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C.7.2 The Package Task_Attributes

Static Semantics

The following language-defined generic library package exists: 
with Ada.Task_Identification; use Ada.Task_Identification;
   type Attribute is private;
   Initial_Value : in Attribute;
package Ada.Task_Attributes is
   type Attribute_Handle is access all Attribute;
   function Value(T : Task_Id := Current_Task)
     return Attribute;
   function Reference(T : Task_Id := Current_Task)
     return Attribute_Handle;
   procedure Set_Value(Val : in Attribute;
                       T : in Task_Id := Current_Task);
   procedure Reinitialize(T : in Task_Id := Current_Task);
end Ada.Task_Attributes;

Dynamic Semantics

When an instance of Task_Attributes is elaborated in a given active partition, an object of the actual type corresponding to the formal type Attribute is implicitly created for each task (of that partition) that exists and is not yet terminated. This object acts as a user-defined attribute of the task. A task created previously in the partition and not yet terminated has this attribute from that point on. Each task subsequently created in the partition will have this attribute when created. In all these cases, the initial value of the given attribute is Initial_Value.
The Value operation returns the value of the corresponding attribute of T.
The Reference operation returns an access value that designates the corresponding attribute of T.
The Set_Value operation performs any finalization on the old value of the attribute of T and assigns Val to that attribute (see 5.2 and 7.6).
The effect of the Reinitialize operation is the same as Set_Value where the Val parameter is replaced with Initial_Value.
Implementation Note: In most cases, the attribute memory can be reclaimed at this point. 
For all the operations declared in this package, Tasking_Error is raised if the task identified by T is terminated. Program_Error is raised if the value of T is Null_Task_Id.
   {AI95-00237-01} After a task has terminated, all of its attributes are finalized, unless they have been finalized earlier. When the master of an instantiation of Ada.Task_Attributes is finalized, the corresponding attribute of each task is finalized, unless it has been finalized earlier. 
Reason: This is necessary so that a task attribute does not outlive its type. For instance, that's possible if the instantiation is nested, and the attribute is on a library-level task. 
Ramification: The task owning an attribute cannot, in general, finalize that attribute. That's because the attributes are finalized after the task is terminated; moreover, a task may have attributes as soon as it is created; the task may never even have been activated. 

Bounded (Run-Time) Errors

   {8652/0071} {AI95-00165-01} If the package Ada.Task_Attributes is instantiated with a controlled type and the controlled type has user-defined Adjust or Finalize operations that in turn access task attributes by any of the above operations, then a call of Set_Value of the instantiated package constitutes a bounded error. The call may perform as expected or may result in forever blocking the calling task and subsequently some or all tasks of the partition. 

Erroneous Execution

It is erroneous to dereference the access value returned by a given call on Reference after a subsequent call on Reinitialize for the same task attribute, or after the associated task terminates. 
Reason: This allows the storage to be reclaimed for the object associated with an attribute upon Reinitialize or task termination. 
If a value of Task_Id is passed as a parameter to any of the operations declared in this package and the corresponding task object no longer exists, the execution of the program is erroneous.
   {8652/0071} {AI95-00165-01} {AI95-00237-01} An access to a task attribute via a value of type Attribute_Handle is erroneous if executed concurrently with another such access or a call of any of the operations declared in package Task_Attributes. An access to a task attribute is erroneous if executed concurrently with or after the finalization of the task attribute. 
Reason: There is no requirement of atomicity on accesses via a value of type Attribute_Handle. 
Ramification: A task attribute can only be accessed after finalization through a value of type Attribute_Handle. Operations in package Task_Attributes cannot be used to access a task attribute after finalization, because either the master of the instance has been or is in the process of being left (in which case the instance is out of scope and thus cannot be called), or the associated task is already terminated (in which case Tasking_Error is raised for any attempt to call a task attribute operation). 

Implementation Requirements

 {8652/0071} {AI95-00165-01} For a given attribute of a given task, the implementation shall perform the operations declared in this package atomically with respect to any of these operations of the same attribute of the same task. The granularity of any locking mechanism necessary to achieve such atomicity is implementation defined. 
Implementation defined: Granularity of locking for Task_Attributes.
Ramification: Hence, other than by dereferencing an access value returned by Reference, an attribute of a given task can be safely read and updated concurrently by multiple tasks. 
 {AI95-00237-01} After task attributes are finalized, the implementation shall reclaim any storage associated with the attributes. 

Documentation Requirements

The implementation shall document the limit on the number of attributes per task, if any, and the limit on the total storage for attribute values per task, if such a limit exists.
In addition, if these limits can be configured, the implementation shall document how to configure them. 
This paragraph was deleted.
Documentation Requirement: For package Task_Attributes, limits on the number and size of task attributes, and how to configure any limits.


 {AI95-00434-01} The implementation shall document the following metrics: A task calling the following subprograms shall execute at a sufficiently high priority as to not be preempted during the measurement period. This period shall start just before issuing the call and end just after the call completes. If the attributes of task T are accessed by the measurement tests, no other task shall access attributes of that task during the measurement period. For all measurements described here, the Attribute type shall be a scalar type whose size is equal to the size of the predefined type Integer. For each measurement, two cases shall be documented: one where the accessed attributes are of the calling task [(that is, the default value for the T parameter is used)], and the other, where T identifies another, nonterminated, task.
The following calls (to subprograms in the Task_Attributes package) shall be measured: 
a call to Value, where the return value is Initial_Value;
a call to Value, where the return value is not equal to Initial_Value;
a call to Reference, where the return value designates a value equal to Initial_Value;
a call to Reference, where the return value designates a value not equal to Initial_Value;
{AI95-00434-01} a call to Set_Value where the Val parameter is not equal to Initial_Value and the old attribute value is equal to Initial_Value;
a call to Set_Value where the Val parameter is not equal to Initial_Value and the old attribute value is not equal to Initial_Value.
Documentation Requirement: The metrics for the Task_Attributes package.

Implementation Permissions

An implementation need not actually create the object corresponding to a task attribute until its value is set to something other than that of Initial_Value, or until Reference is called for the task attribute. Similarly, when the value of the attribute is to be reinitialized to that of Initial_Value, the object may instead be finalized and its storage reclaimed, to be recreated when needed later. While the object does not exist, the function Value may simply return Initial_Value, rather than implicitly creating the object. 
Discussion: The effect of this permission can only be observed if the assignment operation for the corresponding type has side effects. 
Implementation Note: {AI95-00114-01} This permission means that even though every task has every attribute, storage need only be allocated for those attributes for which function Reference has been invoked or set to a value other than that of Initial_Value.
An implementation is allowed to place restrictions on the maximum number of attributes a task may have, the maximum size of each attribute, and the total storage size allocated for all the attributes of a task.

Implementation Advice

 {AI95-00434-01} Some implementations are targeted to domains in which memory use at run time must be completely deterministic. For such implementations, it is recommended that the storage for task attributes will be pre-allocated statically and not from the heap. This can be accomplished by either placing restrictions on the number and the size of the attributes of a task, or by using the pre-allocated storage for the first N attribute objects, and the heap for the others. In the latter case, N should be documented.
Implementation Advice: If the target domain requires deterministic memory use at run time, storage for task attributes should be pre-allocated statically and the number of attributes pre-allocated should be documented.
Discussion: We don't mention “restrictions on the size and number” (that is, limits) in the text for the Annex, because it is covered by the Documentation Requirement above, and we try not to repeat requirements in the Annex (they're enough work to meet without having to do things twice). 
   {AI95-00237-01} Finalization of task attributes and reclamation of associated storage should be performed as soon as possible after task termination. 
Implementation Advice: Finalization of task attributes and reclamation of associated storage should be performed as soon as possible after task termination.
Reason: {AI95-00237-01} This is necessary because the normative wording only says that attributes are finalized “after” task termination. Without this advice, waiting until the instance is finalized would meet the requirements (it is after termination, but may be a very long time after termination). We can't say anything more specific than this, as we do not want to require the overhead of an interaction with the tasking system to be done at a specific point. 
13  An attribute always exists (after instantiation), and has the initial value. It need not occupy memory until the first operation that potentially changes the attribute value. The same holds true after Reinitialize.
14  The result of the Reference function should be used with care; it is always safe to use that result in the task body whose attribute is being accessed. However, when the result is being used by another task, the programmer must make sure that the task whose attribute is being accessed is not yet terminated. Failing to do so could make the program execution erroneous.

Wording Changes from Ada 95

{8652/0071} {AI95-00165-01} Corrigendum: Clarified that use of task attribute operations from within a task attribute operation (by an Adjust or Finalize call) is a bounded error, and that concurrent use of attribute handles is erroneous.
{AI95-00237-01} Clarified the wording so that the finalization takes place after the termination of the task or when the instance is finalized (whichever is sooner). 

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