9.2 Task Execution - Task Activation
The execution of a task of a
given task type consists of the execution of the corresponding task_body
initial part of this execution is called the activation
task; it consists of the elaboration of the declarative_part
of the task_body
Should an exception be propagated by the elaboration
of its declarative_part
the activation of the task is defined to have failed
, and it becomes
a completed task.
A task object (which represents one task) can be a part of a stand-alone
object, of an object created by an allocator
or of an anonymous object of a limited type, or a coextension of one
of these. All tasks that are part or coextensions of any of the stand-alone
objects created by the elaboration of object_declaration
of formal objects of mode in
) of a single declarative region are
activated together. All tasks that are part or coextensions of a single
object that is not a stand-alone object are activated together.
The initialization of an
can indirectly include the creation of other objects that contain tasks.
For example, the default expression for a subcomponent of an object created
by an allocator
might call a function that evaluates a completely different allocator
Tasks created by the two allocators are not
For tasks that are part or coextensions of a single object that is not
a stand-alone object, activations are initiated after completing any
initialization of the outermost object enclosing these tasks, prior to
performing any other operation on the outermost object. In particular,
for tasks that are part or coextensions of the object created by the
evaluation of an allocator
the activations are initiated as the last step of evaluating the allocator
prior to returning the new access value. For tasks that are part or coextensions
of an object that is the result of a function call, the activations are
not initiated until after the function returns.
The intent is that “temporary” objects with task parts (or
coextensions) are treated similarly to an object created by an allocator.
The “whole” object is initialized, and then all of the task
parts (including the coextensions) are activated together. Each such
“whole” object has its own task activation sequence, involving
the activating task being suspended until all the new tasks complete
that created the new tasks and initiated their activations (the activator
is blocked until all of these activations complete (successfully or not).
Once all of these activations are complete, if the
activation of any of the tasks has failed [(due to the propagation of
an exception)], Tasking_Error is raised in the activator, at the place
at which it initiated the activations. Otherwise, the activator proceeds
with its execution normally. Any tasks that are aborted prior to completing
their activation are ignored when determining whether to raise Tasking_Error.
Note that a task created
by an allocator
does not necessarily depend on its activator; in such a case the activator's
termination can precede the termination of the newly created task.
Discussion: Tasking_Error is raised only
once, even if two or more of the tasks being activated fail their activation.
To be honest:
The pragma Partition_Elaboration_Policy (see H.6
can be used to defer task activation to a later point, thus changing
many of these rules.
If the master that directly encloses the point where the activation of
a task T
would be initiated, completes before the activation of
is initiated, T
becomes terminated and is never activated.
Furthermore, if a return statement is left such that the return object
is not returned to the caller, any task that was created as a part of
the return object or one of its coextensions immediately becomes terminated
and is never activated.
The first case can only happen if the activation point of T is not reached
due to an exception being raised or a task or statement being aborted.
Note that this is exclusive; if the master completes normally and starts
finalization, we're already past the activation point.
The second case can happen with an exception being raised in a return
statement, by an exit or goto from an extended_return_statement
or by a return statement being aborted. Any tasks created for the return
object of such a return statement are never activated.
5 An entry of a task can be called before
the task has been activated.
6 If several tasks are activated together,
the execution of any of these tasks need not await the end of the activation
of the other tasks.
7 A task can become completed during its
activation either because of an exception or because it is aborted (see
Example of task
procedure P is
A, B : Server; -- elaborate the task objects A, B
C : Server; -- elaborate the task object C
-- the tasks A, B, C are activated together before the first statement
Wording Changes from Ada 83
We have replaced the term suspended with
blocked, since we didn't want to consider a task blocked when
it was simply competing for execution resources. "Suspended"
is sometimes used more generally to refer to tasks that are not actually
running on some processor, due to the lack of resources.
This subclause has been rewritten in an attempt to improve presentation.
Wording Changes from Ada 95
Adjusted the wording for activating tasks to handle the case of anonymous
function return objects. This is critical; we don't want to be waiting
for the tasks in a return object when we exit the function normally.
Wording Changes from Ada 2005
Corrected the wording that handles tasks that are
never activated to ensure that no lookahead is implied and to make it
clear that tasks created by return statements that never return are never
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe