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;
generic
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.
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.
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.
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.
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.
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.
Metrics
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;
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 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.
Finalization of task attributes and reclamation
of associated storage should be performed as soon as possible after task
termination.
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.
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