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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.
2/2
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-to-object type. The user may define new pool types, and may override the choice of pool for an access-to-object 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);
6/2
    type Root_Storage_Pool is
        abstract new Ada.Finalization.Limited_Controlled with private;
    pragma Preelaborable_Initialization(Root_Storage_Pool);
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.
12/2
 For every access-to-object subtype S, the following representation 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 nonderived access-to-object type via an attribute_definition_clause; the name in a Storage_Pool clause shall denote a variable.
16/3
 An allocator of a type T that does not support subpools allocates storage from T's storage pool. If the storage pool is a user-defined object, then the storage is allocated by calling Allocate as described below. Allocators for types that support subpools are described in 13.11.4.
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). 
18/4
 If Storage_Size is specified for an access type T, an implementation-defined pool P is used for the type. The Storage_Size of P is at least that requested, and the storage for P 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 freezing point of type T. The storage pool P is used only for allocators returning type T or other access types specified to use T'Storage_Pool. Storage_Error is raised by an allocator returning such a type if the storage space of P is exhausted (additional memory is not allocated).
18.1/4
   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. 

Implementation Requirements

21.1/3
   The Allocate procedure of a user-defined storage pool object P may be called by the implementation only to allocate storage for a type T whose pool is P, only at the following points:
21.2/3
During the execution of an allocator of type T;
21.3/3
During the execution of a return statement for a function whose result is built-in-place in the result of an allocator of type T;
21.4/3
During the execution of an assignment operation with a target of an allocated object of type T with a part that has an unconstrained discriminated subtype with defaults.
21.5/3
   For each of the calls of Allocate described above, P (equivalent to T'Storage_Pool) is passed 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 of T. The Alignment parameter is a nonzero integral multiple of D'Alignment if D is a specific type, and otherwise is a nonzero integral multiple of the alignment of the specific type identified by the tag of the object being created; it is unspecified if there is no such value. The Alignment parameter is no more than D'Max_Alignment_For_Allocation. The result returned in the Storage_Address parameter is used 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 construct that contained the call.
21.6/3
   The number of calls to Allocate needed to implement an allocator for any particular type is unspecified. The number of calls to Deallocate needed to implement an instance of Unchecked_Deallocation (see 13.11.2) for any particular object is the same as the number of Allocate calls for that object.
21.7/3
   The Deallocate procedure of a user-defined storage pool object P may be called by the implementation to deallocate storage for a type T whose pool is P only at the places when an Allocate call is allowed for P, during the execution of an instance of Unchecked_Deallocation for T, or as part of the finalization of the collection of T. For such a call of Deallocate, P (equivalent to T'Storage_Pool) is passed as the Pool parameter. The value of the Storage_Address parameter for a call to Deallocate is the value returned in the Storage_Address parameter of the corresponding successful call to Allocate. The values of the Size_In_Storage_Elements and Alignment parameters are the same values passed to the corresponding Allocate call. Any exception propagated by Deallocate is propagated by the construct that contained the call.

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. 
25/2
 The storage pool used for an allocator of an anonymous access type should be determined as follows:
25.1/2
If the allocator is defining a coextension (see 3.10.2) of an object being created by an outer allocator, then the storage pool used for the outer allocator should also be used for the coextension;
25.2/2
For other access discriminants and access parameters, the storage pool should be created at the point of the allocator, and be reclaimed when the allocated object becomes inaccessible;
25.3/3
If the allocator defines the result of a function with an access result, the storage pool is determined as though the allocator were in place of the call of the function. If the call is the operand of a type conversion, the storage pool is that of the target access type of the conversion. If the call is itself defining the result of a function with an access result, this rule is applied recursively;
25.4/2
Otherwise, a default storage pool should be created at the point where the anonymous access type is elaborated; such a storage pool need not support deallocation of individual objects. 
NOTES
26
27  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 nontrivial 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
28  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: 
28
If the allocators are used in different tasks, they require mutual exclusion.
29
If they are used inside protected objects, they cannot block.
30
If they are used by interrupt handlers (see C.3, “Interrupt Support”), the mutual exclusion mechanism has to work properly in that context. 
31
29  The primitives Allocate, Deallocate, and Storage_Size are declared as abstract (see 3.9.3), and therefore they have to be overridden when a new (nonabstract) storage pool type is declared. 

Examples

32
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:
33
Pool_Object : Some_Storage_Pool_Type;
34
type T is access Designated;
for T'Storage_Pool use Pool_Object;
35
Another access type may be added to an existing storage pool, via: 
36
for T2'Storage_Pool use T'Storage_Pool;
37
The semantics of this is implementation defined for a standard storage pool. 
38/3
 As usual, a derivative of Root_Storage_Pool may define additional operations. For example, consider the Mark_Release_Pool_Type defined in 13.11.6, that has two additional operations, Mark and Release, the following is a possible use: 
39/3
type Mark_Release_Pool_Type
   (Pool_Size : Storage_Elements.Storage_Count)
        is new Subpools.Root_Storage_Pool_With_Subpools with private;
           -- As defined in package MR_Pool, see 13.11.6
40
...
41/3
Our_Pool : Mark_Release_Pool_Type (Pool_Size => 2000);
My_Mark : MR_Pool.Subpool_Handle; -- See 13.11.6
42/3
type Acc is access ...;
for Acc'Storage_Pool use Our_Pool;
...
43/3
My_Mark := Mark(Our_Pool);
... -- Allocate objects using “new (My_Mark) Designated(...)”.
Release(My_Mark); -- Finalize objects and reclaim storage.

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