- 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 defining_identifier [known_discriminant_part] [is task_definition];
task defining_identifier [is task_definition];
task_item ::= entry_declaration | representation_clause
task body defining_identifier is
- If a task_identifier appears at the end of a task_definition or task_body, it shall repeat the defining_identifier.
- A task declaration requires a completion, which shall be a task_body, and
every task_body shall be the completion of some task declaration.
- 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.
- 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
- As part of the initialization of a task object, any representation_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
- 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);
task type Keyboard_Driver(ID : Keyboard_ID := New_ID) is
entry Read (C : out Character);
entry Write(C : in Character);
- Examples of declarations of single tasks:
task Controller is
entry Request(Level)(D : Item); -- a family of entries
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);
-- Email comments, additions, corrections, gripes, kudos, etc. to:
Magnus Kempe -- Magnus.Kempe@di.epfl.ch
Page last generated: 95-03-12