names for this type:
Real_Time.Duration, Fine_Duration, Interval, Time_Interval_Length,
Time_Measure, and more. Each of these names had some problems, and
we've finally settled for Time_Span.
21 Time_First and Time_Last are the smallest and largest values of the Time
type, respectively. Similarly, Time_Span_First and Time_Span_Last are the
smallest and largest values of the Time_Span type, respectively.
22 A value of type Seconds_Count represents an elapsed time, measured in
seconds, since the epoch.
Dynamic Semantics
23 Time_Unit is the smallest amount of real time representable by the Time
type; it is expressed in seconds. Time_Span_Unit is the difference between two
successive values of the Time type. It is also the smallest positive value of
type Time_Span. Time_Unit and Time_Span_Unit represent the same real time
duration. A clock tick is a real time interval during which the clock value
(as observed by calling the Clock function) remains constant. Tick is the
average length of such intervals.
24/2 {AI95-00432-01} The function To_Duration converts the value TS to a value
of type Duration. Similarly, the function To_Time_Span converts the value D to
a value of type Time_Span. For To_Duration, the result is rounded to the
nearest value of type Duration (away from zero if exactly halfway between two
values). If the result is outside the range of Duration, Constraint_Error is
raised. For To_Time_Span, the value of D is first rounded to the nearest
integral multiple of Time_Unit, away from zero if exactly halfway between two
multiples. If the rounded value is outside the range of Time_Span,
Constraint_Error is raised. Otherwise, the value is converted to the type
Time_Span.
25 To_Duration(Time_Span_Zero) returns 0.0, and To_Time_Span(0.0) returns
Time_Span_Zero.
26/2 {AI95-00386-01} {AI95-00432-01} The functions Nanoseconds, Microseconds,
Milliseconds, Seconds, and Minutes convert the input parameter to a value of
the type Time_Span. NS, US, MS, S, and M are interpreted as a number of
nanoseconds, microseconds, milliseconds, seconds, and minutes respectively.
The input parameter is first converted to seconds and rounded to the nearest
integral multiple of Time_Unit, away from zero if exactly halfway between two
multiples. If the rounded value is outside the range of Time_Span,
Constraint_Error is raised. Otherwise, the rounded value is converted to the
type Time_Span.
26.a/2 This paragraph was deleted.{AI95-00432-01}
27 The effects of the operators on Time and Time_Span are as for the
operators defined for integer types.
27.a Implementation Note: Though time values are modeled by integers,
the types Time and Time_Span need not be implemented as integers.
28 The function Clock returns the amount of time since the epoch.
29 The effects of the Split and Time_Of operations are defined as follows,
treating values of type Time, Time_Span, and Seconds_Count as mathematical
integers. The effect of Split(T,SC,TS) is to set SC and TS to values such that
T*Time_Unit = SC*1.0 + TS*Time_Unit, and 0.0 <= TS*Time_Unit < 1.0. The value
returned by Time_Of(SC,TS) is the value T such that T*Time_Unit = SC*1.0 +
TS*Time_Unit.
Implementation Requirements
30 The range of Time values shall be sufficient to uniquely represent the
range of real times from program start-up to 50 years later. Tick shall be no
greater than 1 millisecond. Time_Unit shall be less than or equal to 20
microseconds.
30.a Implementation Note: The required range and accuracy of Time are
such that 32-bits worth of seconds and 32-bits worth of ticks in a
second could be used as the representation.
31 Time_Span_First shall be no greater than -3600 seconds, and Time_Span_Last
shall be no less than 3600 seconds.
31.a Reason: This is equivalent to ± one hour and there is still room
for a two-microsecond resolution.
32 A clock jump is the difference between two successive distinct values of
the clock (as observed by calling the Clock function). There shall be no
backward clock jumps.
Documentation Requirements
33 The implementation shall document the values of Time_First, Time_Last,
Time_Span_First, Time_Span_Last, Time_Span_Unit, and Tick.
33.a/2 Documentation Requirement: The values of Time_First, Time_Last,
Time_Span_First, Time_Span_Last, Time_Span_Unit, and Tick for
package Real_Time.
34 The implementation shall document the properties of the underlying time
base used for the clock and for type Time, such as the range of values
supported and any relevant aspects of the underlying hardware or operating
system facilities used.
34.a.1/2 Documentation Requirement: The properties of the underlying time
base used in package Real_Time.
34.a Discussion: If there is an underlying operating system, this might
include information about which system call is used to implement
the clock. Otherwise, it might include information about which
hardware clock is used.
35 The implementation shall document whether or not there is any
synchronization with external time references, and if such synchronization
exists, the sources of synchronization information, the frequency of
synchronization, and the synchronization method applied.
35.a.1/2 Documentation Requirement: Any synchronization of package
Real_Time with external time references.
36/3 {AI05-0299-1} The implementation shall document any aspects of the
external environment that could interfere with the clock behavior as defined
in this subclause.
36.a.1/2 Documentation Requirement: Any aspects of the external environment
that could interfere with package Real_Time.
36.a Discussion: For example, the implementation is allowed to rely on
the time services of an underlying operating system, and this
operating system clock can implement time zones or allow the clock
to be reset by an operator. This dependence has to be documented.
Metrics
37/3 {AI05-0299-1} For the purpose of the metrics defined in this subclause,
real time is defined to be the International Atomic Time (TAI).
38 The implementation shall document the following metrics:
39 * An upper bound on the real-time duration of a clock tick. This is a
value D such that if t1 and t2 are any real times such that t1 < t2
and Clock(t1) = Clock(t2) then t2 - t1 <= D.
40 * An upper bound on the size of a clock jump.
41 * An upper bound on the drift rate of Clock with respect to real time.
This is a real number D such that
42 E*(1-D) <= (Clock(t+E) - Clock(t)) <= E*(1+D)
provided that: Clock(t) + E*(1+D) <= Time_Last.
43 * where Clock(t) is the value of Clock at time t, and E is a real time
duration not less than 24 hours. The value of E used for this metric
shall be reported.
43.a Reason: This metric is intended to provide a measurement of the
long term (cumulative) deviation; therefore, 24 hours is the lower
bound on the measurement period. On some implementations, this is
also the maximum period, since the language does not require that
the range of the type Duration be more than 24 hours. On those
implementations that support longer-range Duration, longer
measurements should be performed.
44 * An upper bound on the execution time of a call to the Clock function,
in processor clock cycles.
45 * Upper bounds on the execution times of the operators of the types Time
and Time_Span, in processor clock cycles.
45.a Implementation Note: A fast implementation of the Clock function
involves repeated reading until you get the same value twice. It
is highly improbable that more than three reads will be necessary.
Arithmetic on time values should not be significantly slower than
64-bit arithmetic in the underlying machine instruction set.
45.a.1/2 Documentation Requirement: The metrics for package Real_Time.
Implementation Permissions
46 Implementations targeted to machines with word size smaller than 32 bits
need not support the full range and granularity of the Time and Time_Span
types.
46.a Discussion: These requirements are based on machines with a word
size of 32 bits.
46.b Since the range and granularity are implementation defined, the
supported values need to be documented.
Implementation Advice
47 When appropriate, implementations should provide configuration mechanisms
to change the value of Tick.
47.a.1/2 Implementation Advice: When appropriate, mechanisms to change the
value of Tick should be provided.
47.a Reason: This is often needed when the compilation system was
originally targeted to a particular processor with a particular
interval timer, but the customer uses the same processor with a
different interval timer.
47.b Discussion: Tick is a deferred constant and not a named number
specifically for this purpose.
47.c Implementation Note: This can be achieved either by pre-run-time
configuration tools, or by having Tick be initialized (in the
package private part) by a function call residing in a board
specific module.
48 It is recommended that Calendar.Clock and Real_Time.Clock be implemented
as transformations of the same time base.
48.a.1/2 Implementation Advice: Calendar.Clock and Real_Time.Clock should
be transformations of the same time base.
49 It is recommended that the "best" time base which exists in the underlying
system be available to the application through Clock. "Best" may mean highest
accuracy or largest range.
49.a.1/2 Implementation Advice: The "best" time base which exists in the
underlying system should be available to the application through
Real_Time.Clock.
NOTES
50/3 35 {AI05-0299-1} The rules in this subclause do not imply that the
implementation can protect the user from operator or installation
errors which could result in the clock being set incorrectly.
51 36 Time_Unit is the granularity of the Time type. In contrast, Tick
represents the granularity of Real_Time.Clock. There is no requirement
that these be the same.
Incompatibilities With Ada 95
51.a/3 {AI95-00386-01} {AI05-0005-1} Functions Seconds and Minutes are
added to Real_Time. If Real_Time is referenced in a use_clause,
and an entity E with a defining_identifier of Seconds or Minutes
is defined in a package that is also referenced in a use_clause,
the entity E may no longer be use-visible, resulting in errors.
This should be rare and is easily fixed if it does occur.
Wording Changes from Ada 95
51.b/2 {AI95-00432-01} Added wording explaining how and when many of
these functions can raise Constraint_Error. While there always was
an intent to raise Constraint_Error if the values did not fit,
there never was any wording to that effect, and since Time_Span
was a private type, the normal numeric type rules do not apply to
it.
D.9 Delay Accuracy
1/3 {AI05-0299-1} [This subclause specifies performance requirements for the
delay_statement. The rules apply both to delay_relative_statement and to delay_-
until_statement. Similarly, they apply equally to a simple delay_statement and
to one which appears in a delay_alternative.]
Dynamic Semantics
2 The effect of the delay_statement for Real_Time.Time is defined in terms
of Real_Time.Clock:
3 * If C(1) is a value of Clock read before a task executes a
delay_relative_statement with duration D, and C(2) is a value of Clock
read after the task resumes execution following that delay_statement,
then C(2) - C(1) >= D.
4 * If C is a value of Clock read after a task resumes execution following
a delay_until_statement with Real_Time.Time value T, then C >= T.
5 A simple delay_statement with a negative or zero value for the expiration
time does not cause the calling task to be blocked; it is nevertheless a
potentially blocking operation (see 9.5.1).
6/3 {AI05-0004-1} When a delay_statement appears in a delay_alternative of a
timed_entry_call the selection of the entry call is attempted, regardless of
the specified expiration time. When a delay_statement appears in a
select_alternative, and a call is queued on one of the open entries, the
selection of that entry call proceeds, regardless of the value of the delay
expression.
6.a Ramification: The effect of these requirements is that one has to
always attempt a rendezvous, regardless of the value of the delay
expression. This can be tested by issuing a timed_entry_call with
an expiration time of zero, to an open entry.
Documentation Requirements
7 The implementation shall document the minimum value of the delay
expression of a delay_relative_statement that causes the task to actually be
blocked.
7.a/2 Documentation Requirement: The minimum value of the delay
expression of a delay_relative_statement that causes a task to
actually be blocked.
8 The implementation shall document the minimum difference between the value
of the delay expression of a delay_until_statement and the value of
Real_Time.Clock, that causes the task to actually be blocked.
8.a/2 This paragraph was deleted.
8.b/2 Documentation Requirement: The minimum difference between the
value of the delay expression of a delay_until_statement and the
value of Real_Time.Clock, that causes the task to actually be
blocked.
Metrics
9 The implementation shall document the following metrics:
10 * An upper bound on the execution time, in processor clock cycles, of a
delay_relative_statement whose requested value of the delay expression
is less than or equal to zero.
11 * An upper bound on the execution time, in processor clock cycles, of a
delay_until_statement whose requested value of the delay expression is
less than or equal to the value of Real_Time.Clock at the time of
executing the statement. Similarly, for Calendar.Clock.
12 * An upper bound on the lateness of a delay_relative_statement, for a
positive value of the delay expression, in a situation where the task
has sufficient priority to preempt the processor as soon as it becomes
ready, and does not need to wait for any other execution resources.
The upper bound is expressed as a function of the value of the delay
expression. The lateness is obtained by subtracting the value of the
delay expression from the actual duration. The actual duration is
measured from a point immediately before a task executes the
delay_statement to a point immediately after the task resumes
execution following this statement.
13 * An upper bound on the lateness of a delay_until_statement, in a
situation where the value of the requested expiration time is after
the time the task begins executing the statement, the task has
sufficient priority to preempt the processor as soon as it becomes
ready, and it does not need to wait for any other execution resources.
The upper bound is expressed as a function of the difference between
the requested expiration time and the clock value at the time the
statement begins execution. The lateness of a delay_until_statement is
obtained by subtracting the requested expiration time from the real
time that the task resumes execution following this statement.
13.a/2 Documentation Requirement: The metrics for delay statements.
Wording Changes from Ada 83
14.a The rules regarding a timed_entry_call with a very small positive
Duration value, have been tightened to always require the check
whether the rendezvous is immediately possible.
Wording Changes from Ada 95
14.b/2 {AI95-00355-01} The note about "voluntary round-robin', while
still true, has been deleted as potentially confusing as it is
describing a different kind of round-robin than is defined by the
round-robin dispatching policy.
D.10 Synchronous Task Control
1/3 {AI05-0299-1} [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
2 The following language-defined package exists:
3/2 {AI95-00362-01} package Ada.Synchronous_Task_Control is
pragma Preelaborate(Synchronous_Task_Control);
4 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;
5 The type Suspension_Object is a by-reference type.
5.a/2 Implementation Note: {AI95-00318-02} The implementation can ensure
this by, for example, making the full view an explicitly limited
record type.
5.1/3 {AI05-0168-1} The following language-defined package exists:
5.2/3 {AI05-0168-1} 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
6/2 {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.
6.a 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.
7/2 {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.
8 Current_State returns the current state of the object.
8.a Discussion: This state can change immediately after the operation
returns.
9/2 {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.
10 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).
10.1/3 {AI05-0168-1} {AI05-0269-1} 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
11 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.
NOTES
12/3 37 {AI05-0168-1} More complex schemes, such as setting the deadline
relative to when Set_True is called, can be programmed using a
protected object.
Extensions to Ada 95
12.a/2 {AI95-00362-01} Synchronous_Task_Control is now Preelaborated, so
it can be used in preelaborated units.
Extensions to Ada 2005
12.b/3 {AI05-0168-1} Child package Ada.Synchronous_Task_Control.EDF is
new.
D.10.1 Synchronous Barriers
1/3 {AI05-0174-1} {AI05-0299-1} This subclause introduces a language-defined
package to synchronously release a group of tasks after the number of blocked
tasks reaches a specified count value.
Static Semantics
2/3 {AI05-0174-1} The following language-defined library package exists:
3/3 package Ada.Synchronous_Barriers is
pragma Preelaborate(Synchronous_Barriers);
4/3 subtype Barrier_Limit
is Positive range 1 .. implementation-defined;
4.a.1/3 Implementation defined: The value of Barrier_Limit'Last in
Synchronous_Barriers.
5/3 type Synchronous_Barrier
(Release_Threshold : Barrier_Limit) is limited private;
6/3 procedure Wait_For_Release
(The_Barrier : in out Synchronous_Barrier;
Notified : out Boolean);
7/3 private
-- not specified by the language
end Ada.Synchronous_Barriers;
8/3 {AI05-0174-1} Type Synchronous_Barrier needs finalization (see 7.6).
Dynamic Semantics
9/3 {AI05-0174-1} Each call to Wait_For_Release blocks the calling task until
the number of blocked tasks associated with the Synchronous_Barrier object is
equal to Release_Threshold, at which time all blocked tasks are released.
Notified is set to True for one of the released tasks, and set to False for
all other released tasks.
10/3 {AI05-0174-1} The mechanism for determining which task sets Notified to
True is implementation defined.
11/3 {AI05-0174-1} Once all tasks have been released, a Synchronous_Barrier
object may be reused to block another Release_Threshold number of tasks.
12/3 {AI05-0174-1} As the first step of the finalization of a
Synchronous_Barrier, each blocked task is unblocked and Program_Error is
raised at the place of the call to Wait_For_Release.
13/3 {AI05-0174-1} It is implementation defined whether an abnormal task which
is waiting on a Synchronous_Barrier object is aborted immediately or aborted
when the tasks waiting on the object are released.
13.a.1/3 Implementation defined: When an aborted task that is waiting on a
Synchronous_Barrier is aborted.
14/3 {AI05-0174-1} Wait_For_Release is a potentially blocking operation (see
9.5.1).
Bounded (Run-Time) Errors
15/3 {AI05-0174-1} It is a bounded error to call Wait_For_Release on a
Synchronous_Barrier object after that object is finalized. If the error is
detected, Program_Error is raised. Otherwise, the call proceeds normally,
which may leave a task blocked forever.
Extensions to Ada 2005
15.a/3 {AI05-0174-1} The package Ada.Synchronous_Barriers is new.
D.11 Asynchronous Task Control
1/3 {AI05-0299-1} [This subclause 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 state.]
Static Semantics
2 The following language-defined library package exists:
3/2 {AI95-00362-01} with Ada.Task_Identification;
package Ada.Asynchronous_Task_Control is
pragma Preelaborate(Asynchronous_Task_Control);
procedure Hold(T : in Ada.Task_Identification.Task_Id);
procedure Continue(T : in Ada.Task_Identification.Task_Id);
function Is_Held(T : Ada.Task_Identification.Task_Id)
return Boolean;
end Ada.Asynchronous_Task_Control;
Dynamic Semantics
4/2 {AI95-00357-01} After the 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 whose
value is below the base priority of the idle task.
4.a Discussion: The held state should not be confused with the blocked
state as defined in 9.2; the task is still ready.
4.1/2 {AI95-00357-01} For any priority below System.Any_Priority'First, the
task dispatching policy is FIFO_Within_Priorities.
4.b/2 To be honest: This applies even if a Task_Dispatching_Policy
specifies the policy for all of the priorities of the partition.
4.c/2 Ramification: A 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 EDF_Across_Priorities).
5/2 {AI95-00357-01} 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.
5.a 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.
6/2 {AI95-00357-01} The Continue operation resets the state of T to not-held;
its active priority is then reevaluated as determined by the task dispatching
policy associated with its base priority.
7 The Is_Held function returns True if and only if T is in the held state.
7.a Discussion: Note that the state of T can be changed immediately
after Is_Held returns.
8 As part of these operations, a check is made that the task identified by T
is not terminated. Tasking_Error is raised if the check fails. Program_Error
is raised if the value of T is Null_Task_Id.
Erroneous Execution
9 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.
Implementation Permissions
10 An implementation need not support Asynchronous_Task_Control if it is
infeasible to support it in the target environment.
10.a 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 be problematic.
NOTES
11 38 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.
12 39 The effect of calling Get_Priority and Set_Priority on a Held task
is the same as on any other task.
13 40 Calling Hold on a held task or Continue on a non-held task has no
effect.
14 41 The rules affecting queuing are derived from the above rules, in
addition to the normal priority rules:
15 * 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.
16 * 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.
17 * 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.
18/1 * {8652/0077} {AI95-00111-01} If a task becomes held while waiting
in a selective_accept, and an entry call is issued to one of the
open entries, the corresponding accept_alternative 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 Continue.
19 * 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
19.a/2 {AI95-00362-01} Asynchronous_Task_Control is now Preelaborated, so
it can be used in preelaborated units.
Wording Changes from Ada 95
19.b/2 {8652/0077} {AI95-00111-01} Corrigendum: Corrected to eliminate
the use of the undefined term "accept body".
19.c/2 {AI95-00357-01} 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 policy).
D.12 Other Optimizations and Determinism Rules
1/3 {AI05-0299-1} [This subclause describes various requirements for improving
the response and determinism in a real-time system.]
Implementation Requirements
2 If the implementation blocks interrupts (see C.3) not as a result of
direct user action (e.g. an execution of a protected action) there shall be an
upper bound on the duration of this blocking.
2.a Ramification: The implementation shall not allow itself to be
interrupted when it is in a state where it is unable to support
all the language-defined operations permitted in the execution of
interrupt handlers. (see 9.5.1).
3 The implementation shall recognize entry-less protected types. The
overhead of acquiring the execution resource of an object of such a type (see
9.5.1) shall be minimized. In particular, there should not be any overhead due
to evaluating entry_barrier conditions.
3.a Implementation Note: Ideally the overhead should just be a
spin-lock.
4 Unchecked_Deallocation shall be supported for terminated tasks that are
designated by access types, and shall have the effect of releasing all the
storage associated with the task. This includes any run-time system or heap
storage that has been implicitly allocated for the task by the implementation.
Documentation Requirements
5 The implementation shall document the upper bound on the duration of
interrupt blocking caused by the implementation. If this is different for
different interrupts or interrupt priority levels, it should be documented for
each case.
5.a/2 This paragraph was deleted.
5.b/2 Documentation Requirement: The upper bound on the duration of
interrupt blocking caused by the implementation.
Metrics
6 The implementation shall document the following metric:
7 * The overhead associated with obtaining a mutual-exclusive access to an
entry-less protected object. This shall be measured in the following
way:
8 For a protected object of the form:
9 protected Lock is
procedure Set;
function Read return Boolean;
private
Flag : Boolean := False;
end Lock;
10 protected body Lock is
procedure Set is
begin
Flag := True;
end Set;
function Read return Boolean
Begin
return Flag;
end Read;
end Lock;
11 The execution time, in processor clock cycles, of a call to Set. This
shall be measured between the point just before issuing the call, and
the point just after the call completes. The function Read shall be
called later to verify that Set was indeed called (and not optimized
away). The calling task shall have sufficiently high priority as to
not be preempted during the measurement period. The protected object
shall have sufficiently high ceiling priority to allow the task to
call Set.
12 For a multiprocessor, if supported, the metric shall be reported for
the case where no contention (on the execution resource) exists [from
tasks executing on other processors].
12.a/2 Documentation Requirement: The metrics for entry-less protected
objects.
D.13 The Ravenscar Profile
1/3 {AI95-00249-01} {AI05-0246-1} {AI05-0299-1} [This subclause defines the
Ravenscar profile.]
Paragraphs 2 and 3 were moved to 13.12, "
Pragma Restrictions and Pragma Profile".
Legality Rules
4/3 {AI95-00249-01} {AI05-0246-1} The profile_identifier Ravenscar is a usage
profile (see 13.12). For usage profile Ravenscar, there shall be no
profile_pragma_argument_associations.
Static Semantics
5/3 {AI95-00249-01} {AI05-0246-1} The usage profile Ravenscar is equivalent to
the following set of pragmas:
6/4 {AI95-00249-01} {AI95-00297-01} {AI95-00394-01} {AI05-0171-1}
{AI05-0246-1} {AI12-0055-1} {AI12-0073-1}
pragma Task_Dispatching_Policy (FIFO_Within_Priorities);
pragma Locking_Policy (Ceiling_Locking);
pragma Detect_Blocking;
pragma Restrictions (
No_Abort_Statements,
No_Dynamic_Attachment,
No_Dynamic_CPU_Assignment,
No_Dynamic_Priorities,
No_Implicit_Heap_Allocations,
No_Local_Protected_Objects,
No_Local_Timing_Events,
No_Protected_Type_Allocators,
No_Relative_Delay,
No_Requeue_Statements,
No_Select_Statements,
No_Specific_Termination_Handlers,
No_Task_Allocators,
No_Task_Hierarchy,
No_Task_Termination,
Simple_Barriers,
Max_Entry_Queue_Length => 1,
Max_Protected_Entries => 1,
Max_Task_Entries => 0,
No_Dependence => Ada.Asynchronous_Task_Control,
No_Dependence => Ada.Calendar,
No_Dependence => Ada.Execution_Time.Group_Budgets,
No_Dependence => Ada.Execution_Time.Timers,
No_Dependence => Ada.Synchronous_Barriers,
No_Dependence => Ada.Task_Attributes,
No_Dependence => System.Multiprocessors.Dispatching_Domains);
6.a/3 Discussion: The Ravenscar profile is named for the location of the
meeting that defined its initial version. The name is now in
widespread use, so we stick with existing practice, rather than
using a more descriptive name.
Paragraph 7 was deleted.
Implementation Requirements
8/4 This paragraph was deleted.{AI05-0171-1} {AI05-0229-1} {AI12-0055-1}
Implementation Advice
9/3 {AI05-0171-1} On a multiprocessor system, an implementation should support
a fully partitioned approach. Each processor should have separate and disjoint
ready queues.
9.a.1/3 Implementation Advice: On a multiprocessor system, each processor
should have a separate and disjoint ready queue.
NOTES
10/3 42 {AI95-00249-01} {AI05-0246-1} The effect of the
Max_Entry_Queue_Length => 1 restriction applies only to protected
entry queues due to the accompanying restriction of Max_Task_Entries
=> 0.
11/4 43 {AI12-0055-1} When the Ravenscar profile is in effect (via the
effect of the No_Dynamic_CPU_Assignment restriction), all of the tasks
in the partition will execute on a single CPU unless the programmer
explicitly uses aspect CPU to specify the CPU assignments for tasks.
The use of multiple CPUs requires care, as many guarantees of single
CPU scheduling no longer apply.
12/4 44 {AI12-0055-1} It is not recommended to specify the CPU of a task
to be Not_A_Specific_CPU when the Ravenscar profile is in effect. How
a partition executes strongly depends on the assignment of tasks to
CPUs.
Extensions to Ada 95
12.a/3 {AI95-00249-01} {AI05-0246-1} The Ravenscar profile is new; it was
moved here by Ada 2012.
Wording Changes from Ada 2005
12.b/3 {AI05-0171-1} How Ravenscar behaves on a multiprocessor system is
now defined.
Incompatibilities With Ada 2012
12.c/4 {AI05-0073-1} Corrigendum: The Ravenscar profile no longer allows
the use of package Synchronous_Barriers, as this package violates
the fundamental Ravenscar requirement that each waiting point can
only block (and release) a single task. This is incompatible with
the published Ada 2012 standard, but it is unlikely that any
existing Ravenscar runtime ever usefully supported barriers.
12.d/4 {AI05-0055-1} Corrigendum:The Ravenscar profile (via the effect of
the new restriction No_Dynamic_CPU_Assignment) no longer allows
setting the CPU aspect of a task to a non-static value. While this
was allowed, an implementation would have had to come up with a
creative interpretation of the Ada 2012 requirement to define the
association of tasks to processors statically. As such, the new
check is more likely to catch bugs than break a working program.
D.14 Execution Time
1/3 {AI95-00307-01} {AI05-0299-1} This subclause describes a language-defined
package to measure execution time.
Static Semantics
2/2 {AI95-00307-01} The following language-defined library package exists:
3/2 with Ada.Task_Identification;
with Ada.Real_Time; use Ada.Real_Time;
package Ada.Execution_Time is
4/2 type CPU_Time is private;
CPU_Time_First : constant CPU_Time;
CPU_Time_Last : constant CPU_Time;
CPU_Time_Unit : constant := implementation-defined-real-number;
CPU_Tick : constant Time_Span;
5/2 function Clock
(T : Ada.Task_Identification.Task_Id
:= Ada.Task_Identification.Current_Task)
return CPU_Time;
6/2 function "+" (Left : CPU_Time; Right : Time_Span) return CPU_Time;
function "+" (Left : Time_Span; Right : CPU_Time) return CPU_Time;
function "-" (Left : CPU_Time; Right : Time_Span) return CPU_Time;
function "-" (Left : CPU_Time; Right : CPU_Time) return Time_Span;
7/2 function "<" (Left, Right : CPU_Time) return Boolean;
function "<=" (Left, Right : CPU_Time) return Boolean;
function ">" (Left, Right : CPU_Time) return Boolean;
function ">=" (Left, Right : CPU_Time) return Boolean;
8/2 procedure Split
(T : in CPU_Time; SC : out Seconds_Count; TS : out Time_Span);
9/2 function Time_Of (SC : Seconds_Count;
TS : Time_Span := Time_Span_Zero) return CPU_Time;
9.1/3 {AI05-0170-1} Interrupt_Clocks_Supported
: constant Boolean := implementation-defined;
9.2/3 {AI05-0170-1} Separate_Interrupt_Clocks_Supported
: constant Boolean :=
implementation-defined;
9.3/3 {AI05-0170-1} function Clock_For_Interrupts return CPU_Time;
10/2 private
... -- not specified by the language
end Ada.Execution_Time;
11/3 {AI95-00307-01} {AI05-0170-1} {AI05-0269-1} The execution time or CPU
time of a given task is defined as the time spent by the system executing that
task, including the time spent executing run-time or system services on its
behalf. The mechanism used to measure execution time is implementation
defined. The Boolean constant Interrupt_Clocks_Supported is set to True if the
implementation separately accounts for the execution time of interrupt
handlers. If it is set to False it is implementation defined which task, if
any, is charged the execution time that is consumed by interrupt handlers. The
Boolean constant Separate_Interrupt_Clocks_Supported is set to True if the
implementation separately accounts for the execution time of individual
interrupt handlers (see D.14.3).
11.a/2 Discussion: The implementation-defined properties above and of the
values declared in the package are repeated in
Documentation Requirements, so we don't mark them as
implementation-defined.
12/2 {AI95-00307-01} The type CPU_Time represents the execution time of a
task. The set of values of this type corresponds one-to-one with an
implementation-defined range of mathematical integers.
13/2 {AI95-00307-01} The CPU_Time value I represents the half-open
execution-time interval that starts with I*CPU_Time_Unit and is limited by
(I+1)*CPU_Time_Unit, where CPU_Time_Unit is an implementation-defined real
number. For each task, the execution time value is set to zero at the creation
of the task.
13.a/2 Ramification: Since it is implementation-defined which task is
charged execution time for system services, the execution time
value may become nonzero even before the start of the activation
of the task.
14/2 {AI95-00307-01} CPU_Time_First and CPU_Time_Last are the smallest and
largest values of the CPU_Time type, respectively.
14.1/3 {AI05-0170-1} The execution time value for the function
Clock_For_Interrupts is initialized to zero.
Dynamic Semantics
15/2 {AI95-00307-01} CPU_Time_Unit is the smallest amount of execution time
representable by the CPU_Time type; it is expressed in seconds. A CPU clock
tick is an execution time interval during which the clock value (as observed
by calling the Clock function) remains constant. CPU_Tick is the average
length of such intervals.
16/2 {AI95-00307-01} The effects of the operators on CPU_Time and Time_Span
are as for the operators defined for integer types.
17/2 {AI95-00307-01} The function Clock returns the current execution time of
the task identified by T; Tasking_Error is raised if that task has terminated;
Program_Error is raised if the value of T is Task_Identification.Null_Task_Id.
18/2 {AI95-00307-01} The effects of the Split and Time_Of operations are
defined as follows, treating values of type CPU_Time, Time_Span, and
Seconds_Count as mathematical integers. The effect of Split (T, SC, TS) is to
set SC and TS to values such that T*CPU_Time_Unit = SC*1.0 + TS*CPU_Time_Unit,
and 0.0 <= TS*CPU_Time_Unit < 1.0. The value returned by Time_Of(SC,TS) is the
execution-time value T such that T*CPU_Time_Unit=SC*1.0 + TS*CPU_Time_Unit.
18.1/3 {AI05-0170-1} The function Clock_For_Interrupts returns the total
cumulative time spent executing within all interrupt handlers. This time is
not allocated to any task execution time clock. If Interrupt_Clocks_Supported
is set to False the function raises Program_Error.
Erroneous Execution
19/2 {AI95-00307-01} For a call of Clock, if the task identified by T no
longer exists, the execution of the program is erroneous.
Implementation Requirements
20/2 {AI95-00307-01} The range of CPU_Time values shall be sufficient to
uniquely represent the range of execution times from the task start-up to 50
years of execution time later. CPU_Tick shall be no greater than 1
millisecond.
Documentation Requirements
21/2 {AI95-00307-01} The implementation shall document the values of
CPU_Time_First, CPU_Time_Last, CPU_Time_Unit, and CPU_Tick.
21.a/2 Documentation Requirement: The values of CPU_Time_First,
CPU_Time_Last, CPU_Time_Unit, and CPU_Tick of package
Execution_Time.
22/2 {AI95-00307-01} The implementation shall document the properties of the
underlying mechanism used to measure execution times, such as the range of
values supported and any relevant aspects of the underlying hardware or
operating system facilities used.
22.a/3 Documentation Requirement: The properties of the mechanism used to
implement package Execution_Time, including the values of the
constants defined in the package.
Metrics
23/2 {AI95-00307-01} The implementation shall document the following metrics:
24/2 * An upper bound on the execution-time duration of a clock tick. This
is a value D such that if t1 and t2 are any execution times of a given
task such that t1 < t2 and Clock(t1) = Clock(t2) then t2 - t1 <= D.
25/2 * An upper bound on the size of a clock jump. A clock jump is the
difference between two successive distinct values of an execution-time
clock (as observed by calling the Clock function with the same
Task_Id).
26/2 * An upper bound on the execution time of a call to the Clock function,
in processor clock cycles.
27/2 * Upper bounds on the execution times of the operators of the type
CPU_Time, in processor clock cycles.
27.a/2 Documentation Requirement: The metrics for execution time.
Implementation Permissions
28/2 {AI95-00307-01} Implementations targeted to machines with word size
smaller than 32 bits need not support the full range and granularity of the
CPU_Time type.
Implementation Advice
29/2 {AI95-00307-01} When appropriate, implementations should provide
configuration mechanisms to change the value of CPU_Tick.
29.a/2 Implementation Advice: When appropriate, implementations should
provide configuration mechanisms to change the value of
Execution_Time.CPU_Tick.
Extensions to Ada 95
29.b/2 {AI95-00307-01} The package Execution_Time is new.
Incompatibilities With Ada 2005
29.c/3 {AI05-0170-1} Function Clock_For_Interrupts, and constants
Interrupt_Clocks_Supported and Separate_Interrupt_Clocks_Supported
are added to Execution_Time. If Execution_Time is referenced in a
use_clause, and an entity E with a defining_identifier of one of
the added entities is defined in a package that is also referenced
in a use_clause, the entity E may no longer be use-visible,
resulting in errors. This should be rare and is easily fixed if it
does occur.
Wording Changes from Ada 2005
29.d/3 {AI05-0170-1} If Interrupt_Clocks_Supported is True, it is now
possible to determine the execution time of interrupt handlers.
This is not an inconsistency, as not charging any task for such
time was a legitimate implementation for Ada 2005.
D.14.1 Execution Time Timers
1/3 {AI95-00307-01} {AI05-0299-1} This subclause describes a language-defined
package that provides a facility for calling a handler when a task has used a
defined amount of CPU time.
Static Semantics
2/2 {AI95-00307-01} The following language-defined library package exists:
3/2 with System;
package Ada.Execution_Time.Timers is
4/2 type Timer (T : not null access constant
Ada.Task_Identification.Task_Id) is
tagged limited private;
5/2 type Timer_Handler is
access protected procedure (TM : in out Timer);
6/2 Min_Handler_Ceiling : constant System.Any_Priority :=
implementation-defined;
7/2 procedure Set_Handler (TM : in out Timer;
In_Time : in Time_Span;
Handler : in Timer_Handler);
procedure Set_Handler (TM : in out Timer;
At_Time : in CPU_Time;
Handler : in Timer_Handler);
function Current_Handler (TM : Timer) return Timer_Handler;
procedure Cancel_Handler (TM : in out Timer;
Cancelled : out Boolean);
8/2 function Time_Remaining (TM : Timer) return Time_Span;
9/2 Timer_Resource_Error : exception;
10/2 private
... -- not specified by the language
end Ada.Execution_Time.Timers;
11/2 {AI95-00307-01} The type Timer represents an execution-time event for a
single task and is capable of detecting execution-time overruns. The access
discriminant T identifies the task concerned. The type Timer needs
finalization (see 7.6).
12/2 {AI95-00307-01} An object of type Timer is said to be set if it is
associated with a nonnull value of type Timer_Handler and cleared otherwise.
All Timer objects are initially cleared.
13/2 {AI95-00307-01} The type Timer_Handler identifies a protected procedure
to be executed by the implementation when the timer expires. Such a protected
procedure is called a handler.
13.a/2 Discussion: Type Timer is tagged. This makes it possible to share
a handler between several events. In simple cases, 'Access can be
used to compare the parameter with a specific timer object (this
works because a tagged type is a by-reference type). In more
complex cases, a type extension of type Timer can be declared; a
double type conversion can be used to access the extension data.
An example of how this can be done can be found for the similar
type Timing_Event, see D.15.
Dynamic Semantics
14/2 {AI95-00307-01} When a Timer object is created, or upon the first call of
a Set_Handler procedure with the timer as parameter, the resources required to
operate an execution-time timer based on the associated execution-time clock
are allocated and initialized. If this operation would exceed the available
resources, Timer_Resource_Error is raised.
15/3 {AI95-00307-01} {AI05-0264-1} The procedures Set_Handler associate the
handler Handler with the timer TM: if Handler is null, the timer is cleared;
otherwise, it is set. The first procedure Set_Handler loads the timer TM with
an interval specified by the Time_Span parameter. In this mode, the timer TM
expires when the execution time of the task identified by TM.T.all has
increased by In_Time; if In_Time is less than or equal to zero, the timer
expires immediately. The second procedure Set_Handler loads the timer TM with
the absolute value specified by At_Time. In this mode, the timer TM expires
when the execution time of the task identified by TM.T.all reaches At_Time; if
the value of At_Time has already been reached when Set_Handler is called, the
timer expires immediately.
15.a/2 Implementation Note: Since an access-to-constant can designate a
variable, the Task_Id value designated by the discriminant of a
Timer object can be changed after the object is created. Thus, an
implementation cannot use the value of the Task_Id other than
where this Standard specifies. For instance, the Task_Id should be
read when the timer is set, but it should not be used when the
timer expires (as it may designate a different task at that
point).
16/2 {AI95-00307-01} A call of a procedure Set_Handler for a timer that is
already set replaces the handler and the (absolute or relative) execution
time; if Handler is not null, the timer remains set.
17/2 {AI95-00307-01} When a timer expires, the associated handler is executed,
passing the timer as parameter. The initial action of the execution of the
handler is to clear the event.
18/3 {AI95-00307-01} {AI05-0264-1} The function Current_Handler returns the
handler associated with the timer TM if that timer is set; otherwise, it
returns null.
19/3 {AI95-00307-01} {AI05-0264-1} The procedure Cancel_Handler clears the
timer if it is set. Cancelled is assigned True if the timer was set prior to
it being cleared; otherwise, it is assigned False.
20/3 {AI95-00307-01} {AI05-0264-1} The function Time_Remaining returns the
execution time interval that remains until the timer TM would expire, if that
timer is set; otherwise, it returns Time_Span_Zero.
21/2 {AI95-00307-01} The constant Min_Handler_Ceiling is the minimum ceiling
priority required for a protected object with a handler to ensure that no
ceiling violation will occur when that handler is invoked.
22/2 {AI95-00307-01} As part of the finalization of an object of type Timer,
the timer is cleared.
23/2 {AI95-00307-01} For all the subprograms defined in this package,
Tasking_Error is raised if the task identified by TM.T.all has terminated, and
Program_Error is raised if the value of TM.T.all is
Task_Identification.Null_Task_Id.
24/2 {AI95-00307-01} An exception propagated from a handler invoked as part of
the expiration of a timer has no effect.
Erroneous Execution
25/2 {AI95-00307-01} For a call of any of the subprograms defined in this
package, if the task identified by TM.T.all no longer exists, the execution of
the program is erroneous.
Implementation Requirements
26/2 {AI95-00307-01} For a given Timer object, the implementation shall
perform the operations declared in this package atomically with respect to any
of these operations on the same Timer object. The replacement of a handler by
a call of Set_Handler shall be performed atomically with respect to the
execution of the handler.
26.a/2 Reason: This prevents various race conditions. In particular it
ensures that if an event occurs when Set_Handler is changing the
handler then either the new or old handler is executed in response
to the appropriate event. It is never possible for a new handler
to be executed in response to an old event
27/2 {AI95-00307-01} When an object of type Timer is finalized, the system
resources used by the timer shall be deallocated.
Implementation Permissions
28/3 {AI95-00307-01} {AI05-0264-1} Implementations may limit the number of
timers that can be defined for each task. If this limit is exceeded, then
Timer_Resource_Error is raised.
NOTES
29/2 45 {AI95-00307-01} A Timer_Handler can be associated with several
Timer objects.
Extensions to Ada 95
29.a/2 {AI95-00307-01} The package Execution_Time.Timers is new.
D.14.2 Group Execution Time Budgets
1/3 {AI95-00354-01} {AI05-0299-1} This subclause describes a language-defined
package to assign execution time budgets to groups of tasks.
Static Semantics
2/2 {AI95-00354-01} The following language-defined library package exists:
3/3 {AI05-0169-1} with System;
with System.Multiprocessors;
package Ada.Execution_Time.Group_Budgets is
4/3 {AI05-0092-1} {AI05-0169-1} type Group_Budget
(CPU : System.Multiprocessors.CPU :=
System.Multiprocessors.CPU'First)
is tagged limited private;
5/2 type Group_Budget_Handler is access
protected procedure (GB : in out Group_Budget);
6/2 type Task_Array is array (Positive range <>) of
Ada.Task_Identification.Task_Id;
7/2 Min_Handler_Ceiling : constant System.Any_Priority :=
implementation-defined;
7.a.1/3 Implementation defined: The value of Min_Handler_Ceiling in
Execution_Time.Group_Budgets.
8/2 procedure Add_Task (GB : in out Group_Budget;
T : in Ada.Task_Identification.Task_Id);
procedure Remove_Task (GB: in out Group_Budget;
T : in Ada.Task_Identification.Task_Id);
function Is_Member (GB : Group_Budget;
T : Ada.Task_Identification.Task_Id) return Boolean;
function Is_A_Group_Member
(T : Ada.Task_Identification.Task_Id) return Boolean;
function Members (GB : Group_Budget) return Task_Array;
9/2 procedure Replenish (GB : in out Group_Budget; To : in Time_Span);
procedure Add (GB : in out Group_Budget; Interval : in Time_Span);
function Budget_Has_Expired (GB : Group_Budget) return Boolean;
function Budget_Remaining (GB : Group_Budget) return Time_Span;
10/2 procedure Set_Handler (GB : in out Group_Budget;
Handler : in Group_Budget_Handler);
function Current_Handler (GB : Group_Budget)
return Group_Budget_Handler;
procedure Cancel_Handler (GB : in out Group_Budget;
Cancelled : out Boolean);
11/2 Group_Budget_Error : exception;
12/2 private
-- not specified by the language
end Ada.Execution_Time.Group_Budgets;
13/2 {AI95-00354-01} The type Group_Budget represents an execution time budget
to be used by a group of tasks. The type Group_Budget needs finalization (see
7.6). A task can belong to at most one group. Tasks of any priority can be
added to a group.
14/2 {AI95-00354-01} An object of type Group_Budget has an associated
nonnegative value of type Time_Span known as its budget, which is initially
Time_Span_Zero. The type Group_Budget_Handler identifies a protected procedure
to be executed by the implementation when the budget is exhausted, that is,
reaches zero. Such a protected procedure is called a handler.
15/2 {AI95-00354-01} An object of type Group_Budget also includes a handler,
which is a value of type Group_Budget_Handler. The handler of the object is
said to be set if it is not null and cleared otherwise. The handler of all
Group_Budget objects is initially cleared.
15.a/2 Discussion: Type Group_Budget is tagged. This makes it possible to
share a handler between several events. In simple cases, 'Access
can be used to compare the parameter with a specific group budget
object (this works because a tagged type is a by-reference type).
In more complex cases, a type extension of type Group_Budget can
be declared; a double type conversion can be used to access the
extension data. An example of how this can be done can be found
for the similar type Timing_Event, see D.15.
Dynamic Semantics
16/2 {AI95-00354-01} The procedure Add_Task adds the task identified by T to
the group GB; if that task is already a member of some other group,
Group_Budget_Error is raised.
17/2 {AI95-00354-01} The procedure Remove_Task removes the task identified by
T from the group GB; if that task is not a member of the group GB,
Group_Budget_Error is raised. After successful execution of this procedure,
the task is no longer a member of any group.
18/3 {AI95-00354-01} {AI05-0264-1} The function Is_Member returns True if the
task identified by T is a member of the group GB; otherwise, it returns False.
19/3 {AI95-00354-01} {AI05-0264-1} The function Is_A_Group_Member returns True
if the task identified by T is a member of some group; otherwise, it returns
False.
20/2 {AI95-00354-01} The function Members returns an array of values of type
Task_Identification.Task_Id identifying the members of the group GB. The order
of the components of the array is unspecified.
21/3 {AI95-00354-01} {AI05-0092-1} {AI05-0169-1} The procedure Replenish loads
the group budget GB with To as the Time_Span value. The exception
Group_Budget_Error is raised if the Time_Span value To is nonpositive. Any
execution on CPU of any member of the group of tasks results in the budget
counting down, unless exhausted. When the budget becomes exhausted (reaches
Time_Span_Zero), the associated handler is executed if the handler of group
budget GB is set. Nevertheless, the tasks continue to execute.
22/2 {AI95-00354-01} The procedure Add modifies the budget of the group GB. A
positive value for Interval increases the budget. A negative value for
Interval reduces the budget, but never below Time_Span_Zero. A zero value for
Interval has no effect. A call of procedure Add that results in the value of
the budget going to Time_Span_Zero causes the associated handler to be
executed if the handler of the group budget GB is set.
23/3 {AI95-00354-01} {AI05-0264-1} The function Budget_Has_Expired returns
True if the budget of group GB is exhausted (equal to Time_Span_Zero);
otherwise, it returns False.
24/2 {AI95-00354-01} The function Budget_Remaining returns the remaining
budget for the group GB. If the budget is exhausted it returns Time_Span_Zero.
This is the minimum value for a budget.
25/3 {AI95-00354-01} {AI05-0264-1} The procedure Set_Handler associates the
handler Handler with the Group_Budget GB: if Handler is null, the handler of
Group_Budget is cleared; otherwise, it is set.
26/2 {AI95-00354-01} A call of Set_Handler for a Group_Budget that already has
a handler set replaces the handler; if Handler is not null, the handler for
Group_Budget remains set.
27/3 {AI95-00354-01} {AI05-0264-1} The function Current_Handler returns the
handler associated with the group budget GB if the handler for that group
budget is set; otherwise, it returns null.
28/3 {AI95-00354-01} {AI05-0264-1} The procedure Cancel_Handler clears the
handler for the group budget if it is set. Cancelled is assigned True if the
handler for the group budget was set prior to it being cleared; otherwise, it
is assigned False.
29/2 {AI95-00354-01} The constant Min_Handler_Ceiling is the minimum ceiling
priority required for a protected object with a handler to ensure that no
ceiling violation will occur when that handler is invoked.
30/2 {AI95-00354-01} The precision of the accounting of task execution time to
a Group_Budget is the same as that defined for execution-time clocks from the
parent package.
31/2 {AI95-00354-01} As part of the finalization of an object of type
Group_Budget all member tasks are removed from the group identified by that
object.
32/3 {AI95-00354-01} {AI05-0264-1} If a task is a member of a Group_Budget
when it terminates, then as part of the finalization of the task it is removed
from the group.
33/2 {AI95-00354-01} For all the operations defined in this package,
Tasking_Error is raised if the task identified by T has terminated, and
Program_Error is raised if the value of T is Task_Identification.Null_Task_Id.
34/2 {AI95-00354-01} An exception propagated from a handler invoked when the
budget of a group of tasks becomes exhausted has no effect.
Erroneous Execution
35/2 {AI95-00354-01} For a call of any of the subprograms defined in this
package, if the task identified by T no longer exists, the execution of the
program is erroneous.
Implementation Requirements
36/2 {AI95-00354-01} For a given Group_Budget object, the implementation shall
perform the operations declared in this package atomically with respect to any
of these operations on the same Group_Budget object. The replacement of a
handler, by a call of Set_Handler, shall be performed atomically with respect
to the execution of the handler.
36.a/2 Reason: This prevents various race conditions. In particular it
ensures that if the budget is exhausted when Set_Handler is
changing the handler then either the new or old handler is
executed and the exhausting event is not lost.
NOTES
37/2 46 {AI95-00354-01} Clearing or setting of the handler of a group
budget does not change the current value of the budget. Exhaustion or
loading of a budget does not change whether the handler of the group
budget is set or cleared.
38/2 47 {AI95-00354-01} A Group_Budget_Handler can be associated with
several Group_Budget objects.
Extensions to Ada 95
38.a/2 {AI95-00354-01} The package Execution_Time.Group_Budgets is new.
Inconsistencies With Ada 2005
38.b/3 {AI05-0169-1} A Group_Budget is now defined to work on a single
processor. If an implementation managed to make this package work
for programs running on a multiprocessor system, and a program
depends on that fact, it could fail when ported to Ada 2012. We
believe it is unlikely that such an implementation exists because
of the difficulty of signalling other processors when the time
reaches zero; in any case, depending on such an implementation is
not portable.
D.14.3 Execution Time of Interrupt Handlers
1/3 {AI05-0170-1} {AI05-0299-1} This subclause describes a language-defined
package to measure the execution time of interrupt handlers.
Static Semantics
2/3 {AI05-0170-1} The following language-defined library package exists:
3/3 with Ada.Interrupts;
package Ada.Execution_Time.Interrupts is
function Clock (Interrupt : Ada.Interrupts.Interrupt_Id)
return CPU_Time;
function Supported (Interrupt : Ada.Interrupts.Interrupt_Id)
return Boolean;
end Ada.Execution_Time.Interrupts;
4/3 {AI05-0170-1} The execution time or CPU time of a given interrupt
Interrupt is defined as the time spent by the system executing interrupt
handlers identified by Interrupt, including the time spent executing run-time
or system services on its behalf. The mechanism used to measure execution time
is implementation defined. Time spent executing interrupt handlers is distinct
from time spent executing any task.
4.a/3 Discussion: The implementation-defined mechanism here is the same
as that covered by the Documentation Requirements of D.14, so we
don't repeat that requirement here.
5/3 {AI05-0170-1} For each interrupt, the execution time value is initially
set to zero.
Dynamic Semantics
6/3 {AI05-0170-1} The function Clock returns the current cumulative execution
time of the interrupt identified by Interrupt. If
Separate_Interrupt_Clocks_Supported is set to False the function raises
Program_Error.
7/3 {AI05-0170-1} {AI05-0264-1} The function Supported returns True if the
implementation is monitoring the execution time of the interrupt identified by
Interrupt; otherwise, it returns False. For any Interrupt_Id Interrupt for
which Supported(Interrupt) returns False, the function Clock(Interrupt) will
return a value equal to Ada.Execution_Time.Time_Of(0).
Extensions to Ada 2005
7.a/3 {AI05-0170-1} The package Execution_Time.Interrupts is new.
D.15 Timing Events
1/3 {AI95-00297-01} {AI05-0299-1} This subclause describes a language-defined
package to allow user-defined protected procedures to be executed at a
specified time without the need for a task or a delay statement.
Static Semantics
2/2 {AI95-00297-01} The following language-defined library package exists:
3/2 package Ada.Real_Time.Timing_Events is
4/2 type Timing_Event is tagged limited private;
type Timing_Event_Handler
is access protected procedure (Event : in out Timing_Event);
5/2 procedure Set_Handler (Event : in out Timing_Event;
At_Time : in Time;
Handler : in Timing_Event_Handler);
procedure Set_Handler (Event : in out Timing_Event;
In_Time : in Time_Span;
Handler : in Timing_Event_Handler);
function Current_Handler (Event : Timing_Event)
return Timing_Event_Handler;
procedure Cancel_Handler (Event : in out Timing_Event;
Cancelled : out Boolean);
6/2 function Time_Of_Event (Event : Timing_Event) return Time;
7/2 private
... -- not specified by the language
end Ada.Real_Time.Timing_Events;
8/2 {AI95-00297-01} The type Timing_Event represents a time in the future when
an event is to occur. The type Timing_Event needs finalization (see 7.6).
9/2 {AI95-00297-01} An object of type Timing_Event is said to be set if it is
associated with a nonnull value of type Timing_Event_Handler and cleared
otherwise. All Timing_Event objects are initially cleared.
10/2 {AI95-00297-01} The type Timing_Event_Handler identifies a protected
procedure to be executed by the implementation when the timing event occurs.
Such a protected procedure is called a handler.
10.a/2 Discussion: Type Timing_Event is tagged. This makes it possible to
share a handler between several events. In simple cases, 'Access
can be used to compare the parameter with a specific timing event
object (this works because a tagged type is a by-reference type).
In more complex cases, a type extension of type Timing_Event can
be declared; a double type conversion can be used to access the
extension data. For example:
10.b/2 type Toaster_Timing_Event is new Timing_Event with record
Slot : Natural;
end record;
10.c/2 ...
10.d/2 protected body Toaster is
10.e/2 procedure Timer (Event : in out Timing_Event) is
begin
Pop_Up_Toast (Toaster_Timing_Event(Timing_Event'Class(Event)).Slot);
end Timer;
10.f/2 ...
end Toaster;
10.g/2 The extra conversion to the class-wide type is necessary to make
the conversions legal. While this usage is clearly ugly, we think
that the need for this sort of usage will be rare, so we can live
with it. It's certainly better than having no way to associate
data with an event.
Dynamic Semantics
11/3 {AI95-00297-01} {AI05-0264-1} The procedures Set_Handler associate the
handler Handler with the event Event: if Handler is null, the event is
cleared; otherwise, it is set. The first procedure Set_Handler sets the
execution time for the event to be At_Time. The second procedure Set_Handler
sets the execution time for the event to be Real_Time.Clock + In_Time.
12/2 {AI95-00297-01} A call of a procedure Set_Handler for an event that is
already set replaces the handler and the time of execution; if Handler is not
null, the event remains set.
13/2 {AI95-00297-01} As soon as possible after the time set for the event, the
handler is executed, passing the event as parameter. The handler is only
executed if the timing event is in the set state at the time of execution. The
initial action of the execution of the handler is to clear the event.
13.a/2 Reason: The second sentence of this paragraph is because of a
potential race condition. The time might expire and yet before the
handler is executed, some task could call Cancel_Handler (or
equivalently call Set_Handler with a null parameter) and thus
clear the handler.
14/2 {AI95-00297-01} If the Ceiling_Locking policy (see D.3) is in effect when
a procedure Set_Handler is called, a check is made that the ceiling priority
of Handler.all is Interrupt_Priority'Last. If the check fails, Program_Error
is raised.
15/3 {AI95-00297-01} {AI05-0094-1} {AI05-0264-1} If a procedure Set_Handler is
called with zero or negative In_Time or with At_Time indicating a time in the
past, then the handler is executed as soon as possible after the completion of
the call of Set_Handler.
15.a/3 Ramification: {AI05-0094-1} The handler will still be executed.
Under no circumstances is a scheduled call of a handler lost.
15.b/3 Discussion: {AI05-0094-1} We say "as soon as possible" so that we
do not deadlock if we are executing the handler when Set_Handler
is called. In that case, the current invocation of the handler
must complete before the new handler can start executing.
16/3 {AI95-00297-01} {AI05-0264-1} The function Current_Handler returns the
handler associated with the event Event if that event is set; otherwise, it
returns null.
17/3 {AI95-00297-01} {AI05-0264-1} The procedure Cancel_Handler clears the
event if it is set. Cancelled is assigned True if the event was set prior to
it being cleared; otherwise, it is assigned False.
18/3 {AI95-00297-01} {AI05-0264-1} The function Time_Of_Event returns the time
of the event if the event is set; otherwise, it returns Real_Time.Time_First.
19/2 {AI95-00297-01} As part of the finalization of an object of type
Timing_Event, the Timing_Event is cleared.
19.a/2 Implementation Note: This is the only finalization defined by the
language that has a visible effect; but an implementation may have
other finalization that it needs to perform. Implementations need
to ensure that the event is cleared before anything else is
finalized that would prevent a set event from being triggered.
20/2 {AI95-00297-01} If several timing events are set for the same time, they
are executed in FIFO order of being set.
21/2 {AI95-00297-01} An exception propagated from a handler invoked by a
timing event has no effect.
Implementation Requirements
22/2 {AI95-00297-01} For a given Timing_Event object, the implementation shall
perform the operations declared in this package atomically with respect to any
of these operations on the same Timing_Event object. The replacement of a
handler by a call of Set_Handler shall be performed atomically with respect to
the execution of the handler.
22.a/2 Reason: This prevents various race conditions. In particular it
ensures that if an event occurs when Set_Handler is changing the
handler then either the new or old handler is executed in response
to the appropriate event. It is never possible for a new handler
to be executed in response to an old event.
Metrics
23/2 {AI95-00297-01} The implementation shall document the following metric:
24/3 * {AI05-0210-1} An upper bound on the lateness of the execution of a
handler. That is, the maximum time between the time specified for the
event and when a handler is actually invoked assuming no other handler
or task is executing during this interval.
24.a/2 Documentation Requirement: The metrics for timing events.
Implementation Advice
25/2 {AI95-00297-01} The protected handler procedure should be executed
directly by the real-time clock interrupt mechanism.
25.a/2 Implementation Advice: For a timing event, the handler should be
executed directly by the real-time clock interrupt mechanism.
NOTES
26/2 48 {AI95-00297-01} Since a call of Set_Handler is not a potentially
blocking operation, it can be called from within a handler.
27/2 49 {AI95-00297-01} A Timing_Event_Handler can be associated with
several Timing_Event objects.
Extensions to Ada 95
27.a/2 {AI95-00297-01} The package Real_Time.Timing_Events is new.
Wording Changes from Ada 2005
27.b/3 {AI05-0094-1} Correction: Reworded to eliminate a deadlock
condition if the event time is in the past and a handler is
currently executing. This is technically an inconsistency, but
only if a program is depending on deadlocking; since it is
impossible to imagine how that could be useful, we have not
documented this as an inconsistency.
27.c/3 {AI05-0210-1} Correction: Clarified the metric for lateness of a
timing event to exclude interference from other handlers and
tasks. This change might change the documentation of an
implementation, but not the implementation itself, so there is no
inconsistency.
D.16 Multiprocessor Implementation
1/3 {AI05-0171-1} {AI05-0299-1} This subclause allows implementations on
multiprocessor platforms to be configured.
Static Semantics
2/3 {AI05-0171-1} The following language-defined library package exists:
3/3 package System.Multiprocessors is
pragma Preelaborate(Multiprocessors);
4/3 type CPU_Range is range 0 .. implementation-defined;
Not_A_Specific_CPU : constant CPU_Range := 0;
subtype CPU is CPU_Range range 1 .. CPU_Range'Last;
4.a.1/3 Implementation defined: The value of CPU_Range'Last in
System.Multiprocessors.
5/3 function Number_Of_CPUs return CPU;
end System.Multiprocessors;
6/3 {AI05-0171-1} A call of Number_Of_CPUs returns the number of processors
available to the program. Within a given partition, each call on
Number_Of_CPUs will return the same value.
7/3 {AI05-0229-1} For a task type (including the anonymous type of a
single_task_declaration) or subprogram, the following language-defined
representation aspect may be specified:
8/3 CPU The aspect CPU is an expression, which shall be of type
System.Multiprocessors.CPU_Range.
8.a/3 Aspect Description for CPU: Processor on which a given task should
run.
Legality Rules
9/3 {AI05-0171-1} {AI05-0229-1} If the CPU aspect is specified for a
subprogram, the expression shall be static.
10/3 {AI05-0229-1} The CPU aspect shall not be specified on a task interface
type.
Dynamic Semantics
11/4 {AI05-0171-1} {AI05-0229-1} {AI12-0081-1} The expression specified for
the CPU aspect of a task type is evaluated each time an object of the task
type is created (see 9.1). The CPU value is then associated with the task
object.
12/3 {AI05-0171-1} {AI05-0229-1} The CPU aspect has no effect if it is
specified for a subprogram other than the main subprogram; the CPU value is
not associated with any task.
13/3 {AI05-0171-1} {AI05-0229-1} The CPU value is associated with the
environment task if the CPU aspect is specified for the main subprogram. If
the CPU aspect is not specified for the main subprogram it is implementation
defined on which processor the environment task executes.
13.a.1/3 Implementation defined: The processor on which the environment
task executes in the absence of a value for the aspect CPU.
14/3 {AI05-0171-1} {AI05-0264-1} The CPU value determines the processor on
which the task will activate and execute; the task is said to be assigned to
that processor. If the CPU value is Not_A_Specific_CPU, then the task is not
assigned to a processor. A task without a CPU aspect specified will activate
and execute on the same processor as its activating task if the activating
task is assigned a processor. If the CPU value is not in the range of
System.Multiprocessors.CPU_Range or is greater than Number_Of_CPUs the task is
defined to have failed, and it becomes a completed task (see 9.2).
Extensions to Ada 2005
14.a/3 {AI05-0171-1} {AI05-0229-1} The package System.Multiprocessors and
the CPU aspect are new.
Wording Changes from Ada 2012
14.b/4 {AI12-0081-1} Corrigendum: Clarified when the CPU aspect
expression is evaluated.
D.16.1 Multiprocessor Dispatching Domains
1/3 {AI05-0167-1} {AI05-0299-1} This subclause allows implementations on
multiprocessor platforms to be partitioned into distinct dispatching domains
during program startup.
Static Semantics
2/3 {AI05-0167-1} The following language-defined library package exists:
3/3 with Ada.Real_Time;
with Ada.Task_Identification;
package System.Multiprocessors.Dispatching_Domains is
4/3 Dispatching_Domain_Error : exception;
5/3 type Dispatching_Domain (<>) is limited private;
6/3 System_Dispatching_Domain : constant Dispatching_Domain;
7/4 {AI12-0033-1} function Create
(First : CPU; Last : CPU_Range) return Dispatching_Domain;
8/3 function Get_First_CPU (Domain : Dispatching_Domain) return CPU;
9/4 {AI12-0033-1} function Get_Last_CPU
(Domain : Dispatching_Domain) return CPU_Range;
9.1/4 {AI12-0033-1} type CPU_Set is array(CPU range <>) of Boolean;
9.2/4 {AI12-0033-1} function Create
(Set : CPU_Set) return Dispatching_Domain;
9.3/4 {AI12-0033-1} function Get_CPU_Set
(Domain : Dispatching_Domain) return CPU_Set;
10/3 function Get_Dispatching_Domain
(T : Ada.Task_Identification.Task_Id :=
Ada.Task_Identification.Current_Task)
return Dispatching_Domain;
11/3 procedure Assign_Task
(Domain : in out Dispatching_Domain;
CPU : in CPU_Range := Not_A_Specific_CPU;
T : in Ada.Task_Identification.Task_Id :=
Ada.Task_Identification.Current_Task);
12/3 procedure Set_CPU
(CPU : in CPU_Range;
T : in Ada.Task_Identification.Task_Id :=
Ada.Task_Identification.Current_Task);
13/3 function Get_CPU
(T : Ada.Task_Identification.Task_Id :=
Ada.Task_Identification.Current_Task)
return CPU_Range;
14/3 procedure Delay_Until_And_Set_CPU
(Delay_Until_Time : in Ada.Real_Time.Time; CPU : in CPU_Range);
15/3 private
... -- not specified by the language
end System.Multiprocessors.Dispatching_Domains;
16/4 {AI05-0167-1} {AI12-0082-1} A dispatching domain represents a set of
processors on which a task may execute. Each processor is contained within
exactly one dispatching domain. An object of type Dispatching_Domain
identifies a dispatching domain. System_Dispatching_Domain identifies a domain
that contains the processor or processors on which the environment task
executes. At program start-up all processors are contained within this domain.
17/3 {AI05-0167-1} For a task type (including the anonymous type of a
single_task_declaration), the following language-defined representation aspect
may be specified:
18/3 Dispatching_Domain
The value of aspect Dispatching_Domain is an expression, which
shall be of type Dispatching_Domains.Dispatching_Domain. This
aspect is the domain to which the task (or all objects of the
task type) are assigned.
18.a/3 Aspect Description for Dispatching_Domain: Domain (group of
processors) on which a given task should run.
Legality Rules
19/3 {AI05-0167-1} The Dispatching_Domain aspect shall not be specified for a
task interface.
Dynamic Semantics
20/4 {AI05-0167-1} {AI12-0033-1} The expression specified for the
Dispatching_Domain aspect of a task type is evaluated each time an object of
the task type is created (see 9.1). If the identified dispatching domain is
empty, then Dispatching_Domain_Error is raised; otherwise the newly created
task is assigned to the domain identified by the value of the expression.
21/3 {AI05-0167-1} If a task is not explicitly assigned to any domain, it is
assigned to that of the activating task. A task always executes on some CPU in
its domain.
22/4 {AI05-0167-1} {AI12-0082-1} If both the dispatching domain and CPU are
specified for a task, and the CPU value is not contained within the set of
processors for the domain (and is not Not_A_Specific_CPU), the activation of
the task is defined to have failed, and it becomes a completed task (see 9.2).
23/4 {AI05-0167-1} {AI12-0033-1} The function Create with First and Last
parameters creates and returns a dispatching domain containing all the
processors in the range First .. Last. The function Create with a Set
parameter creates and returns a dispatching domain containing the processors
for which Set(I) is True. These processors are removed from
System_Dispatching_Domain. A call of Create will raise
Dispatching_Domain_Error if any designated processor is not currently in
System_Dispatching_Domain, or if the system cannot support a distinct domain
over the processors identified, or if a processor has a task assigned to it,
or if the allocation would leave System_Dispatching_Domain empty. A call of
Create will raise Dispatching_Domain_Error if the calling task is not the
environment task, or if Create is called after the call to the main subprogram.
24/4 {AI05-0167-1} {AI12-0033-1} The function Get_First_CPU returns the first
CPU in Domain, or CPU'First if Domain is empty; Get_Last_CPU returns the last
CPU in Domain, or CPU_Range'First if Domain is empty. The function
Get_CPU_Set(D) returns an array whose low bound is Get_First_CPU(D), whose
high bound is Get_Last_CPU(D), with True values in the Set corresponding to
the CPUs that are in the given Domain.
25/4 {AI05-0167-1} {AI12-0082-1} The function Get_Dispatching_Domain returns
the dispatching domain on which the task is assigned.
26/4 {AI05-0167-1} {AI05-0278-1} {AI12-0033-1} A call of the procedure
Assign_Task assigns task T to the CPU within the dispatching domain Domain.
Task T can now execute only on CPU, unless CPU designates Not_A_Specific_CPU
in which case it can execute on any processor within Domain. The exception
Dispatching_Domain_Error is propagated if Domain is empty, T is already
assigned to a dispatching domain other than System_Dispatching_Domain, or if
CPU is not one of the processors of Domain (and is not Not_A_Specific_CPU). A
call of Assign_Task is a task dispatching point for task T unless T is inside
of a protected action, in which case the effect on task T is delayed until its
next task dispatching point. If T is the Current_Task the effect is immediate
if T is not inside a protected action, otherwise the effect is as soon as
practical. Assigning a task already assigned to System_Dispatching_Domain to
that domain has no effect.
27/4 {AI05-0167-1} {AI05-0278-1} {AI12-0082-1} A call of procedure Set_CPU
assigns task T to the CPU. Task T can now execute only on CPU, unless CPU
designates Not_A_Specific_CPU, in which case it can execute on any processor
within its dispatching domain. The exception Dispatching_Domain_Error is
propagated if CPU is not one of the processors of the dispatching domain on
which T is assigned (and is not Not_A_Specific_CPU). A call of Set_CPU is a
task dispatching point for task T unless T is inside of a protected action, in
which case the effect on task T is delayed until its next task dispatching
point. If T is the Current_Task the effect is immediate if T is not inside a
protected action, otherwise the effect is as soon as practical.
28/3 {AI05-0167-1} The function Get_CPU returns the processor assigned to task
T, or Not_A_Specific_CPU if the task is not assigned to a processor.
29/4 {AI05-0167-1} {AI12-0082-1} A call of Delay_Until_And_Set_CPU delays the
calling task for the designated time and then assigns the task to the
specified processor when the delay expires. The exception
Dispatching_Domain_Error is propagated if P is not one of the processors of
the calling task's dispatching domain (and is not Not_A_Specific_CPU).
Implementation Requirements
30/3 {AI05-0167-1} The implementation shall perform the operations
Assign_Task, Set_CPU, Get_CPU and Delay_Until_And_Set_CPU atomically with
respect to any of these operations on the same dispatching_domain, processor
or task.
30.1/4 {AI12-0048-1} Any task that belongs to the system dispatching domain
can execute on any CPU within that domain, unless the assignment of the task
has been specified.
30.a/4 Reason: This ensures that priorities and deadlines are respected
within the system dispatching domain. There is no such guarantee
between different domains.
30.b/4 We only need to talk about the system dispatching domain here,
because Assign_Task and Set_CPU already have such wording for
tasks that are assigned explicitly to a dispatching domain and
specify Not_a_Specific_CPU.
30.c/4 Ramification: If no dispatching domains are created, all tasks can
execute on all processors. (As always, implementation-defined
dispatching policies may have other rules, so a partition that
does not specify any language-defined dispatching policy may do
anything at all and in particular does not need to follow this
rule.
30.d/4 Discussion: A task can be assigned to a specific CPU by specifying
the aspect CPU for a task, or by calling a dynamic operation like
Set_CPU or Assign_Task.
Implementation Advice
31/3 {AI05-0167-1} Each dispatching domain should have separate and disjoint
ready queues.
31.a/3 Implementation Advice: Each dispatching domain should have
separate and disjoint ready queues.
31.b/4 To be honest: {AI12-0048-1} "Ready queue" here doesn't mean the
conceptual "ready queue" as defined in D.2.1 (one per processor);
this rule is talking about the ready queues used by the
implementation.
Documentation Requirements
32/3 {AI05-0167-1} The implementation shall document the processor(s) on which
the clock interrupt is handled and hence where delay queue and ready queue
manipulations occur. For any Interrupt_Id whose handler can execute on more
than one processor the implementation shall also document this set of
processors.
32.a/3 Documentation Requirement: The processor(s) on which the clock
interrupt is handled; the processors on which each Interrupt_Id
can be handled.
Implementation Permissions
33/3 {AI05-0167-1} An implementation may limit the number of dispatching
domains that can be created and raise Dispatching_Domain_Error if an attempt
is made to exceed this number.
Extensions to Ada 2005
33.a/3 {AI05-0167-1} {AI05-0278-1} The package
System.Multiprocessors.Dispatching_Domains and the aspect
Dispatching_Domains are new.
Inconsistencies With Ada 2012
33.b/4 {AI12-0033-1} Corrigendum: We now explicitly allow empty
dispatching domains, as it would be difficult to avoid declaring
them when a system is configured at runtime. Therefore, assigning
a task to an empty domain now raises Dispatching_Domain_Error;
creating such a domain should not raise Dispatching_Domain_Error.
If an implementation does something different in these cases, and
a program depends on that difference, the program could
malfunction. This seems very unlikely (if no exception is ever
raised, the task assigned to the empty domain could never run; if
the exception is raised earlier, the program can't do anything
useful).
Incompatibilities With Ada 2012
33.c/4 {AI05-0033-1} Corrigendum: The subtypes of the parameter or result
of several routines were changed to support empty domains. These
changes will cause rules requiring subtype conformance to fail on
these routines (such as 'Access). We believe such uses are
unlikely. In addition, type CPU_Set and function Get_CPU_Set,
along with an overloaded Create are newly added to this package.
If Multiprocessors.Dispatching_Domains is referenced in a
use_clause, and an entity E with the same defining_identifier as a
new entity in this package is defined in a package that is also
referenced in a use_clause, the entity E may no longer be
use-visible, resulting in errors. This should be rare and is
easily fixed if it does occur.
Wording Changes from Ada 2012
33.d/4 {AI12-0048-1} Corrigendum: Added wording to clarify that all tasks
can execute on all CPUs of the system dispatching domain by
default.
33.e/4 {AI12-0082-1} Corrigndum: Added a definition to clarify that a
"dispatching domain" is a concept which is identified by an object
of type Dispatching_Domain; more than one object might identify
the same dispatching domain (for instance, the result of function
Get_Dispatching_Domain is a different object but identifies the
same dispatching domain).
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