Annex D
(normative)
Real-Time Systems
1 {real-time systems} {embedded systems} 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 {extensions to Ada 83} This Annex is new to Ada 95.
D.1 Task Priorities
1 [This clause specifies the priority model for real-time systems. In
addition, the methods for specifying priorities are defined.]
Syntax
2 The form of a pragma Priority is as follows:
3 pragma Priority(expression);
4 The form of a pragma Interrupt_Priority is as follows:
5 pragma Interrupt_Priority[(expression)];
Name Resolution Rules
6 {expected type (Priority pragma argument) [partial]}
{expected type (Interrupt_Priority pragma argument) [partial]} The expected
type for the expression in a Priority or Interrupt_Priority pragma is Integer.
Legality Rules
7 A Priority pragma is allowed only immediately within a task_definition, a
protected_definition, or the declarative_part of a subprogram_body. An
Interrupt_Priority pragma is allowed only immediately within a
task_definition or a protected_definition. At most one such pragma shall
appear within a given construct.
8 For a Priority pragma that appears in the declarative_part of a
subprogram_body, 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.
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 The priority specified by a Priority or Interrupt_Priority pragma is the
value of the expression in the pragma, if any. If there is no expression in an
Interrupt_Priority pragma, the priority value is Interrupt_Priority'Last.
Dynamic Semantics
14 A Priority pragma has no effect if it occurs in the declarative_part of
the subprogram_body of a subprogram other than the main subprogram.
15 {task priority} {priority} {priority inheritance} {base priority}
{active priority} 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 The effect of specifying such a pragma in a protected_definition is
discussed in D.3.
17 {creation (of a task object)} The expression in a Priority or
Interrupt_Priority pragma that appears in a task_definition is evaluated for
each task object (see 9.1). For a Priority pragma, the value of the
expression is converted to the subtype Priority; for an Interrupt_Priority
pragma, this value is converted to the subtype Any_Priority. The priority
value is then associated with the task object whose task_definition contains
the pragma. {implicit subtype conversion (pragma Priority) [partial]}
{implicit subtype conversion (pragma Interrupt_Priority) [partial]}
18 Likewise, the priority value is associated with the environment task if
the pragma appears in the declarative_part of the main subprogram.
19 The initial value of a task's base priority is specified by default or by
means of a Priority or Interrupt_Priority pragma. [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 a pragma is the base priority of the task that creates
it at the time of creation (see 9.1). If a pragma Priority does not apply to
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 6 An implementation may provide a non-standard mode in which tasks
inherit priorities under conditions other than those specified above.
29.a Ramification: The use of a Priority or Interrupt_Priority pragma
does not require the package System to be named in a with_clause
for the enclosing compilation_unit.
Extensions to Ada 83
29.b {extensions to Ada 83} 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.
D.2 Priority Scheduling
1/2 {AI95-00321-01} [This clause 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/2 {AI95-00321-01} This introduction is simplified in order to
reflect the rearrangement and expansion of this clause.
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:
1.2/2 package Ada.Dispatching is
pragma Pure(Dispatching);
Dispatching_Policy_Error : exception;
end Ada.Dispatching;
1.3/2 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} {dispatching, task}
{task dispatching point [distributed]} {dispatching point [distributed]} 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} {ready queue} {head (of a queue)} {tail (of a queue)}
{ready task} {task dispatching policy [partial]}
{dispatching policy for tasks [partial]} 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} {running task} 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.
7/2 This paragraph was deleted.{AI95-00321-01}
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 Ramification: 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
a pragma, 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 7 Section 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. {blocked [partial]}
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/2 {AI95-00321-01} 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 may not involve
preemption).
D.2.2 Task Dispatching Pragmas
0.1/2 {AI95-00355-01} [This clause 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
1 The form of a pragma Task_Dispatching_Policy is as follows:
2 pragma Task_Dispatching_Policy(policy_identifier);
2.1/2 {AI95-00355-01} The form of a pragma Priority_Specific_Dispatching is
as follows:
2.2/2 pragma Priority_Specific_Dispatching (
policy_identifier, first_priority_expression,
last_priority_expression);
Name Resolution Rules
2.3/2 {AI95-00355-01} The expected type for first_priority_expression and
last_priority_expression is Integer.
Legality Rules
3/2 {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.
3.a/2 This paragraph was deleted.
3.1/2 {AI95-00355-01} The policy_identifier used in a pragma
Priority_Specific_Dispatching shall be the name of a task dispatching policy.
3.2/2 {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
3.3/2 {AI95-00355-01} Pragma Task_Dispatching_Policy specifies the single task
dispatching policy.
3.4/2 {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.
3.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.
3.5/2 {AI95-00355-01} 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
4/2 {AI95-00355-01} {configuration pragma (Task_Dispatching_Policy)
[partial]} {pragma, configuration (Task_Dispatching_Policy) [partial]} A
Task_Dispatching_Policy pragma is a configuration pragma. A
Priority_Specific_Dispatching pragma is a configuration pragma.
{configuration pragma (Priority_Specific_Dispatching) [partial]}
{pragma, configuration (Priority_Specific_Dispatching) [partial]}
4.1/2 {AI95-00355-01} The priority ranges specified in more than one
Priority_Specific_Dispatching pragma within the same partition shall not be
overlapping.
4.2/2 {AI95-00355-01} If a partition contains one or more
Priority_Specific_Dispatching pragmas it shall not contain a
Task_Dispatching_Policy pragma.
5/2 This paragraph was deleted.{AI95-00333-01}
Dynamic Semantics
6/2 {AI95-00355-01} {task dispatching policy} [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.
6.1/2 {AI95-00355-01} {unspecified [partial]} If neither pragma applies to any
of the program units comprising a partition, the task dispatching policy for
that partition is unspecified.
6.2/2 {AI95-00355-01} 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 non-empty ready
queue for that processor with a higher priority than the priority of the
running task.
6.a/2 Discussion: 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 may not require any preemption. Note that policy
Non_Preemptive_FIFO_Within_Priorities is not allowed in a priority
specific dispatching pragma.
6.3/2 {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.
6.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
13.1/2 {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.
16.a/2 This paragraph was deleted.
Implementation Permissions
17/2 {AI95-00256-01} Implementations are allowed to define other task
dispatching policies, but need not support more than one task dispatching
policy per partition.
18/2 {AI95-00355-01} An implementation need not support pragma
Priority_Specific_Dispatching if it is infeasible to support it in the target
environment.
18.a/2 Implementation defined: Implementation defined task dispatching
policies.
NOTES
Paragraphs 19 through 21 were deleted.
Extensions to Ada 95
21.a/2 {AI95-00333-01} {extensions to Ada 95} 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.
21.b/2 {AI95-00355-01} Pragma Priority_Specific_Dispatching is new; it
allows specifying different policies for different priorities.
Wording Changes from Ada 95
21.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.
21.d/2 {AI95-00321-01} 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 may
not involve preemption).
D.2.3 Preemptive Dispatching
1/2 {AI95-00321-01} [This clause 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} {task dispatching point [partial]} {dispatching point
[partial]} 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.{preempt (a running task)}
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} 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
1/2 {AI95-00298-01} [This clause 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.
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.
9/2 For this policy, a non-blocking delay_statement is the only non-blocking
event that is a task dispatching point (see D.2.1).{task dispatching point
[partial]} {dispatching point [partial]}
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
11/2 {AI95-00298-01} 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 to execute within a protected object without
raising its active priority provided the associated protected unit does not
contain pragma Interrupt_Priority, Interrupt_Handler, or Attach_Handler.
Extensions to Ada 95
11.a/2 {AI95-00298-01} {AI95-00355-01} {extensions to Ada 95} Policy
Non_Preemptive_FIFO_Within_Priorities is new.
D.2.5 Round Robin Dispatching
1/2 {AI95-00355-01} [This clause 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.
8/2 {AI95-00355-01} 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} {extensions to Ada 95} 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.
2/2 {AI95-00357-01} This clause defines a package for representing the
deadline of a task and a dispatching policy that defines Earliest Deadline
First (EDF) dispatching. A pragma is defined to assign an initial deadline to
a task.
2.a/2 Discussion: This pragma 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 pragma Priority is used to
give an initial priority to a task.
Language Design Principles
2.b/2 {AI95-00357-01} 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 clause implement this scheme
including the case where the newly released task should execute
before some existing tasks but not preempt the currently executing
task.
Syntax
3/2 {AI95-00357-01} The form of a pragma Relative_Deadline is as follows:
4/2 pragma Relative_Deadline (relative_deadline_expression);
Name Resolution Rules
5/2 {AI95-00357-01} The expected type for relative_deadline_expression is
Real_Time.Time_Span.
Legality Rules
6/2 {AI95-00357-01} A Relative_Deadline pragma is allowed only immediately
within a task_definition or the declarative_part of a subprogram_body. At most
one such pragma shall appear within a given construct.
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;
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
12/2 {AI95-00357-01} A Relative_Deadline pragma has no effect if it occurs in
the declarative_part of the subprogram_body of a subprogram other than the
main subprogram.
13/2 {AI95-00357-01} The initial absolute deadline of a task containing pragma
Relative_Deadline is the value of Real_Time.Clock +
relative_deadline_expression, where the call of Real_Time.Clock is made between task creation
and the start of its activation. If there is no Relative_Deadline pragma 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.]
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 non-empty 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;
26/2 * 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.
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} {bounded error (cause) [partial]} 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} {erroneous execution (cause) [partial]} 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
33/2 18 {AI95-00357-01} 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} {extensions to Ada 95} Policy
EDF_Across_Priorities and package Dispatching.EDF are new.
D.3 Priority Ceiling Locking
1 [This clause 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 {configuration pragma (Locking_Policy) [partial]}
{pragma, configuration (Locking_Policy) [partial]} A Locking_Policy pragma is a
configuration pragma.
Dynamic Semantics
6/2 {8652/0073} {AI95-00091-01} {AI95-00327-01} {locking policy} [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. {Priority (of a protected object)}
6.a/2 Implementation defined: The locking policy if no Locking_Policy
pragma applies to any unit of a partition.
6.1/2 {AI95-00327-01} The expression of a Priority or Interrupt_Priority
pragma (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 pragma applies to a protected object, the initial priority
is specified by the locking policy.
{implicit subtype conversion (pragma Priority) [partial]}
{implicit subtype conversion (pragma Interrupt_Priority) [partial]}
7 There is one predefined locking policy, Ceiling_Locking; this policy is
defined as follows:
8/2 * {AI95-00327-01} {ceiling priority (of a protected object)} Every
protected object has a ceiling priority, which is determined by either
a Priority or Interrupt_Priority pragma 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.
10/2 * {AI95-00327-01} If an Interrupt_Handler or Attach_Handler pragma (see
C.3.1) appears in a protected_definition without an Interrupt_Priority
pragma, 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.
11/2 * {AI95-00327-01} If no pragma Priority, Interrupt_Priority,
Interrupt_Handler, or Attach_Handler is specified in the
protected_definition, 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 * {Ceiling_Check [partial]} {check, language-defined (Ceiling_Check)}
{Program_Error (raised by failure of run-time check)} 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. {bounded error (cause) [partial]} 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;
{Program_Error (raised by failure of run-time check)}
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
{Program_Error (raised by failure of run-time check)}
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 non-interrupt 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} {extensions to Ada 95} 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.
D.4 Entry Queuing Policies
1/1 {8652/0074} {AI95-00068-01} [{queuing policy} This clause 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 {configuration pragma (Queuing_Policy) [partial]}
{pragma, configuration (Queuing_Policy) [partial]} A Queuing_Policy pragma is a
configuration pragma.
Dynamic Semantics
6 {queuing policy} [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 * {priority of an entry call} 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
1/2 {AI95-00327-01} [This clause describes how the priority of an entity can
be modified or queried at run time.]
Wording Changes from Ada 95
1.a/2 {AI95-00327-01} This clause is turned into two subclauses. This
clause introduction is new.
D.5.1 Dynamic Priorities for Tasks
1 [This clause 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 (raised by failure of run-time check)} 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 (raised by failure of run-time check)} 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.
Bounded (Run-Time) Errors
11/2 This paragraph was deleted.{AI95-00327-01}
11.a/2 This paragraph was deleted.
Erroneous Execution
12 {erroneous execution (cause) [partial]} 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 29 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, setting or
querying the priority of a completed or an abnormal task is allowed,
so long as the task is not yet terminated.
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} {extensions to Ada 95} 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/2 {AI95-00327-01} This Ada 95 clause was turned into a subclause.
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
1/2 {AI95-00327-01} This clause 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
5/2 {AI95-00327-01} 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.
6/2 {AI95-00445-01} If the locking policy Ceiling_Locking is in effect, then
for a protected object P with either an Attach_Handler or Interrupt_Handler
pragma applying to 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.{Program_Error (raised by failure of run-time check)}
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} {extensions to Ada 95} The ability
to dynamically change and query the priority of a protected object
is new.
D.6 Preemptive Abort
1 [This clause 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 [This clause 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 {Restrictions (No_Task_Hierarchy)} No_Task_Hierarchy
All (nonenvironment) tasks depend directly on the environment
task of the partition.
4/2 {8652/0042} {AI95-00130-01} {AI95-00360-01}
{Restrictions (No_Nested_Finalization)} No_Nested_Finalization
Objects of a type that needs finalization (see 7.6) and access
types that designate a type that needs finalization shall be
declared only at library level.
4.a/1 This paragraph was deleted.{8652/0042} {AI95-00130-01}
5 {Restrictions (No_Abort_Statements)} No_Abort_Statements
There are no abort_statements, and there are no calls on
Task_Identification.Abort_Task.
6 {Restrictions (No_Terminate_Alternatives)} No_Terminate_Alternatives
There are no selective_accepts with terminate_alternatives.
7 {Restrictions (No_Task_Allocators)} No_Task_Allocators
There are no allocators for task types or types containing
task subcomponents.
8 {Restrictions (No_Implicit_Heap_Allocations)} 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. {Restrictions (No_Dynamic_Priorities)}
10/2 {AI95-00305-01} {AI95-00394-01} {Restrictions (No_Dynamic_Attachment)}
No_Dynamic_Attachment
There is no call to any of the operations defined in package
Interrupts (Is_Reserved, Is_Attached, Current_Handler,
Attach_Handler, Exchange_Handler, Detach_Handler, and
Reference).
10.1/2 {AI95-00305-01} {Restrictions (No_Local_Protected_Objects)}
No_Local_Protected_Objects
Protected objects shall be declared only at library level.
10.2/2 {AI95-00297-01} {Restrictions (No_Local_Timing_Events)}
No_Local_Timing_Events
Timing_Events shall be declared only at library level.
10.3/2 {AI95-00305-01} {Restrictions (No_Protected_Type_Allocators)}
No_Protected_Type_Allocators
There are no allocators for protected types or types
containing protected type subcomponents.
10.4/2 {AI95-00305-01} {Restrictions (No_Relative_Delay)} No_Relative_Delay
There are no delay_relative_statements.
10.5/2 {AI95-00305-01} {Restrictions (No_Requeue_Statements)}
No_Requeue_Statements
There are no requeue_statements.
10.6/2 {AI95-00305-01} {Restrictions (No_Select_Statements)}
No_Select_Statements
There are no select_statements.
10.7/2 {AI95-00394-01} {Restrictions (No_Specific_Termination_Handlers)}
No_Specific_Termination_Handlers
There are no calls to the Set_Specific_Handler and
Specific_Handler subprograms in Task_Termination.
10.8/2 {AI95-00305-01} {Restrictions (Simple_Barriers)} Simple_Barriers
The Boolean expression in an entry barrier shall be either a
static Boolean expression or a Boolean component of the
enclosing protected object.
11 The following restriction_parameter_identifiers are language defined:
12 {Restrictions (Max_Select_Alternatives)} Max_Select_Alternatives
Specifies the maximum number of alternatives in a
selective_accept.
13 {Restrictions (Max_Task_Entries)} 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.
{Restrictions (Max_Protected_Entries)}
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} {Restrictions (No_Task_Termination)}
No_Task_Termination
All tasks are non-terminating. 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} {Restrictions (Max_Storage_At_Blocking)}
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; {Storage_Check
[partial]} {check, language-defined (Storage_Check)}
{Storage_Error (raised by failure of run-time check)}
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}
{Restrictions (Max_Asynchronous_Select_Nesting)}
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; {Storage_Check [partial]}
{check, language-defined (Storage_Check)}
{Storage_Error (raised by failure of run-time check)}
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} {Restrictions (Max_Tasks)} 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;
{Storage_Check [partial]}
{check, language-defined (Storage_Check)}
{Storage_Error (raised by failure of run-time check)}
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} {Restrictions (Max_Entry_Queue_Length)}
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.{Program_Error (raised by failure of run-time check)}
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} {incompatibilities with Ada 95} 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}
{extensions to Ada 95} 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.
D.8 Monotonic Time
1 [This clause 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 {real time} 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 {epoch} {unspecified [partial]} 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. {clock tick} 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 {clock jump} 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.
36/1 The implementation shall document any aspects of the external environment
that could interfere with the clock behavior as defined in this clause.
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 For the purpose of the metrics defined in this clause, 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 * {drift rate} 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 35 The rules in this clause 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/2 {AI95-00386-01} {incompatibilities with Ada 95} Functions Seconds
and Minutes are newly 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 [This clause 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 {potentially blocking operation (delay_statement) [partial]}
{blocking, potentially (delay_statement) [partial]} 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).
6/2 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 * {lateness} {actual duration} 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.
NOTES
14/2 This paragraph was deleted.{AI95-00355-01}
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 [This clause describes a language-defined private semaphore (suspension
object), which can be used for two-stage suspend operations and as a simple
building block for implementing higher-level queues.]
Static Semantics
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.
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 {potentially blocking operation (Suspend_Until_True) [partial]}
{blocking, potentially (Suspend_Until_True) [partial]}
{Program_Error (raised by failure of run-time check)} Program_Error is raised
upon calling Suspend_Until_True if another task is already waiting on that
suspension object. Suspend_Until_True is a potentially blocking operation (see
9.5.1).
Implementation Requirements
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.
Extensions to Ada 95
11.a/2 {AI95-00362-01} {extensions to Ada 95} Synchronous_Task_Control is
now Preelaborated, so it can be used in preelaborated units.
D.11 Asynchronous Task Control
1 [This clause introduces a language-defined package to do asynchronous
suspend/resume on tasks. It uses a conceptual held priority value to represent
the task's held 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} {task state (held) [partial]} {held priority} {idle task}
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 (raised by failure of run-time check)}
Tasking_Error is raised if the check fails.
{Program_Error (raised by failure of run-time check)} Program_Error is raised
if the value of T is Null_Task_Id.
Erroneous Execution
9 {erroneous execution (cause) [partial]} 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 37 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 38 The effect of calling Get_Priority and Set_Priority on a Held task
is the same as on any other task.
13 39 Calling Hold on a held task or Continue on a non-held task has no
effect.
14 40 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} {extensions to Ada 95} 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 [This clause 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 Run-time Profiles
1/2 {AI95-00249-01} [This clause specifies a mechanism for defining run-time
profiles.]
Syntax
2/2 {AI95-00249-01} The form of a pragma Profile is as follows:
3/2 pragma Profile (profile_identifier {,
profile_pragma_argument_association});
Legality Rules
4/2 {AI95-00249-01} The profile_identifier shall be the name of a run-time
profile. The semantics of any profile_pragma_argument_associations are defined
by the run-time profile specified by the profile_identifier.
Static Semantics
5/2 {AI95-00249-01} A profile is equivalent to the set of configuration
pragmas that is defined for each run-time profile.
Post-Compilation Rules
6/2 {AI95-00249-01} {configuration pragma (Profile) [partial]}
{pragma, configuration (Profile) [partial]} A pragma Profile is a configuration
pragma. There may be more than one pragma Profile for a partition.
Extensions to Ada 95
6.a/2 {AI95-00249-01} {extensions to Ada 95} Pragma Profile is new.
D.13.1 The Ravenscar Profile
1/2 {AI95-00249-01} [This clause defines the Ravenscar profile.]{Ravenscar}
Legality Rules
2/2 {AI95-00249-01} The profile_identifier Ravenscar is a run-time profile.
For run-time profile Ravenscar, there shall be no
profile_pragma_argument_associations.
Static Semantics
3/2 {AI95-00249-01} The run-time profile Ravenscar is equivalent to the
following set of pragmas:
4/2 {AI95-00249-01} {AI95-00297-01} {AI95-00394-01}
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_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_Budget,
No_Dependence => Ada.Execution_Time.Timers,
No_Dependence => Ada.Task_Attributes);
4.a/2 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.
NOTES
5/2 41 {AI95-00249-01} 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.
Extensions to Ada 95
5.a/2 {AI95-00296-01} {extensions to Ada 95} The Ravenscar profile is
new.
D.14 Execution Time
1/2 {AI95-00307-01} This clause 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;
10/2 private
... -- not specified by the language
end Ada.Execution_Time;
11/2 {AI95-00307-01} {execution time (of a task)} {CPU time (of a task)} 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. It is implementation defined which task, if any, is
charged the execution time that is consumed by interrupt handlers and run-time
services on behalf of the system.
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 non-zero 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.
Dynamic Semantics
15/2 {AI95-00307-01} {CPU clock tick} 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.
Erroneous Execution
19/2 {AI95-00307-01} {erroneous execution (cause) [partial]} 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/2 Documentation Requirement: The properties of the mechanism used to
implement package Execution_Time.
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} {extensions to Ada 95} The package Execution_Time
is new.
D.14.1 Execution Time Timers
1/2 {AI95-00307-01} This clause 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 non-null value of type Timer_Handler and cleared otherwise.
All Timer objects are initially cleared. {set (execution timer object)
[partial]} {clear (execution timer object) [partial]}
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. {handler (execution timer) [partial]}
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.
15/2 {AI95-00307-01} 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.{expires (execution timer)}
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.
18/2 {AI95-00307-01} The function Current_Handler returns the handler
associated with the timer TM if that timer is set; otherwise it returns null.
19/2 {AI95-00307-01} 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.
20/2 {AI95-00307-01} 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} {erroneous execution (cause) [partial]} 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
28/2 {AI95-00307-01} 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 42 {AI95-00307-01} A Timer_Handler can be associated with several
Timer objects.
Extensions to Ada 95
29.a/2 {AI95-00307-01} {extensions to Ada 95} The package
Execution_Time.Timers is new.
D.14.2 Group Execution Time Budgets
1/2 {AI95-00354-01} This clause 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:
3/2 with System;
package Ada.Execution_Time.Group_Budgets is
4/2 type Group_Budget 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;
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.{budget}
{exhaust (a budget)} {handler (group budget) [partial]}
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. {set (group budget object)
[partial]} {clear (group budget object) [partial]}
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.
18/2 {AI95-00354-01} The function Is_Member returns True if the task
identified by T is a member of the group GB; otherwise it return False.
19/2 {AI95-00354-01} 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.
21/2 {AI95-00354-01} 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 non-positive. Any execution 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.
23/2 {AI95-00354-01} 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.
25/2 {AI95-00354-01} 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.
27/2 {AI95-00354-01} 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.
28/2 {AI95-00354-01} 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.
32/2 {AI95-00354-01} 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} {erroneous execution (cause) [partial]} 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 43 {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 44 {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} {extensions to Ada 95} The package
Execution_Time.Group_Budgets is new.
D.15 Timing Events
1/2 {AI95-00297-01} This clause 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 non-null value of type Timing_Event_Handler and cleared
otherwise. All Timing_Event objects are initially cleared.
{set (timing event object) [partial]} {clear (timing event object) [partial]}
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. {handler (timing event)
[partial]}
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
11/2 {AI95-00297-01} 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.
15/2 {AI95-00297-01} 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 immediately by the task executing the call of Set_Handler.
The timing event Event is cleared.
16/2 {AI95-00297-01} The function Current_Handler returns the handler
associated with the event Event if that event is set; otherwise it returns
null.
17/2 {AI95-00297-01} 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.
18/2 {AI95-00297-01} 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:
24/2 * An upper bound on the lateness of the execution of a handler. That
is, the maximum time between when a handler is actually executed and
the time specified when the event was set.
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 45 {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 46 {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} {extensions to Ada 95} The package
Real_Time.Timing_Events is new.
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