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C.7.1 The Package Task_Identification

Static Semantics

1
The following language-defined library package exists: 
2/2
{AI95-00362-01} package Ada.Task_Identification is
   pragma Preelaborate(Task_Identification);

   type Task_Id is private;
   pragma Preelaborable_Initialization (Task_Id);

   Null_Task_Id : constant Task_Id;
   function  "=" (Left, Right : Task_Id) return Boolean;
3/1
{8652/0070} {AI95-00101-01}    function  Image        (T : Task_Id) return String;
   function  Current_Task return Task_Id;
   procedure Abort_Task   (T : in out Task_Id);
4
   function  Is_Terminated(T : Task_Id) return Boolean;
   function  Is_Callable  (T : Task_Id) return Boolean;
private
   ... -- not specified by the language
end Ada.Task_Identification;

Dynamic Semantics

5
A value of the type Task_Id identifies an existent task. The constant Null_Task_Id does not identify any task. Each object of the type Task_Id is default initialized to the value of Null_Task_Id.
6
The function "=" returns True if and only if Left and Right identify the same task or both have the value Null_Task_Id.
7
The function Image returns an implementation-defined string that identifies T. If T equals Null_Task_Id, Image returns an empty string. 
7.a
Implementation defined: The result of the Task_Identification.Image attribute.
8
The function Current_Task returns a value that identifies the calling task.
9
The effect of Abort_Task is the same as the abort_statement for the task identified by T. [In addition, if T identifies the environment task, the entire partition is aborted, See E.1.]
10
The functions Is_Terminated and Is_Callable return the value of the corresponding attribute of the task identified by T.
10.a.1/1
Ramification: {8652/0115} {AI95-00206-01} These routines can be called with an argument identifying the environment task. Is_Terminated will always be False for such a call, but Is_Callable (usually True) could be False if the environment task is waiting for the termination of dependent tasks. Thus, a dependent task can use Is_Callable to determine if the main subprogram has completed.
11
For a prefix T that is of a task type [(after any implicit dereference)], the following attribute is defined: 
12
T'Identity
Yields a value of the type Task_Id that identifies the task denoted by T.
13
For a prefix E that denotes an entry_declaration, the following attribute is defined: 
14
E'Caller
Yields a value of the type Task_Id that identifies the task whose call is now being serviced. Use of this attribute is allowed only inside an entry_body or accept_statement corresponding to the entry_declaration denoted by E. 
15
{Program_Error (raised by failure of run-time check)} Program_Error is raised if a value of Null_Task_Id is passed as a parameter to Abort_Task, Is_Terminated, and Is_Callable.
16
{potentially blocking operation (Abort_Task) [partial]} {blocking, potentially (Abort_Task) [partial]} Abort_Task is a potentially blocking operation (see 9.5.1).

Bounded (Run-Time) Errors

17/2
 {AI95-00237-01} {bounded error (cause) [partial]} It is a bounded error to call the Current_Task function from an entry body, or an interrupt handler, or finalization of a task attribute. {Program_Error (raised by failure of run-time check)} Program_Error is raised, or an implementation-defined value of the type Task_Id is returned. 
17.a/2
Implementation defined: The value of Current_Task when in a protected entry, or interrupt handler, or finalization of a task attribute.
17.b
Implementation Note: This value could be Null_Task_Id, or the ID of some user task, or that of an internal task created by the implementation. 
17.c/2
Ramification: {AI95-00237-01} An entry barrier is syntactically part of an entry_body, so a call to Current_Task from an entry barrier is also covered by this rule. 

Erroneous Execution

18
{erroneous execution (cause) [partial]} If a value of Task_Id is passed as a parameter to any of the operations declared in this package (or any language-defined child of this package), and the corresponding task object no longer exists, the execution of the program is erroneous.

Documentation Requirements

19
The implementation shall document the effect of calling Current_Task from an entry body or interrupt handler. 
19.a/2
This paragraph was deleted.Implementation defined: The effect of calling Current_Task from an entry body or interrupt handler.
19.b/2
Documentation Requirement: The effect of calling Current_Task from an entry body or interrupt handler.
NOTES
20
10  This package is intended for use in writing user-defined task scheduling packages and constructing server tasks. Current_Task can be used in conjunction with other operations requiring a task as an argument such as Set_Priority (see D.5).
21
11  The function Current_Task and the attribute Caller can return a Task_Id value that identifies the environment task.

Extensions to Ada 95

21.a/2
{AI95-00362-01} Task_Identification is now preelaborated, so it can be used in preelaborated units. 

Wording Changes from Ada 95

21.b/2
{8652/0070} {AI95-00101-01} Corrigendum: Corrected the mode of the parameter to Abort_Task to in.
21.c/2
{AI95-00237-01} Corrected the wording to include finalization of a task attribute in the bounded error case; we don't want to specify which task does these operations. 

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