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.
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.
Metrics
{
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).
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
