C.7.2 The Package Task_Attributes

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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.

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.

Bounded (Run-Time) Errors

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.

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.

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.

Implementation Requirements

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.

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.


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:

  • 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;
  • 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.

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.

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.

Copyright © 1992,1993,1994,1995 Intermetrics, Inc.
Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual