9.1 Task Units and Task Objects

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A task unit is declared by a task declaration, which has a corresponding task_body. A task declaration may be a task_type_declaration, in which case it declares a named task type; alternatively, it may be a single_task_declaration, in which case it defines an anonymous task type, as well as declaring a named task object of that type.


task_type_declaration ::=
    task type defining_identifier [known_discriminant_part] [is task_definition];

single_task_declaration ::=
    task defining_identifier [is task_definition];

task_definition ::=
   [ private
   end [task_identifier]

task_item ::= entry_declaration | aspect_clause

task_body ::=
    task body defining_identifier is
    end [task_identifier];

If a task_identifier appears at the end of a task_definition or task_body, it shall repeat the defining_identifier.

Legality Rules

A task declaration requires a completion, which shall be a task_body, and every task_body shall be the completion of some task declaration.

Static Semantics

A task_definition defines a task type and its first subtype. The first list of task_items of a task_definition, together with the known_discriminant_part, if any, is called the visible part of the task unit. The optional list of task_items after the reserved word private is called the private part of the task unit.

For a task declaration without a task_definition, a task_definition without task_items is assumed.

Dynamic Semantics

The elaboration of a task declaration elaborates the task_definition. The elaboration of a single_task_declaration also creates an object of an (anonymous) task type.

The elaboration of a task_definition creates the task type and its first subtype; it also includes the elaboration of the entry_declarations in the given order.

As part of the initialization of a task object, any aspect_clauses and any per-object constraints associated with entry_declarations of the corresponding task_definition are elaborated in the given order.

The elaboration of a task_body has no effect other than to establish that tasks of the type can from then on be activated without failing the Elaboration_Check.

The execution of a task_body is invoked by the activation of a task of the corresponding type (see 9.2).

The content of a task object of a given task type includes:

  • The values of the discriminants of the task object, if any;
  • An entry queue for each entry of the task object;
  • A representation of the state of the associated task.


2  Within the declaration or body of a task unit, the name of the task unit denotes the current instance of the unit (see 8.6), rather than the first subtype of the corresponding task type (and thus the name cannot be used as a subtype_mark).

3  The notation of a selected_component can be used to denote a discriminant of a task (see 4.1.3). Within a task unit, the name of a discriminant of the task type denotes the corresponding discriminant of the current instance of the unit.

4  A task type is a limited type (see 7.5), and hence has neither an assignment operation nor predefined equality operators. If an application needs to store and exchange task identities, it can do so by defining an access type designating the corresponding task objects and by using access values for identification purposes. Assignment is available for such an access type as for any access type. Alternatively, if the implementation supports the Systems Programming Annex, the Identity attribute can be used for task identification (see C.7).


Examples of declarations of task types:

task type Server is 
    entry Next_Work_Item(WI in Work_Item); 
    entry Shut_Down;
end Server;

task type Keyboard_Driver(ID Keyboard_ID := New_ID) is 
    entry Read (C out Character); 
    entry Write(C in  Character);
end Keyboard_Driver;

Examples of declarations of single tasks:

task Controller is 
    entry Request(Level)(D : Item);  --  a family of entries
end Controller;

task Parser is 
    entry Next_Lexeme(L in  Lexical_Element); 
    entry Next_Action(A out Parser_Action);

task User;  --  has no entries

Examples of task objects:

Agent    : Server; 
Teletype : Keyboard_Driver(TTY_ID); 
Pool     : array(1 .. 10) of Keyboard_Driver;

Example of access type designating task objects:

type Keyboard is access Keyboard_Driver; 
Terminal : Keyboard := new Keyboard_Driver(Term_ID);

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