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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. 

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