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13.11 Storage Management

  1. Each access-to-object type has an associated storage pool. The storage allocated by an allocator comes from the pool; instances of Unchecked_Deallocation return storage to the pool. Several access types can share the same pool.
  2. A storage pool is a variable of a type in the class rooted at Root_Storage_Pool, which is an abstract limited controlled type. By default, the implementation chooses a standard storage pool for each access type. The user may define new pool types, and may override the choice of pool for an access type by specifying Storage_Pool for the type.

    Legality Rules

  3. If Storage_Pool is specified for a given access type, Storage_Size shall not be specified for it.

    Static Semantics

  4. The following language-defined library package exists: 
  5. with Ada.Finalization;
with System.Storage_Elements;
package System.Storage_Pools is
    pragma Preelaborate(System.Storage_Pools);

  6.     type Root_Storage_Pool is
        abstract new
          Ada.Finalization.Limited_Controlled with private;

  7.     procedure Allocate(
      Pool : in out Root_Storage_Pool;
      Storage_Address : out Address;
      Size_In_Storage_Elements : in Storage_Elements.Storage_Count;
      Alignment : in Storage_Elements.Storage_Count) is abstract;

  8.     procedure Deallocate(
      Pool : in out Root_Storage_Pool;
      Storage_Address : in Address;
      Size_In_Storage_Elements : in Storage_Elements.Storage_Count;
      Alignment : in Storage_Elements.Storage_Count) is abstract;

  9.     function Storage_Size(Pool : Root_Storage_Pool)
        return Storage_Elements.Storage_Count is abstract;

  10. private
   ... -- not specified by the language
end System.Storage_Pools;
  11. A storage pool type (or pool type) is a descendant of Root_Storage_Pool. The elements of a storage pool are the objects allocated in the pool by allocators.
  12. For every access subtype S, the following attributes are defined:
  13. S'Storage_Pool 
    Denotes the storage pool of the type of S. The type of this
attribute is Root_Storage_Pool'Class.
  14. S'Storage_Size 
    Yields the result of calling Storage_Size(S'Storage_Pool),
which is intended to be a measure of the number of storage
elements reserved for the pool. The type of this attribute
is universal_integer.
  15. Storage_Size or Storage_Pool may be specified for a non-derived access-to-object type via an attribute_definition_clause; the name in a Storage_Pool clause shall denote a variable.
  16. An allocator of type T allocates storage from T's storage pool. If the storage pool is a user-defined object, then the storage is allocated by calling Allocate, passing T'Storage_Pool as the Pool parameter. The Size_In_Storage_Elements parameter indicates the number of storage elements to be allocated, and is no more than D'Max_Size_In_Storage_Elements, where D is the designated subtype. The Alignment parameter is D'Alignment. The result returned in the Storage_Address parameter is used by the allocator as the address of the allocated storage, which is a contiguous block of memory of Size_In_Storage_Elements storage elements. Any exception propagated by Allocate is propagated by the allocator.
  17. If Storage_Pool is not specified for a type defined by an access_to_object_definition, then the implementation chooses a standard storage pool for it in an implementation-defined manner. In this case, the exception Storage_Error is raised by an allocator if there is not enough storage. It is implementation defined whether or not the implementation provides user-accessible names for the standard pool type(s).
  18. If Storage_Size is specified for an access type, then the Storage_Size of this pool is at least that requested, and the storage for the pool is reclaimed when the master containing the declaration of the access type is left. If the implementation cannot satisfy the request, Storage_Error is raised at the point of the attribute_definition_clause. If neither Storage_Pool nor Storage_Size are specified, then the meaning of Storage_Size is implementation defined.
  19. If Storage_Pool is specified for an access type, then the specified pool is used.
  20. The effect of calling Allocate and Deallocate for a standard storage pool directly (rather than implicitly via an allocator or an instance of Unchecked_Deallocation) is unspecified.

    Erroneous Execution

  21. If Storage_Pool is specified for an access type, then if Allocate can satisfy the request, it should allocate a contiguous block of memory, and return the address of the first storage element in Storage_Address. The block should contain Size_In_Storage_Elements storage elements, and should be aligned according to Alignment. The allocated storage should not be used for any other purpose while the pool element remains in existence. If the request cannot be satisfied, then Allocate should propagate an exception (such as Storage_Error). If Allocate behaves in any other manner, then the program execution is erroneous.

    Documentation Requirements

  22. An implementation shall document the set of values that a user-defined Allocate procedure needs to accept for the Alignment parameter. An implementation shall document how the standard storage pool is chosen, and how storage is allocated by standard storage pools.

    Implementation Advice

  23. An implementation should document any cases in which it dynamically allocates heap storage for a purpose other than the evaluation of an allocator.
  24. A default (implementation-provided) storage pool for an access-to-constant type should not have overhead to support deallocation of individual objects.
  25. A storage pool for an anonymous access type should be created at the point of an allocator for the type, and be reclaimed when the designated object becomes inaccessible.

    NOTES

  26. (23) A user-defined storage pool type can be obtained by extending the Root_Storage_Pool type, and overriding the primitive subprograms Allocate, Deallocate, and Storage_Size. A user-defined storage pool can then be obtained by declaring an object of the type extension. The user can override Initialize and Finalize if there is any need for non-trivial initialization and finalization for a user-defined pool type. For example, Finalize might reclaim blocks of storage that are allocated separately from the pool object itself.
  27. (24) The writer of the user-defined allocation and deallocation procedures, and users of allocators for the associated access type, are responsible for dealing with any interactions with tasking. In particular:
    1. If the allocators are used in different tasks, they require mutual exclusion.
    2. If they are used inside protected objects, they cannot block.
    3. If they are used by interrupt handlers, See section C.3 Interrupt Support, the mutual exclusion mechanism has to work properly in that context.

  1. (25) The primitives Allocate, Deallocate, and Storage_Size are declared as abstract, See section 3.9.3 Abstract Types and Subprograms, and therefore they have to be overridden when a new (non-abstract) storage pool type is declared.

    Examples

  2. To associate an access type with a storage pool object, the user first declares a pool object of some type derived from Root_Storage_Pool. Then, the user defines its Storage_Pool attribute, as follows: 
  3. Pool_Object : Some_Storage_Pool_Type;

  4. type T is access Designated;
for T'Storage_Pool use Pool_Object;
  5. Another access type may be added to an existing storage pool, via: 
  6. for T2'Storage_Pool use T'Storage_Pool;
  7. The semantics of this is implementation defined for a standard storage pool.
  8. As usual, a derivative of Root_Storage_Pool may define additional operations. For example, presuming that Mark_Release_Pool_Type has two additional operations, Mark and Release, the following is a possible use: 
  9. type Mark_Release_Pool_Type
   (Pool_Size : Storage_Elements.Storage_Count;
    Block_Size : Storage_Elements.Storage_Count)
        is new Root_Storage_Pool with limited private;

  10. ...

  11. MR_Pool : Mark_Release_Pool_Type (Pool_Size => 2000,
                                  Block_Size => 100);

  12. type Acc is access ...;
for Acc'Storage_Pool use MR_Pool;
...

  13. Mark(MR_Pool);
... -- Allocate objects using ``new Designated(...)''.
Release(MR_Pool); -- Reclaim the storage.

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