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9.3 Task Dependence - Termination of Tasks
Dynamic Semantics
1
{dependence
(of a task on a master)} {task
(dependence)} {task (completion)}
{task (termination)} Each
task (other than an environment task -- see
10.2)
depends on one or more masters (see
7.6.1),
as follows:
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- If the task is created by the evaluation of an allocator
for a given access type, it depends on each master that includes the
elaboration of the declaration of the ultimate ancestor of the given
access type.
3
- If the task is created by the elaboration of an object_declaration,
it depends on each master that includes this elaboration.
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{dependence (of a task on another
task)} Furthermore, if a task depends
on a given master, it is defined to depend on the task that executes
the master, and (recursively) on any master of that task.
4.a
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A task is said to be
completed when
the execution of its corresponding
task_body
is completed. A task is said to be
terminated when any finalization of
the
task_body has been performed (see
7.6.1).
[The first step of finalizing a master (including a
task_body)
is to wait for the termination of any tasks dependent on the master.]
{blocked
(waiting for dependents to terminate) [partial]} The
task executing the master is blocked until all the dependents have terminated.
[Any remaining finalization is then performed and the master is left.]
6/1
Completion
of a task (and the corresponding
task_body)
can occur when the task is blocked at a
select_statement
with an
an open
terminate_alternative
(see
9.7.1); the open
terminate_alternative
is selected if and only if the following conditions are satisfied:
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- The task depends on some completed master;
8
- Each task that depends on the master considered is either
already terminated or similarly blocked at a select_statement
with an open terminate_alternative.
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When both conditions are satisfied, the task considered
becomes completed, together with all tasks that depend on the master
considered that are not yet completed.
9.a
Ramification: Any required
finalization is performed after the selection of terminate_alternatives.
The tasks are not callable during the finalization. In some ways it is
as though they were aborted.
10
8 The full view of a limited
private type can be a task type, or can have subcomponents of a task
type. Creation of an object of such a type creates dependences according
to the full type.
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9 An object_renaming_declaration
defines a new view of an existing entity and hence creates no further
dependence.
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10 The rules given for the
collective completion of a group of tasks all blocked on select_statements
with open terminate_alternatives
ensure that the collective completion can occur only when there are no
remaining active tasks that could call one of the tasks being collectively
completed.
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11 If two or more tasks are
blocked on select_statements with
open terminate_alternatives, and
become completed collectively, their finalization actions proceed concurrently.
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12 The
completion of a task can occur due to any of the following:
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- the raising of an exception during the elaboration of the
declarative_part of the corresponding
task_body;
16
- the completion of the handled_sequence_of_statements
of the corresponding task_body;
17
- the selection of an open terminate_alternative
of a select_statement in the corresponding
task_body;
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Examples
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Example of task
dependence:
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declare
type Global is access Server; -- see 9.1
A, B : Server;
G : Global;
begin
-- activation of A and B
declare
type Local is access Server;
X : Global := new Server; -- activation of X.all
L : Local := new Server; -- activation of L.all
C : Server;
begin
-- activation of C
G := X; -- both G and X designate the same task object
...
end; -- await termination of C and L.all (but not X.all)
...
end; -- await termination of A, B, and G.all
Wording Changes from Ada 83
20.a
20.b
Tasks that used to depend on library packages
in Ada 83, now depend on the (implicit) task_body
of the environment task (see 10.2). Therefore, the
environment task has to wait for them before performing library level finalization
and terminating the partition. In Ada 83 the requirement to wait for tasks that
depended on library packages was not as clear.
20.c
What was "collective termination"
is now "collective completion" resulting from selecting terminate_alternatives.
This is because finalization still occurs for such tasks, and this happens
after selecting the terminate_alternative,
but before termination.
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