C.7.3 The Package Task_Termination
The following language-defined library package
package Ada.Task_Termination is
type Cause_Of_Termination is (Normal, Abnormal, Unhandled_Exception);
type Termination_Handler is access protected procedure
(Cause : in Cause_Of_Termination;
T : in Ada.Task_Identification.Task_Id;
X : in Ada.Exceptions.Exception_Occurrence);
(Handler: in Termination_Handler);
function Current_Task_Fallback_Handler return Termination_Handler;
(T : in Ada.Task_Identification.Task_Id;
Handler : in Termination_Handler);
function Specific_Handler (T : Ada.Task_Identification.Task_Id)
Termination_Handler identifies a protected procedure to be executed by
the implementation when a task terminates. Such a protected procedure
is called a handler. In all cases T identifies the task that is
terminating. If the task terminates due to completing the last statement
of its body, or as a result of waiting on a terminate alternative, and
the finalization of the task completes normally, then
Cause is set to Normal and X is set to Null_Occurrence. If the task terminates
because it is being aborted, then Cause is set to Abnormal; and X is set to Null_Occurrence if the finalization of the task completes normally.
If the task terminates because of an exception raised by the execution
of its task_body,
then Cause is set to Unhandled_Exception; and X is set to the associated exception
occurrence if the finalization of the task
completes normally. Independent of how the task completes, if finalization of the task propagates
an exception, then Cause is either Unhandled_Exception or Abnormal, and
X is an exception occurrence that identifies the Program_Error exception.
task has two termination handlers, a fall-back handler and a specific
handler. The specific handler applies only to the task itself, while
the fall-back handler applies only to the dependent tasks of the task.
A handler is said to be set if it is associated with a non-null
value of type Termination_Handler, and cleared otherwise. When
a task is created, its specific handler and fall-back handler are cleared.
The procedure Set_Dependents_Fallback_Handler changes
the fall-back handler for the calling task:; if Handler is null, that fall-back handler is cleared;, otherwise, it is set to be Handler.all. If a fall-back handler had previously
been set it is replaced.
The function Current_Task_Fallback_Handler returns
the fall-back handler that is currently set for the calling task, if
one is set; otherwise, it returns null.
The procedure Set_Specific_Handler changes the
specific handler for the task identified by T:; if Handler is null, that specific handler is cleared;, otherwise, it is set to be Handler.all. If a specific handler had previously
been set it is replaced.
This package cannot portably be used to set a handler
on the program as a whole. It is possible to call Set_Specific_Handler
with the environment task's ID. But any call to the handler would necessarily
be a Bounded (Run-Time) Error, as the handler is called after the task's
finalization has completed. In the case of the environment task, that
includes any possible protected objects, and calling a protected object
after it is finalized is a Bounded (Run-Time) Error (see 9.4).
This might work in a particular implementation, but it cannot be depended
The function Specific_Handler returns the specific
handler that is currently set for the task identified by T, if one is
set; otherwise, it returns null.
As part of the finalization of a task_body,
after performing the actions specified in 7.6
for finalization of a master, the specific handler for the task, if one
is set, is executed. If the specific handler is cleared, a search for
a fall-back handler proceeds by recursively following the master relationship
for the task. If a task is found whose fall-back handler is set, that
handler is executed; otherwise, no handler is executed.
For Set_Specific_Handler or Specific_Handler, Tasking_Error
is raised if the task identified by T has already terminated. Program_Error
is raised if the value of T is Ada.Task_Identification.Null_Task_Id.
An exception propagated from a handler that is
invoked as part of the termination of a task has no effect.
For a call of Set_Specific_Handler or Specific_Handler,
if the task identified by T no longer exists, the execution of the program
Extensions to Ada 95
Wording Changes from Ada 2005
Correction: Specified what is passed to
the handler if the finalization of the task fails after it is completed.
This was not specified at all in Ada 2005, so there is a possibility
that some program depended on some other behavior of an implementation.
But as this case is very unlikely (and only occurs when there is already
a significant bug in the program - so should not occur in fielded systems),
we're not listing this as an inconsistency.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe