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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.
    {Tasking_Error (raised by failure of run-time check)} For all the operations declared in this package, Tasking_Error is raised if the task identified by T is terminated. {Program_Error (raised by failure of run-time check)} Program_Error is raised if the value of T is Null_Task_ID.

Bounded (Run-Time) Errors

        {8652/0071} {bounded error (cause) [partial]} 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

    {erroneous execution (cause) [partial]} 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.
    {erroneous execution (cause) [partial]} 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} {erroneous execution (cause) [partial]} Accesses to task attributes via a value of type Attribute_Handle are erroneous if executed concurrently with each other or with calls of any of the operations declared in package Task_Attributes.
Reason: There is no requirement of atomicity on accesses via a value of type Attribute_Handle.

Implementation Requirements

      {8652/0071} For a given attribute of a given task, t The implementation shall perform the operations declared in this package each of the above operations for a given attribute of a given task atomically with respect to any of these operations of other of the above operations for 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.
    When a task terminates, the implementation shall finalize all attributes of the task, and reclaim any other 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.
Implementation defined: Limits on the number and size of task attributes, and how to configure them. Implementation-defined aspects of Task_Attributes.


    The implementation shall document the following metrics: A task calling the following subprograms shall execute in 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 whose size is equal to the size of the predefined integer size. 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, non-terminated, task.
    The following calls (to subprograms in the Task_Attributes package) shall be measured:

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: This permission means that even though every task has every attribute, storage need only be allocated for those attributes that have been Reference'd 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

    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 task's attributes, 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.
12  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.
13  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.
14  As specified in C.7.1, if the parameter T (in a call on a subprogram of an instance of this package) identifies a nonexistent task, the execution of the program is erroneous.

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