D.7 Tasking Restrictions
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AI05-0299-1}
[This subclause defines restrictions that can be used with a pragma Restrictions
(see
13.12) to facilitate the construction
of highly efficient tasking run-time systems.]
Static Semantics
The following
restriction_identifiers
are language defined:
No task depends on a master other than the library-level master.
Ramification: {
AI05-0216-1}
This is equivalent to saying “no task depends on a master other
than the master that is the execution of the body of the environment
task of the partition”, but it is much easier to understand. This
is a post-compilation check, which can be checked at compile-time.
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This disallows any function returning an object with a task part or coextension,
even if called at the library level, as such a task would temporarily
depend on a nested master (the master of the return statement), which
is disallowed by this restriction.
Objects of a type that needs finalization (see
7.6)
are declared only at library level. If an access type does not have library-level
accessibility, then there are no
allocators
of the type where the type determined by the
subtype_mark
of the
subtype_indication
or
qualified_expression
needs finalization.
Ramification: {
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The second sentence prevents the declaration of objects of access types
which would require nested finalization. It also prevents the declarations
of coextensions that need finalization in a nested scope. The latter
cannot be done by preventing the declaration of the objects, as it is
not necessarily known if the coextension type needs finalization (it
could be a limited view).
There are no
abort_statements,
and there is no use of a
name
denoting Task_Identification.Abort_Task.
No_Terminate_Alternatives
There are no
allocators
for task types or types containing task subcomponents.
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In the case of an initialized
allocator
of an access type whose designated type is class-wide and limited, a
check is made that the specific type of the allocated object has no task
subcomponents. Program_Error is raised if this check fails.
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.
Implementation defined: Any operations
that implicitly require heap storage allocation.
There are no semantic dependences on the package Dynamic_Priorities,
and no occurrences of the attribute Priority.
There is no use of a
name
denoting any of the operations defined in package Interrupts (Is_Reserved,
Is_Attached, Current_Handler, Attach_Handler, Exchange_Handler, Detach_Handler,
and Reference).
Ramification: {
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This includes 'Access and 'Address of any of these operations, as well
as inherited versions of these operations.
No task has the CPU aspect specified to be a non-static expression. Each
task (including the environment task) that has the CPU aspect specified
as Not_A_Specific_CPU will be assigned to a particular implementation-defined
CPU. The same is true for the environment task when the CPU aspect is
not specified. [Any other task without a CPU aspect will activate and
execute on the same processor as its activating task.]
Proof: The processor of a task without
a CPU aspect is defined in
D.16, and this
restriction guarantees that the activator always has a CPU assigned.
Reason: This restriction prevents any
migration of tasks.
Ramification: If no CPU aspects are specified,
then the program will run on a single CPU, as all of the tasks will be
activated directly or indirectly by the environment task, and the rules
require the same CPU to be used as the activating task.
Implementation defined: When restriction
No_Dynamic_CPU_Assignment applies to a partition, the processor on which
a task with a CPU value of a Not_A_Specific_CPU will execute.
Protected objects are declared only at library level.
Timing_Events are declared only at library level.
There are no
allocators
for protected types or types containing protected type subcomponents.
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In the case of an initialized
allocator
of an access type whose designated type is class-wide and limited, a
check is made that the specific type of the allocated object has no protected
subcomponents. Program_Error is raised if this check fails.
There are no
delay_relative_statements,
and there is no use of a
name
that denotes the Timing_Events.Set_Handler subprogram that has a Time_Span
parameter.
There is no use of a
name
denoting the Set_Specific_Handler and Specific_Handler subprograms in
Task_Termination.
The CPU aspect is specified for the environment task. No CPU aspect is
specified to be statically equal to Not_A_Specific_CPU. If aspect CPU
is specified (dynamically) to the value Not_A_Specific_CPU, then Program_Error
is raised. If Set_CPU or Delay_Until_And_Set_CPU are called with the
CPU parameter equal to Not_A_Specific_CPU, then Program_Error is raised.
Ramification: If this restriction is
used in a context for which restriction No_Dynamic_CPU_Assignment is
in effect, then no runtime check is needed when specifying the CPU aspect.
If the restriction is used with the Ravenscar profile, no runtime checks
are needed.
The Boolean expression in each entry barrier is either a static expression
or a name that statically denotes a component of the enclosing protected
object.
The following
restriction_parameter_identifiers
are language defined:
Specifies the maximum number of alternatives in a
selective_accept.
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.]
Max_Protected_Entries
Specifies the maximum number of entries per protected type. The bounds
of every entry family of a protected unit shall be static, or shall be
defined by a discriminant of a subtype whose corresponding bound is static.
Dynamic Semantics
All tasks are nonterminating. It is implementation-defined what happens
if a task attempts to terminate. If there is a fall-back handler (see
C.7.3) set for the partition it should be called when the first task
attempts to terminate.
Implementation defined: When restriction
No_Task_Termination applies to a partition, what happens when a task
terminates.
The following
restriction_parameter_identifiers
are language defined:
Specifies the maximum portion [(in storage elements)] of a task's Storage_Size
that can be retained by a blocked task. If an implementation chooses
to detect a violation of this restriction, Storage_Error should be raised;
otherwise,
the behavior is implementation defined.
Implementation defined: The behavior
when restriction Max_Storage_At_Blocking is violated.
Specifies the maximum dynamic nesting level of
asynchronous_selects.
A value of zero prevents the use of any
asynchronous_select
and, if a program contains an
asynchronous_select,
it is illegal. If an implementation chooses to detect a violation of
this restriction for values other than zero, Storage_Error should be
raised;
otherwise,
the behavior is implementation defined.
Implementation defined: The behavior
when restriction Max_Asynchronous_Select_Nesting is violated.
Specifies the maximum number of task creations that may be executed over
the lifetime of a partition, not counting the creation of the environment
task. A value of zero prevents any task creation and, if a program contains
a task creation, it is illegal. If an implementation chooses to detect
a violation of this restriction, Storage_Error should be raised;
otherwise,
the behavior is implementation defined.
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.
Implementation Note: We envision an implementation
approach that places TCBs or pointers to them in a fixed-size table,
and never reuses table elements.
Implementation defined: The behavior
when restriction Max_Tasks is violated.
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.
Specifies that an
allocator
using a standard storage pool (see
13.11)
shall not occur within a parameterless library subprogram, nor within
the
handled_sequence_of_statements
of a task body. For the purposes of this rule, an
allocator
of a type derived from a formal access type does not use a standard storage
pool.
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At run time, Storage_Error is raised if an
allocator
using a standard storage pool is evaluated after the elaboration of the
library_items
of the partition has completed.
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.
Implementation defined: Whether the use
of pragma Restrictions results in a reduction in program code or data
size or execution time.
Implementation Advice
When feasible, the implementation should take advantage
of the specified restrictions to produce a more efficient implementation.
Implementation Advice: When feasible,
specified restrictions should be used to produce a more efficient implementation.
34 The above Storage_Checks can be suppressed
with pragma Suppress.
Incompatibilities With Ada 95
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AI95-00360-01}
Amendment Correction: The No_Nested_Finalization
is now defined in terms of types that need finalization. These types
include a variety of language-defined types that
might be implemented
with a controlled type. If the restriction No_Nested_Finalization (see
D.7) applies to the partition, and one of these
language-defined types does not have a controlled part, it will not be
allowed in local objects in Ada 2005 whereas it would be allowed in original
Ada 95. Such code is not portable, as other Ada compilers may have had
a controlled part, and thus would be illegal under the restriction.
Extensions to Ada 95
{
AI95-00297-01}
{
AI95-00305-01}
{
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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
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8652/0042}
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Corrigendum: Clarified that No_Nested_Finalization covered task
and protected parts as well.
{
8652/0076}
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Corrigendum: Changed the description of Max_Tasks and Max_Asynchronous_Select_Nested
to eliminate conflicts with the High Integrity Annex (see
H.4).
{
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Added using of the new Priority attribute to the restriction No_Dynamic_Priorities.
{
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Restriction No_Asynchronous_Control is now obsolescent.
Incompatibilities With Ada 2005
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Correction: Changed so that coextensions of
types that require nested finalization are also prohibited; this is done
by prohibiting
allocators
rather than objects of specific access types. It seems unlikely that
any program depending on this restriction would violate it in this blatant
manner, so it is expected that very few programs will be affected by
this change.
{
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Correction: The restriction No_Relative_Delay was changed to include
the Timing_Events routine that uses a relative delay. This means that
a program that uses that routine and this restriction will now be rejected.
However, such a program violates the spirit and intent of the restriction
and as such the program should never have been allowed. Moreover, it
is unlikely that any program depending on this restriction would violate
it in such an obvious manner, so it is expected that very few programs
will be affected by this change.
{
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Correction: A number of restrictions were changed from "no
calls" on some subprogram to "no use of a
name
that denotes" that subprogram. This closes a hole where renames,
uses as the prefix of 'Access, and the like, would not be rejected by
the restriction, possibly allowing backdoor access to the prohibited
subprogram. A program that uses one of these restrictions and using such
backdoor access will now be rejected; however, it is extremely unlikely
that any program that relies on these restrictions would also use an
end-run around the restriction, so it is expected that very few programs
will be affected by this change.
Extensions to Ada 2005
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Restriction No_Standard_Allocators_After_Elaboration
is newly added to Ada.
Wording Changes from Ada 2005
{
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{
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Correction: Improved the wording of various restrictions to make
it clearer that they prohibit things that would otherwise be legal, and
to word them as definitions, not Legality Rules;.
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Correction: Added wording to explain how No_Task_Allocators and
No_Protected_Type_Allocators are checked for class-wide types. This might
be an extension if the compiler assumed the worst in the past (it is
now a runtime check).
Extensions to Ada 2012
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Corrigendum: Restriction No_Dynamic_CPU_Assignment
is newly added to Ada, for use as part of the Ravenscar profile (see
D.13).
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Corrigendum: Restriction No_Tasks_Unassigned_To_CPU is newly added
to Ada; it ensures that no task is running on an implementation-defined
CPU so that task scheduling can be analyzed.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
