Contents Index Search Previous Next
9.3 Task Dependence - Termination of Tasks
Dynamic Semantics
1
Each
task (other than an environment task -- see
10.2)
depends on one or more masters (see
7.6.1),
as follows:
2
- 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.
4
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.
5
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.
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 open
terminate_alternative
(see
9.7.1); the open
terminate_alternative
is selected if and only if the following conditions are satisfied:
7
- 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.
9
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.
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.
11
9 An object_renaming_declaration
defines a new view of an existing entity and hence creates no further
dependence.
12
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.
13
11 If two or more tasks
are blocked on select_statements
with open terminate_alternatives,
and become completed collectively, their finalization actions proceed
concurrently.
14
12 The
completion of a task can occur due to any of the following:
15
- 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;
18
Examples
19
Example of task
dependence:
20
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
Contents Index Search Previous Next Legal