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13.11 Storage Management

   [ {user-defined storage management} {storage management (user-defined)} {user-defined heap management} {heap management (user-defined)} 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.]
   [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.]
Ramification: By default, the implementation might choose to have a single global storage pool, which is used (by default) by all access types, which might mean that storage is reclaimed automatically only upon partition completion. Alternatively, it might choose to create a new pool at each accessibility level, which might mean that storage is reclaimed for an access type when leaving the appropriate scope. Other schemes are possible.

Legality Rules

   If Storage_Pool is specified for a given access type, Storage_Size shall not be specified for it.
Reason: The Storage_Pool determines the Storage_Size; hence it would not make sense to specify both. Note that this rule is simplified by the fact that the aspects in question cannot be specified for derived types, nor for non-first subtypes, so we don't have to worry about whether, say, Storage_Pool on a derived type overrides Storage_Size on the parent type. For the same reason, ``specified'' means the same thing as ``directly specified'' here.

Static Semantics

   The following language-defined library package exists:
with Ada.Finalization;
with System.Storage_Elements;
package System.Storage_Pools is
    pragma Preelaborate(System.Storage_Pools);
    type Root_Storage_Pool is
        abstract new Ada.Finalization.Limited_Controlled with private;
    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;
    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;
    function Storage_Size(Pool : Root_Storage_Pool)
        return Storage_Elements.Storage_Count is abstract;
   ... -- not specified by the language
end System.Storage_Pools;
Reason: The Alignment parameter is provided to Deallocate because some allocation strategies require it. If it is not needed, it can be ignored.
    {storage pool type} {pool type} A storage pool type (or pool type) is a descendant of Root_Storage_Pool. {storage pool element} {pool element} {element (of a storage pool)} The elements of a storage pool are the objects allocated in the pool by allocators.
Discussion: In most cases, an element corresponds to a single memory block allocated by Allocate. However, in some cases the implementation may choose to associate more than one memory block with a given pool element.
      {8652/0009} For every access subtype S, the following representation attributes are defined:
Denotes the storage pool of the type of S. The type of this attribute is Root_Storage_Pool'Class.
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.
Ramification: Storage_Size is also defined for task subtypes and objects -- see 13.3.
Storage_Size is not a measure of how much un-allocated space is left in the pool. That is, it includes both allocated and unallocated space. Implementations and users may provide a Storage_Available function for their pools, if so desired.
    {specifiable (of Storage_Size for a non-derived access-to-object type) [partial]} {specifiable (of Storage_Pool for a non-derived access-to-object type) [partial]} {Storage_Pool clause} {Storage_Size clause} 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.
    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. {contiguous representation [partial]} {discontiguous representation [partial]} 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.]
Ramification: If the implementation chooses to represent the designated subtype in multiple pieces, one allocator evaluation might result in more than one call upon Allocate. In any case, allocators for the access type obtain all the required storage for an object of the designated type by calling the specified Allocate procedure.
Note that the implementation does not turn other exceptions into Storage_Error.
{8652/0111} If D (the designated type of T) includes subcomponents of other access types, they will be allocated from the storage pools for those types, even if those allocators are executed as part of the allocator of T (as part of the initialization of the object). For instance, an access-to-task type TT may allocate the data structures used to implement the task value from other storage pools. (In particular, the task stack does not necessarily need to be allocated from the storage pool for TT.)
    {standard storage pool} 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. {Storage_Check [partial]} {check, language-defined (Storage_Check)} {Storage_Error (raised by failure of run-time check)} 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).
Implementation defined: The manner of choosing a storage pool for an access type when Storage_Pool is not specified for the type.
Implementation defined: Whether or not the implementation provides user-accessible names for the standard pool type(s).
Ramification: An anonymous access type has no pool. An access-to-object type defined by a derived_type_definition inherits its pool from its parent type, so all access-to-object types in the same derivation class share the same pool. Hence the ``defined by an access_to_object_definition'' wording above.
{contiguous representation [partial]} {discontiguous representation [partial]} There is no requirement that all storage pools be implemented using a contiguous block of memory (although each allocation returns a pointer to a contiguous block of memory).
    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. {Storage_Error (raised by failure of run-time check)} 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.
Implementation defined: The meaning of Storage_Size.
Ramification: The Storage_Size function and attribute will return the actual size, rather than the requested size. Comments about rounding up, zero, and negative on task Storage_Size apply here, as well. See also AI83-00557, AI83-00558, and AI83-00608.
The expression in a Storage_Size clause need not be static.
The reclamation happens after the master is finalized.
Implementation Note: For a pool allocated on the stack, normal stack cut-back can accomplish the reclamation. For a library-level pool, normal partition termination actions can accomplish the reclamation.
    If Storage_Pool is specified for an access type, then the specified pool is used.
    {unspecified [partial]} 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.
Ramification: For example, an allocator might put the pool element on a finalization list. If the user directly Deallocates it, instead of calling an instance of Unchecked_Deallocation, then the implementation would probably try to finalize the object upon master completion, which would be bad news. Therefore, the implementation should define such situations as erroneous.

Erroneous Execution

    {erroneous execution (cause) [partial]} 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

    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 defined: Implementation-defined aspects of storage pools.

Implementation Advice

    An implementation should document any cases in which it dynamically allocates heap storage for a purpose other than the evaluation of an allocator.
Reason: This is ``Implementation Advice'' because the term ``heap storage'' is not formally definable; therefore, it is not testable whether the implementation obeys this advice.
    A default (implementation-provided) storage pool for an access-to-constant type should not have overhead to support deallocation of individual objects.
Ramification: Unchecked_Deallocation is not defined for such types. If the access-to-constant type is library-level, then no deallocation (other than at partition completion) will ever be necessary, so if the size needed by an allocator of the type is known at link-time, then the allocation should be performed statically. If, in addition, the initial value of the designated object is known at compile time, the object can be allocated to read-only memory.
Implementation Note: If the Storage_Size for an access type is specified, the storage pool should consist of a contiguous block of memory, possibly allocated on the stack. The pool should contain approximately this number of storage elements. These storage elements should be reserved at the place of the Storage_Size clause, so that allocators cannot raise Storage_Error due to running out of pool space until the appropriate number of storage elements has been used up. This approximate (possibly rounded-up) value should be used as a maximum; the implementation should not increase the size of the pool on the fly. If the Storage_Size for an access type is specified as zero, then the pool should not take up any storage space, and any allocator for the type should raise Storage_Error.
Ramification: Note that most of this is approximate, and so cannot be (portably) tested. That's why we make it an Implementation Note. There is no particular number of allocations that is guaranteed to succeed, and there is no particular number of allocations that is guaranteed to fail.
    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.
Implementation Note: Normally the "storage pool" for an anonymous access type would not exist as a separate entity. Instead, the designated object of the allocator would be allocated, in the case of an access parameter, as a local aliased variable at the call site, and in the case of an access discriminant, contiguous with the object containing the discriminant. This is similar to the way storage for aggregates is typically managed.
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.
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:
25  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 (non-abstract) storage pool type is declared.
Ramification: Note that the Storage_Pool attribute denotes an object, rather than a value, which is somewhat unusual for attributes.
The calls to Allocate, Deallocate, and Storage_Size are dispatching calls -- this follows from the fact that the actual parameter for Pool is T'Storage_Pool, which is of type Root_Storage_Pool'Class. In many cases (including all cases in which Storage_Pool is not specified), the compiler can determine the tag statically. However, it is possible to construct cases where it cannot.
All access types in the same derivation class share the same pool, whether implementation defined or user defined. This is necessary because we allow type conversions among them (even if they are pool-specific), and we want pool-specific access values to always designate an element of the right pool.
Implementation Note: If an access type has a standard storage pool, then the implementation doesn't actually have to follow the pool interface described here, since this would be semantically invisible. For example, the allocator could conceivably be implemented with inline code.


    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:
Pool_Object : Some_Storage_Pool_Type;
type T is access Designated;
for T'Storage_Pool use Pool_Object;
    Another access type may be added to an existing storage pool, via:
for T2'Storage_Pool use T'Storage_Pool;
    The semantics of this is implementation defined for a standard storage pool.
Reason: For example, the implementation is allowed to choose a storage pool for T that takes advantage of the fact that T is of a certain size. If T2 is not of that size, then the above will probably not work.
    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:
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;
MR_Pool : Mark_Release_Pool_Type (Pool_Size => 2000,
                                  Block_Size => 100);
type Acc is access ...;
for Acc'Storage_Pool use MR_Pool;
... -- Allocate objects using ``new Designated(...)''.
Release(MR_Pool); -- Reclaim the storage.

Extensions to Ada 83

{extensions to Ada 83} User-defined storage pools are new to Ada 95.

Wording Changes from Ada 83

Ada 83 had a concept called a ``collection,'' which is similar to what we call a storage pool. All access types in the same derivation class shared the same collection. In Ada 95, all access types in the same derivation class share the same storage pool, but other (unrelated) access types can also share the same storage pool, either by default, or as specified by the user. A collection was an amorphous collection of objects; a storage pool is a more concrete concept -- hence the different name.
RM83 states the erroneousness of reading or updating deallocated objects incorrectly by missing various cases.

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