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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
   No_Task_Hierarchy
All (nonenvironment) tasks depend directly on the environment task of the partition.
4/2
     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.
5
   No_Abort_Statements
There are no abort_statements, and there are no calls on Task_Identification.Abort_Task.
6
   No_Terminate_Alternatives
There are no selective_accepts with terminate_alternatives.
7
   No_Task_Allocators
There are no allocators for task types or types containing task subcomponents.
8
   No_Implicit_Heap_Allocations
There are no operations that implicitly require heap storage allocation to be performed by the implementation. The operations that implicitly require heap storage allocation are implementation defined.
9/2
     No_Dynamic_Priorities
There are no semantic dependences on the package Dynamic_Priorities, and no occurrences of the attribute Priority.
10/2
      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
        No_Local_Protected_Objects
Protected objects shall be declared only at library level.
10.2/2
        No_Local_Timing_Events
Timing_Events shall be declared only at library level.
10.3/2
        No_Protected_Type_Allocators
There are no allocators for protected types or types containing protected subcomponents.
10.4/2
        No_Relative_Delay
There are no delay_relative_statements.
10.5/2
        No_Requeue_Statements
There are no requeue_statements.
10.6/2
        No_Select_Statements
There are no select_statements.
10.7/2
        No_Specific_Termination_Handlers
There are no calls to the Set_Specific_Handler and Specific_Handler subprograms in Task_Termination.
10.8/2
        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:
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    Max_Select_Alternatives
Specifies the maximum number of alternatives in a selective_accept.
13
    Max_Task_Entries
Specifies the maximum number of entries per task. The bounds of every entry family of a task unit shall be static, or shall be defined by a discriminant of a subtype whose corresponding bound is static. A value of zero indicates that no rendezvous are possible.
14
    Max_Protected_Entries
Specifies the maximum number of entries per protected type. The bounds of every entry family of a protected unit shall be static, or shall be defined by a discriminant of a subtype whose corresponding bound is static.

Dynamic Semantics

15/2
      The following restriction_identifier is language defined:
15.1/2
        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.
16
    The following restriction_parameter_identifiers are language defined:
17/1
      Max_Storage_At_Blocking
Specifies the maximum portion (in storage elements) of a task's Storage_Size that can be retained by a blocked task. If an implementation chooses to detect a violation of this restriction, Storage_Error should be raised; otherwise, the behavior is implementation defined.
18/1
      Max_Asynchronous_Select_Nesting
Specifies the maximum dynamic nesting level of asynchronous_selects. A value of zero prevents the use of any asynchronous_select and, if a program contains an asynchronous_select, it is illegal. If an implementation chooses to detect a violation of this restriction for values other than zero, Storage_Error should be raised; otherwise, the behavior is implementation defined.
19/1
      Max_Tasks
Specifies the maximum number of task creations that may be executed over the lifetime of a partition, not counting the creation of the environment task. A value of zero prevents any task creation and, if a program contains a task creation, it is illegal. If an implementation chooses to detect a violation of this restriction, Storage_Error should be raised; otherwise, the behavior is implementation defined.
19.1/2
        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.
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.

Implementation Advice

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    When feasible, the implementation should take advantage of the specified restrictions to produce a more efficient implementation.
NOTES
22
37  The above Storage_Checks can be suppressed with pragma Suppress.

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