Section 9: Tasks and Synchronization
1 {execution (Ada program) [partial]} The execution of an Ada program
consists of the execution of one or more tasks. {task}
{interaction (between tasks)} Each task represents a separate thread of control
that proceeds independently and concurrently between the points where it
interacts with other tasks. The various forms of task interaction are
described in this section, and include: {parallel processing: See task}
{synchronization} {concurrent processing: See task}
{intertask communication: See also task}
1.a To be honest: The execution of an Ada program consists of the
execution of one or more partitions (see 10.2), each of which in
turn consists of the execution of an environment task and zero or
more subtasks.
2 * the activation and termination of a task;
3 * {protected object} a call on a protected subprogram of a protected
object, providing exclusive read-write access, or concurrent read-only
access to shared data;
4 * a call on an entry, either of another task, allowing for synchronous
communication with that task, or of a protected object, allowing for
asynchronous communication with one or more other tasks using that
same protected object;
5 * a timed operation, including a simple delay statement, a timed entry
call or accept, or a timed asynchronous select statement (see next
item);
6 * an asynchronous transfer of control as part of an asynchronous select
statement, where a task stops what it is doing and begins execution at
a different point in response to the completion of an entry call or
the expiration of a delay;
7 * an abort statement, allowing one task to cause the termination of
another task.
8 In addition, tasks can communicate indirectly by reading and updating
(unprotected) shared variables, presuming the access is properly synchronized
through some other kind of task interaction.
Static Semantics
9 {task unit} The properties of a task are defined by a corresponding task
declaration and task_body, which together define a program unit called a task
unit.
Dynamic Semantics
10 Over time, tasks proceed through various states. {task state (inactive)
[partial]} {inactive (a task state)} {task state (blocked) [partial]}
{blocked (a task state)} {task state (ready) [partial]} {ready (a task state)}
{task state (terminated) [partial]} {terminated (a task state)} A task is
initially inactive; upon activation, and prior to its termination it is either
blocked (as part of some task interaction) or ready to run.
{execution resource (required for a task to run)} While ready, a task competes
for the available execution resources that it requires to run.
10.a Discussion: {task dispatching policy}
{dispatching policy for tasks} The means for selecting which of the
ready tasks to run, given the currently available execution
resources, is determined by the task dispatching policy in effect,
which is generally implementation defined, but may be controlled
by pragmas and operations defined in the Real-Time Annex (see
D.2 and D.5).
NOTES
11 1 Concurrent task execution may be implemented on multicomputers,
multiprocessors, or with interleaved execution on a single physical
processor. On the other hand, whenever an implementation can determine
that the required semantic effects can be achieved when parts of the
execution of a given task are performed by different physical
processors acting in parallel, it may choose to perform them in this
way.
Wording Changes from Ada 83
11.a The introduction has been rewritten.
11.b We use the term "concurrent" rather than "parallel" when talking
about logically independent execution of threads of control. The
term "parallel" is reserved for referring to the situation where
multiple physical processors run simultaneously.
9.1 Task Units and Task Objects
1 {task declaration} 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.
Syntax
2/2 {AI95-00345-01} task_type_declaration ::=
task type defining_identifier [known_discriminant_part] [is
[new interface_list with]
task_definition];
3/2 {AI95-00399-01} single_task_declaration ::=
task defining_identifier [is
[new interface_list with]
task_definition];
4 task_definition ::=
{task_item}
[ private
{task_item}]
end [task_identifier]
5/1 {8652/0009} {AI95-00137-01} task_item ::= entry_declaration
| aspect_clause
6 task_body ::=
task body defining_identifier is
declarative_part
begin
handled_sequence_of_statements
end [task_identifier];
7 If a task_identifier appears at the end of a task_definition or
task_body, it shall repeat the defining_identifier.
Legality Rules
8/2 This paragraph was deleted.{AI95-00345-01}
8.a/2 This paragraph was deleted.
Static Semantics
9 A task_definition defines a task type and its first subtype.
{visible part (of a task unit) [partial]} 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. [{private part (of a task unit)
[partial]} The optional list of task_items after the reserved word private is
called the private part of the task unit.]
9.a Proof: Private part is defined in Section 8.
9.1/1 {8652/0029} {AI95-00116-01} For a task declaration without a
task_definition, a task_definition without task_items is assumed.
9.2/2 {AI95-00345-01} {AI95-00397-01} {AI95-00399-01} {AI95-00419-01} For a
task declaration with an interface_list, the task type inherits user-defined
primitive subprograms from each progenitor type (see 3.9.4), in the same way
that a derived type inherits user-defined primitive subprograms from its
progenitor types (see 3.4). If the first parameter of a primitive inherited
subprogram is of the task type or an access parameter designating the task
type, and there is an entry_declaration for a single entry with the same
identifier within the task declaration, whose profile is type conformant with
the prefixed view profile of the inherited subprogram, the inherited
subprogram is said to be implemented by the conforming task
entry.{implemented (by a task entry) [partial]} {type conformance (required)}
9.b/2 Ramification: The inherited subprograms can only come from an
interface given as part of the task declaration.
Legality Rules
9.3/2 {AI95-00345-01} {requires a completion (task_declaration}) [partial]} A
task declaration requires a completion[, which shall be a task_body,] and
every task_body shall be the completion of some task declaration.
9.c/2 To be honest: The completion can be a pragma Import, if the
implementation supports it.
9.4/2 {AI95-00345-01} {AI95-00399-01} [Each interface_subtype_mark of an
interface_list appearing within a task declaration shall denote a limited
interface type that is not a protected interface.]
9.d/2 Proof: 3.9.4 requires that an interface_list only name interface
types, and limits the descendants of the various kinds of
interface types. Only a limited, task, or synchronized interface
can have a task type descendant. Nonlimited or protected
interfaces are not allowed, as they offer operations that a task
does not have.
9.5/2 {AI95-00397-01} The prefixed view profile of an explicitly declared
primitive subprogram of a tagged task type shall not be type conformant with
any entry of the task type, if the first parameter of the subprogram is of the
task type or is an access parameter designating the task type.
9.e/2 Reason: This prevents the existence of two operations with the
same name and profile which could be called with a prefixed view.
If the operation was inherited, this would be illegal by the
following rules; this rule puts inherited and non-inherited
routines on the same footing. Note that this only applies to
tagged task types (that is, those with an interface in their
declaration); we do that as there is no problem with prefixed view
calls of primitive operations for "normal" task types, and having
this rule apply to all tasks would be incompatible with Ada 95.
9.6/2 {AI95-00345-01} {AI95-00399-01} For each primitive subprogram inherited
by the type declared by a task declaration, at most one of the following shall
apply:
9.7/2 * {AI95-00345-01} the inherited subprogram is overridden with a
primitive subprogram of the task type, in which case the overriding
subprogram shall be subtype conformant with the inherited subprogram
and not abstract; or{subtype conformance (required)}
9.8/2 * {AI95-00345-01} {AI95-00397-01} the inherited subprogram is
implemented by a single entry of the task type; in which case its
prefixed view profile shall be subtype conformant with that of the
task entry. {subtype conformance (required)}
9.f/2 Ramification: An entry may implement two subprograms from the
ancestors, one whose first parameter is of type T and one whose
first parameter is of type access T. That doesn't cause
implementation problems because "implemented by" (unlike "
overridden') probably entails the creation of wrappers.
9.9/2 If neither applies, the inherited subprogram shall be a null procedure.
{generic contract issue [partial]} In addition to the places where
Legality Rules normally apply (see 12.3), these rules also apply in the
private part of an instance of a generic unit.
9.g/2 Reason: Each inherited subprogram can only have a single
implementation (either from overriding a subprogram or
implementing an entry), and must have an implementation unless the
subprogram is a null procedure.
Dynamic Semantics
10 [{elaboration (task declaration) [partial]} The elaboration of a task
declaration elaborates the task_definition.
{elaboration (single_task_declaration) [partial]} The elaboration of a single_-
task_declaration also creates an object of an (anonymous) task type.]
10.a Proof: This is redundant with the general rules for the
elaboration of a full_type_declaration and an object_declaration.
11 {elaboration (task_definition) [partial]} [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.
12/1 {8652/0009} {AI95-00137-01} {initialization (of a task object)
[partial]} 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.
12.a/1 Reason: The only aspect_clauses defined for task entries are ones
that specify the Address of an entry, as part of defining an
interrupt entry. These clearly need to be elaborated per-object,
not per-type. Normally the address will be a function of a
discriminant, if such an Address clause is in a task type rather
than a single task declaration, though it could rely on a
parameterless function that allocates sequential interrupt vectors.
12.b We do not mention representation pragmas, since each pragma may
have its own elaboration rules.
13 {elaboration (task_body) [partial]} 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.
14 [The execution of a task_body is invoked by the activation of a task of
the corresponding type (see 9.2).]
15 The content of a task object of a given task type includes:
16 * The values of the discriminants of the task object, if any;
17 * An entry queue for each entry of the task object;
17.a Ramification: "For each entry" implies one queue for each single
entry, plus one for each entry of each entry family.
18 * A representation of the state of the associated task.
NOTES
19/2 2 {AI95-00382-01} Other than in an access_definition, the name of a
task unit within the declaration or body 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).
19.a/2 Discussion: {AI95-00382-01} It can be used as a subtype_mark in an
anonymous access type. In addition, it is possible to refer to
some other subtype of the task type within its body, presuming
such a subtype has been declared between the
task_type_declaration and the task_body.
20 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.
21/2 4 {AI95-00287-01} A task type is a limited type (see 7.5), and hence
precludes use of assignment_statements and 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.7.1).
Examples
22 Examples of declarations of task types:
23 task type Server is
entry Next_Work_Item(WI : in Work_Item);
entry Shut_Down;
end Server;
24/2 {AI95-00433-01}
task type Keyboard_Driver(ID : Keyboard_ID := New_ID) is
new Serial_Device with -- see 3.9.4
entry Read (C : out Character);
entry Write(C : in Character);
end Keyboard_Driver;
25 Examples of declarations of single tasks:
26 task Controller is
entry Request(Level)(D : Item); -- a family of entries
end Controller;
27 task Parser is
entry Next_Lexeme(L : in Lexical_Element);
entry Next_Action(A : out Parser_Action);
end;
28 task User; -- has no entries
29 Examples of task objects:
30 Agent : Server;
Teletype : Keyboard_Driver(TTY_ID);
Pool : array(1 .. 10) of Keyboard_Driver;
31 Example of access type designating task objects:
32 type Keyboard is access Keyboard_Driver;
Terminal : Keyboard := new Keyboard_Driver(Term_ID);
Extensions to Ada 83
32.a/1 {extensions to Ada 83} The syntax rules for task declarations are
modified to allow a known_discriminant_part, and to allow a
private part. They are also modified to allow entry_declarations
and aspect_clauses to be mixed.
Wording Changes from Ada 83
32.b The syntax rules for tasks have been split up according to task
types and single tasks. In particular: The syntax rules for
task_declaration and task_specification are removed. The syntax
rules for task_type_declaration, single_task_declaration,
task_definition and task_item are new.
32.c The syntax rule for task_body now uses the nonterminal
handled_sequence_of_statements.
32.d The declarative_part of a task_body is now required; that doesn't
make any real difference, because a declarative_part can be empty.
Extensions to Ada 95
32.e/2 {AI95-00345-01} {AI95-00397-01} {AI95-00399-01} {AI95-00419-01}
{extensions to Ada 95} Task types and single tasks can be derived
from one or more interfaces. Entries of the task type can
implement the primitive operations of an interface.
Overriding_indicators can be used to specify whether or not an
entry implements a primitive operation.
Wording Changes from Ada 95
32.f/2 {8652/0029} {AI95-00116-01} Corrigendum: Clarified that a task
type has an implicit empty task_definition if none is given.
32.g/2 {8652/0009} {AI95-00137-01} Corrigendum: Changed representation
clauses to aspect clauses to reflect that they are used for more
than just representation.
32.h/2 {AI95-00287-01} Revised the note on operations of task types to
reflect that limited types do have an assignment operation, but
not copying (assignment_statements).
32.i/2 {AI95-00382-01} Revised the note on use of the name of a task type
within itself to reflect the exception for anonymous access types.
9.2 Task Execution - Task Activation
Dynamic Semantics
1 {execution (task) [partial]} The execution of a task of a given task type
consists of the execution of the corresponding task_body.
{execution (task_body) [partial]} {task (execution)} {activation (of a task)}
{task (activation)} The initial part of this execution is called the activation
of the task; it consists of the elaboration of the declarative_part of the
task_body. {activation failure} 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.
2/2 {AI95-00416-01} 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_declarations (or generic_associations 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.
2.a Discussion: The initialization of an object_declaration or
allocator 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 activated together.
3/2 {AI95-00416-01} For the tasks of a given declarative region, the
activations are initiated within the context of the handled_sequence_of_-
statements (and its associated exception_handlers if any - see 11.2), just
prior to executing the statements of the handled_sequence_of_statements. [For
a package without an explicit body or an explicit
handled_sequence_of_statements, an implicit body or an implicit null_statement
is assumed, as defined in 7.2.]
3.a Ramification: If Tasking_Error is raised, it can be handled by
handlers of the handled_sequence_of_statements.
4/2 {AI95-00416-01} 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.
4.a/2 Discussion: {AI95-00416-01} 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 their activation.
5 {activator (of a task)} {blocked (waiting for activations to complete)
[partial]} The task that created the new tasks and initiated their
activations (the activator) is blocked until all of these activations complete
(successfully or not). {Tasking_Error (raised by failure of run-time check)}
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.
5.a Ramification: 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.
5.b Discussion: Tasking_Error is raised only once, even if two or more
of the tasks being activated fail their activation.
5.c/2 To be honest: {AI95-00265-01} 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.
6 Should the task that created the new tasks never reach the point where it
would initiate the activations (due to an abort or the raising of an
exception), the newly created tasks become terminated and are never activated.
NOTES
7 5 An entry of a task can be called before the task has been activated.
8 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.
9 7 A task can become completed during its activation either because of
an exception or because it is aborted (see 9.8).
Examples
10 Example of task activation:
11 procedure P is
A, B : Server; -- elaborate the task objects A, B
C : Server; -- elaborate the task object C
begin
-- the tasks A, B, C are activated together before the first statement
...
end;
Wording Changes from Ada 83
11.a 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.
11.b This clause has been rewritten in an attempt to improve
presentation.
Wording Changes from Ada 95
11.c/2 {AI95-00416-01} 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.
9.3 Task Dependence - Termination of Tasks
Dynamic Semantics
1 {dependence (of a task on a master)} {task (dependence)}
{task (completion)} {task (termination)} Each task (other than an environment
task - see 10.2) depends on one or more masters (see 7.6.1), as follows:
2 * If the task is created by the evaluation of an allocator for a given
access type, it depends on each master that includes the elaboration
of the declaration of the ultimate ancestor of the given access type.
3 * If the task is created by the elaboration of an object_declaration, it
depends on each master that includes this elaboration.
3.1/2 * {AI95-00416-01} Otherwise, the task depends on the master of the
outermost object of which it is a part (as determined by the
accessibility level of that object - see 3.10.2 and 7.6.1), as well as
on any master whose execution includes that of the master of the
outermost object.
3.a/2 Ramification: {AI95-00416-01} The master of a task created by a
return statement changes when the accessibility of the return
object changes. Note that its activation happens, if at all, only
after the function returns and all accessibility level changes
have occurred.
4 {dependence (of a task on another task)} Furthermore, if a task depends on
a given master, it is defined to depend on the task that executes the master,
and (recursively) on any master of that task.
4.a Discussion: Don't confuse these kinds of dependences with the
dependences among compilation units defined in 10.1.1, "
Compilation Units - Library Units".
5 A task is said to be completed when the execution of its corresponding
task_body is completed. A task is said to be terminated when any finalization
of the task_body has been performed (see 7.6.1). [The first step of finalizing
a master (including a task_body) is to wait for the termination of any tasks
dependent on the master.] {blocked (waiting for dependents to terminate)
[partial]} The task executing the master is blocked until all the dependents
have terminated. [Any remaining finalization is then performed and the master
is left.]
6/1 Completion of a task (and the corresponding task_body) can occur when the
task is blocked at a select_statement with an open terminate_alternative (see
9.7.1); the open terminate_alternative is selected if and only if the
following conditions are satisfied:
7/2 * {AI95-00415-01} The task depends on some completed master; and
8 * Each task that depends on the master considered is either already
terminated or similarly blocked at a select_statement with an open
terminate_alternative.
9 When both conditions are satisfied, the task considered becomes completed,
together with all tasks that depend on the master considered that are not yet
completed.
9.a Ramification: Any required finalization is performed after the
selection of terminate_alternatives. The tasks are not callable
during the finalization. In some ways it is as though they were
aborted.
NOTES
10 8 The full view of a limited private type can be a task type, or can
have subcomponents of a task type. Creation of an object of such a
type creates dependences according to the full type.
11 9 An object_renaming_declaration defines a new view of an existing
entity and hence creates no further dependence.
12 10 The rules given for the collective completion of a group of tasks
all blocked on select_statements with open terminate_alternatives
ensure that the collective completion can occur only when there are no
remaining active tasks that could call one of the tasks being
collectively completed.
13 11 If two or more tasks are blocked on select_statements with open
terminate_alternatives, and become completed collectively, their
finalization actions proceed concurrently.
14 12 The completion of a task can occur due to any of the following:
15 * the raising of an exception during the elaboration of the
declarative_part of the corresponding task_body;
16 * the completion of the handled_sequence_of_statements of the
corresponding task_body;
17 * the selection of an open terminate_alternative of a
select_statement in the corresponding task_body;
18 * the abort of the task.
Examples
19 Example of task dependence:
20 declare
type Global is access Server; -- see 9.1
A, B : Server;
G : Global;
begin
-- activation of A and B
declare
type Local is access Server;
X : Global := new Server; -- activation of X.all
L : Local := new Server; -- activation of L.all
C : Server;
begin
-- activation of C
G := X; -- both G and X designate the same task object
...
end; -- await termination of C and L.all (but not X.all)
...
end; -- await termination of A, B, and G.all
Wording Changes from Ada 83
20.a We have revised the wording to be consistent with the definition
of master now given in 7.6.1, "Completion and Finalization".
20.b Tasks that used to depend on library packages in Ada 83, now
depend on the (implicit) task_body of the environment task (see
10.2). Therefore, the environment task has to wait for them before
performing library level finalization and terminating the
partition. In Ada 83 the requirement to wait for tasks that
depended on library packages was not as clear.
20.c What was "collective termination" is now "collective completion"
resulting from selecting terminate_alternatives. This is because
finalization still occurs for such tasks, and this happens after
selecting the terminate_alternative, but before termination.
Wording Changes from Ada 95
20.d/2 {AI95-00416-01} Added missing wording that explained the master of
tasks that are neither object declarations nor allocators, such as
function returns.
9.4 Protected Units and Protected Objects
1 {protected object} {protected operation} {protected subprogram}
{protected entry} A protected object provides coordinated access to shared
data, through calls on its visible protected operations, which can be
protected subprograms or protected entries. {protected declaration}
{protected unit} {protected declaration} A protected unit is declared by a
protected declaration, which has a corresponding protected_body. A protected
declaration may be a protected_type_declaration, in which case it declares a
named protected type; alternatively, it may be a
single_protected_declaration, in which case it defines an anonymous protected
type, as well as declaring a named protected object of that type.
{broadcast signal: See protected object}
Syntax
2/2 {AI95-00345-01} protected_type_declaration ::=
protected type defining_identifier [known_discriminant_part] is
[new interface_list with]
protected_definition;
3/2 {AI95-00399-01} single_protected_declaration ::=
protected defining_identifier is
[new interface_list with]
protected_definition;
4 protected_definition ::=
{ protected_operation_declaration }
[ private
{ protected_element_declaration } ]
end [protected_identifier]
5/1 {8652/0009} {AI95-00137-01} protected_operation_declaration ::=
subprogram_declaration
| entry_declaration
| aspect_clause
6 protected_element_declaration ::= protected_operation_declaration
| component_declaration
6.a Reason: We allow the operations and components to be mixed because
that's how other things work (for example, package declarations).
We have relaxed the ordering rules for the items inside
declarative_parts and task_definitions as well.
7 protected_body ::=
protected body defining_identifier is
{ protected_operation_item }
end [protected_identifier];
8/1 {8652/0009} {AI95-00137-01} protected_operation_item ::=
subprogram_declaration
| subprogram_body
| entry_body
| aspect_clause
9 If a protected_identifier appears at the end of a
protected_definition or protected_body, it shall repeat the
defining_identifier.
Legality Rules
10/2 This paragraph was deleted.{AI95-00345-01}
10.a/2 This paragraph was deleted.
Static Semantics
11/2 {AI95-00345-01} {AI95-00401-01} A protected_definition defines a
protected type and its first subtype. {visible part (of a protected unit)
[partial]} The list of protected_operation_declarations of a protected_-
definition, together with the known_discriminant_part, if any, is called the
visible part of the protected unit. [{private part (of a protected unit)
[partial]} The optional list of protected_element_declarations after the
reserved word private is called the private part of the protected unit.]
11.a Proof: Private part is defined in Section 8.
11.1/2 {AI95-00345-01} {AI95-00397-01} {AI95-00399-01} {AI95-00419-01} For a
protected declaration with an interface_list, the protected type inherits
user-defined primitive subprograms from each progenitor type (see 3.9.4), in
the same way that a derived type inherits user-defined primitive subprograms
from its progenitor types (see 3.4). If the first parameter of a primitive
inherited subprogram is of the protected type or an access parameter
designating the protected type, and there is a
protected_operation_declaration for a protected subprogram or single entry
with the same identifier within the protected declaration, whose profile is
type conformant with the prefixed view profile of the inherited subprogram,
the inherited subprogram is said to be implemented by the conforming protected
subprogram or entry.{implemented (by a protected subprogram) [partial]}
{implemented (by a protected entry) [partial]} {type conformance (required)}
11.b/2 Ramification: The inherited subprograms can only come from an
interface given as part of the protected declaration.
Legality Rules
11.2/2 {AI95-00345-01} {requires a completion (protected_declaration})
[partial]} A protected declaration requires a completion[, which shall be a
protected_body,] and every protected_body shall be the completion of some
protected declaration.
11.c/2 To be honest: The completion can be a pragma Import, if the
implementation supports it.
11.3/2 {AI95-00345-01} {AI95-00399-01} [Each interface_subtype_mark of an
interface_list appearing within a protected declaration shall denote a limited
interface type that is not a task interface.]
11.d/2 Proof: 3.9.4 requires that an interface_list only name interface
types, and limits the descendants of the various kinds of
interface types. Only a limited, protected, or synchronized
interface can have a protected type descendant. Nonlimited or task
interfaces are not allowed, as they offer operations that a
protected type does not have.
11.4/2 {AI95-00397-01} The prefixed view profile of an explicitly declared
primitive subprogram of a tagged protected type shall not be type conformant
with any protected operation of the protected type, if the first parameter of
the subprogram is of the protected type or is an access parameter designating
the protected type.{type conformance (required)}
11.e/2 Reason: This prevents the existence of two operations with the
same name and profile which could be called with a prefixed view.
If the operation was inherited, this would be illegal by the
following rules; this rule puts inherited and non-inherited
routines on the same footing. Note that this only applies to
tagged protected types (that is, those with an interface in their
declaration); we do that as there is no problem with prefixed view
calls of primitive operations for "normal" protected types, and
having this rule apply to all protected types would be
incompatible with Ada 95.
11.5/2 {AI95-00345-01} {AI95-00399-01} For each primitive subprogram inherited
by the type declared by a protected declaration, at most one of the following
shall apply:
11.6/2 * {AI95-00345-01} the inherited subprogram is overridden with a
primitive subprogram of the protected type, in which case the
overriding subprogram shall be subtype conformant with the inherited
subprogram and not abstract; or{subtype conformance (required)}
11.7/2 * {AI95-00345-01} {AI95-00397-01} the inherited subprogram is
implemented by a protected subprogram or single entry of the protected
type, in which case its prefixed view profile shall be subtype
conformant with that of the protected subprogram or entry.
{subtype conformance (required)}
11.8/2 If neither applies, the inherited subprogram shall be a null procedure.
{generic contract issue [partial]} In addition to the places where
Legality Rules normally apply (see 12.3), these rules also apply in the
private part of an instance of a generic unit.
11.f/2 Reason: Each inherited subprogram can only have a single
implementation (either from overriding a subprogram, implementing
a subprogram, or implementing an entry), and must have an
implementation unless the subprogram is a null procedure.
11.9/2 {AI95-00345-01} If an inherited subprogram is implemented by a
protected procedure or an entry, then the first parameter of the inherited
subprogram shall be of mode out or in out, or an access-to-variable parameter.
11.g/2 Reason: For a protected procedure or entry, the protected object
can be read or written (see 9.5.1}. A subprogram that is
implemented by a protected procedure or entry must have a profile
which reflects that in order to avoid confusion.
11.10/2 {AI95-00397-01} If a protected subprogram declaration has an
overriding_indicator, then at the point of the declaration:
11.11/2 * if the overriding_indicator is overriding, then the subprogram
shall implement an inherited subprogram;
11.12/2 * if the overriding_indicator is not overriding, then the subprogram
shall not implement any inherited subprogram.
11.13/2 {generic contract issue [partial]} In addition to the places where
Legality Rules normally apply (see 12.3), these rules also apply in the
private part of an instance of a generic unit.
11.h/2 Discussion: These rules are subtly different than those for
subprograms (see 8.3.1) because there cannot be "late" inheritance
of primitives from interfaces. Hidden (that is, private)
interfaces are prohibited explicitly (see 7.3), as are hidden
primitive operations (as private operations of public abstract
types are prohibited - see 3.9.3).
Dynamic Semantics
12 [{elaboration (protected declaration) [partial]} The elaboration of a
protected declaration elaborates the protected_definition.
{elaboration (single_protected_declaration) [partial]} The elaboration of a
single_protected_declaration also creates an object of an (anonymous)
protected type.]
12.a Proof: This is redundant with the general rules for the
elaboration of a full_type_declaration and an object_declaration.
13 {elaboration (protected_definition) [partial]} [The elaboration of a
protected_definition creates the protected type and its first subtype;] it
also includes the elaboration of the component_declarations and
protected_operation_declarations in the given order.
14 [{initialization (of a protected object) [partial]} As part of the
initialization of a protected object, any per-object constraints (see 3.8) are
elaborated.]
14.a Discussion: We do not mention pragmas since each pragma has its
own elaboration rules.
15 {elaboration (protected_body) [partial]} The elaboration of a
protected_body has no other effect than to establish that protected operations
of the type can from then on be called without failing the Elaboration_Check.
16 The content of an object of a given protected type includes:
17 * The values of the components of the protected object, including
(implicitly) an entry queue for each entry declared for the protected
object;
17.a Ramification: "For each entry" implies one queue for each single
entry, plus one for each entry of each entry family.
18 * {execution resource (associated with a protected object) [partial]} A
representation of the state of the execution resource associated with
the protected object (one such resource is associated with each
protected object).
19 [The execution resource associated with a protected object has to be
acquired to read or update any components of the protected object; it can be
acquired (as part of a protected action - see 9.5.1) either for concurrent
read-only access, or for exclusive read-write access.]
20 {finalization (of a protected object) [partial]}
{Program_Error (raised by failure of run-time check)} As the first step of the
finalization of a protected object, each call remaining on any entry queue of
the object is removed from its queue and Program_Error is raised at the place
of the corresponding entry_call_statement.
20.a Reason: This is analogous to the raising of Tasking_Error in
callers of a task that completes before accepting the calls. This
situation can only occur due to a requeue (ignoring premature
unchecked_deallocation), since any task that has accessibility to
a protected object is awaited before finalizing the protected
object. For example:
20.b procedure Main is
task T is
entry E;
end T;
20.c task body T is
protected PO is
entry Ee;
end PO;
20.d protected body PO is
entry Ee when False is
begin
null;
end Ee;
end PO;
begin
accept E do
requeue PO.Ee;
end E;
end T;
begin
T.E;
end Main;
20.e The environment task is queued on PO.EE when PO is finalized.
20.f In a real example, a server task might park callers on a local
protected object for some useful purpose, so we didn't want to
disallow this case.
Bounded (Run-Time) Errors
20.1/2 {AI95-00280-01} {bounded error (cause) [partial]} It is a bounded error
to call an entry or subprogram of a protected object after that object is
finalized. If the error is detected, Program_Error is raised. Otherwise, the
call proceeds normally, which may leave a task queued forever.
20.g/2 Reason: This is very similar to the finalization rule. It is a
bounded error so that an implementation can avoid the overhead of
the check if it can ensure that the call still will operate
properly. Such an implementation cannot need to return resources
(such as locks) to an executive that it needs to execute calls.
20.h/2 This case can happen (and has happened in production code) when a
protected object is accessed from the Finalize routine of a type.
For example:
20.i/2 with Ada.Finalization.Controlled;
package Window_Manager is
...
type Root_Window is new Ada.Finalization.Controlled with private;
type Any_Window is access all Root_Window;
...
private
...
procedure Finalize (Object : in out Root_Window);
...
end Window_Manager;
20.j/2 package body Window_Manager is
protected type Lock is
entry Get_Lock;
procedure Free_Lock;
...
end Lock;
20.k/2 Window_Lock : Lock;
20.l/2 procedure Finalize (Object : in out Root_Window) is
begin
Window_Lock.Get_Lock;
...
Window_Lock.Free_Lock;
end Finalize;
...
A_Window : Any_Window := new Root_Window;
end Window_Manager;
20.m/2 The environment task will call Window_Lock for the object
allocated for A_Window when the collection for Any_Window is
finalized, which will happen after the finalization of Window_Lock
(because finalization of the package body will occur before that
of the package specification).
NOTES
21/2 13 {AI95-00382-01} Within the declaration or body of a protected unit
other than in an access_definition, the name of the protected unit
denotes the current instance of the unit (see 8.6), rather than the
first subtype of the corresponding protected type (and thus the name
cannot be used as a subtype_mark).
21.a/2 Discussion: {AI95-00382-01} It can be used as a subtype_mark in an
anonymous access type. In addition, it is possible to refer to
some other subtype of the protected type within its body,
presuming such a subtype has been declared between the
protected_type_declaration and the protected_body.
22 14 A selected_component can be used to denote a discriminant of a
protected object (see 4.1.3). Within a protected unit, the name of a
discriminant of the protected type denotes the corresponding
discriminant of the current instance of the unit.
23/2 15 {AI95-00287-01} A protected type is a limited type (see 7.5), and
hence precludes use of assignment_statements and predefined equality
operators.
24 16 The bodies of the protected operations given in the
protected_body define the actions that take place upon calls to the
protected operations.
25 17 The declarations in the private part are only visible within the
private part and the body of the protected unit.
25.a Reason: Component_declarations are disallowed in a
protected_body because, for efficiency, we wish to allow the
compiler to determine the size of protected objects (when not
dynamic); the compiler cannot necessarily see the body.
Furthermore, the semantics of initialization of such objects would
be problematic - we do not wish to give protected objects complex
initialization semantics similar to task activation.
25.b The same applies to entry_declarations, since an entry involves an
implicit component - the entry queue.
Examples
26 Example of declaration of protected type and corresponding body:
27 protected type Resource is
entry Seize;
procedure Release;
private
Busy : Boolean := False;
end Resource;
28 protected body Resource is
entry Seize when not Busy is
begin
Busy := True;
end Seize;
29 procedure Release is
begin
Busy := False;
end Release;
end Resource;
30 Example of a single protected declaration and corresponding body:
31 protected Shared_Array is
-- Index, Item, and Item_Array are global types
function Component (N : in Index) return Item;
procedure Set_Component(N : in Index; E : in Item);
private
Table : Item_Array(Index) := (others => Null_Item);
end Shared_Array;
32 protected body Shared_Array is
function Component(N : in Index) return Item is
begin
return Table(N);
end Component;
33 procedure Set_Component(N : in Index; E : in Item) is
begin
Table(N) := E;
end Set_Component;
end Shared_Array;
34 Examples of protected objects:
35 Control : Resource;
Flags : array(1 .. 100) of Resource;
Extensions to Ada 83
35.a {extensions to Ada 83} This entire clause is new; protected units
do not exist in Ada 83.
Extensions to Ada 95
35.b/2 {AI95-00345-01} {AI95-00397-01} {AI95-00399-01} {AI95-00401-01}
{AI95-00419-01} {extensions to Ada 95} Protected types and single
protected objects can be derived from one or more interfaces.
Operations declared in the protected type can implement the
primitive operations of an interface. Overriding_indicators can be
used to specify whether or not a protected operation implements a
primitive operation.
Wording Changes from Ada 95
35.c/2 {8652/0009} {AI95-00137-01} Corrigendum: Changed representation
clauses to aspect clauses to reflect that they are used for more
than just representation.
35.d/2 {AI95-00280-01} Described what happens when an operation of a
finalized protected object is called.
35.e/2 {AI95-00287-01} Revised the note on operations of protected types
to reflect that limited types do have an assignment operation, but
not copying (assignment_statements).
35.f/2 {AI95-00382-01} Revised the note on use of the name of a protected
type within itself to reflect the exception for anonymous access
types.
9.5 Intertask Communication
1 {intertask communication} {critical section: See intertask communication}
The primary means for intertask communication is provided by calls on entries
and protected subprograms. Calls on protected subprograms allow coordinated
access to shared data objects. Entry calls allow for blocking the caller until
a given condition is satisfied (namely, that the corresponding entry is open -
see 9.5.3), and then communicating data or control information directly with
another task or indirectly via a shared protected object.
Static Semantics
2 {target object (of a call on an entry or a protected subprogram)} Any call
on an entry or on a protected subprogram identifies a target object for the
operation, which is either a task (for an entry call) or a protected object
(for an entry call or a protected subprogram call). The target object is
considered an implicit parameter to the operation, and is determined by the
operation name (or prefix) used in the call on the operation, as follows:
3 * If it is a direct_name or expanded name that denotes the declaration
(or body) of the operation, then the target object is implicitly
specified to be the current instance of the task or protected unit
immediately enclosing the operation; {internal call} such a call is
defined to be an internal call;
4 * If it is a selected_component that is not an expanded name, then the
target object is explicitly specified to be the task or protected
object denoted by the prefix of the name; {external call} such a call
is defined to be an external call;
4.a Discussion: For example:
4.b protected type Pt is
procedure Op1;
procedure Op2;
end Pt;
4.c PO : Pt;
Other_Object : Some_Other_Protected_Type;
4.d protected body Pt is
procedure Op1 is begin ... end Op1;
4.e procedure Op2 is
begin
Op1; -- An internal call.
Pt.Op1; -- Another internal call.
PO.Op1; -- An external call. It the current instance is PO, then
-- this is a bounded error (see 9.5.1).
Other_Object.Some_Op; -- An external call.
end Op2;
end Pt;
5 * If the name or prefix is a dereference (implicit or explicit) of an
access-to-protected-subprogram value, then the target object is
determined by the prefix of the Access attribute_reference that
produced the access value originally, and the call is defined to be an
external call;
6 * If the name or prefix denotes a subprogram_renaming_declaration, then
the target object is as determined by the name of the renamed entity.
7 {target object (of a requeue_statement)} {internal requeue}
{external requeue} A corresponding definition of target object applies to a
requeue_statement (see 9.5.4), with a corresponding distinction between an
internal requeue and an external requeue.
Legality Rules
7.1/2 {AI95-00345-01} The view of the target protected object associated with
a call of a protected procedure or entry shall be a variable.
Dynamic Semantics
8 Within the body of a protected operation, the current instance (see 8.6)
of the immediately enclosing protected unit is determined by the target object
specified (implicitly or explicitly) in the call (or requeue) on the protected
operation.
8.a To be honest: The current instance is defined in the same way
within the body of a subprogram declared immediately within a
protected_body.
9 Any call on a protected procedure or entry of a target protected object is
defined to be an update to the object, as is a requeue on such an entry.
9.a Reason: Read/write access to the components of a protected object
is granted while inside the body of a protected procedure or
entry. Also, any protected entry call can change the value of the
Count attribute, which represents an update. Any protected
procedure call can result in servicing the entries, which again
might change the value of a Count attribute.
Wording Changes from Ada 95
9.b/2 {AI95-00345-01} Added a Legality Rule to make it crystal-clear
that the protected object of an entry or procedure call must be a
variable. This rule was implied by the Dynamic Semantics here,
along with the Static Semantics of 3.3, but it is much better to
explicitly say it. While many implementations have gotten this
wrong, this is not an incompatibility - allowing updates of
protected constants has always been wrong.
9.5.1 Protected Subprograms and Protected Actions
1 {protected subprogram} {protected procedure} {protected function} A
protected subprogram is a subprogram declared immediately within a
protected_definition. Protected procedures provide exclusive read-write access
to the data of a protected object; protected functions provide concurrent
read-only access to the data.
1.a Ramification: A subprogram declared immediately within a
protected_body is not a protected subprogram; it is an intrinsic
subprogram. See 6.3.1, "Conformance Rules".
Static Semantics
2 Within the body of a protected function (or a function declared
immediately within a protected_body), the current instance of the enclosing
protected unit is defined to be a constant [(that is, its subcomponents may be
read but not updated)]. Within the body of a protected procedure (or a
procedure declared immediately within a protected_body), and within an
entry_body, the current instance is defined to be a variable [(updating is
permitted)].
2.a Ramification: The current instance is like an implicit parameter,
of mode in for a protected function, and of mode in out for a
protected procedure (or protected entry).
Dynamic Semantics
3 {execution (protected subprogram call) [partial]} For the execution of a
call on a protected subprogram, the evaluation of the name or prefix and of
the parameter associations, and any assigning back of in out or out
parameters, proceeds as for a normal subprogram call (see 6.4). If the call is
an internal call (see 9.5), the body of the subprogram is executed as for a
normal subprogram call. If the call is an external call, then the body of the
subprogram is executed as part of a new protected action on the target
protected object; the protected action completes after the body of the
subprogram is executed. [A protected action can also be started by an entry
call (see 9.5.3).]
4 {protected action} A new protected action is not started on a protected
object while another protected action on the same protected object is
underway, unless both actions are the result of a call on a protected
function. This rule is expressible in terms of the execution resource
associated with the protected object:
5 * {protected action (start)}
{acquire (execution resource associated with protected object)}
Starting a protected action on a protected object corresponds to
acquiring the execution resource associated with the protected object,
either for concurrent read-only access if the protected action is for
a call on a protected function, or for exclusive read-write access
otherwise;
6 * {protected action (complete)}
{release (execution resource associated with protected object)}
Completing the protected action corresponds to releasing the
associated execution resource.
7 [After performing an operation on a protected object other than a call on
a protected function, but prior to completing the associated protected action,
the entry queues (if any) of the protected object are serviced (see 9.5.3).]
Bounded (Run-Time) Errors
8 {bounded error (cause) [partial]} During a protected action, it is a
bounded error to invoke an operation that is potentially blocking.
{potentially blocking operation} {blocking, potentially} The following are
defined to be potentially blocking operations:
8.a Reason: Some of these operations are not directly blocking.
However, they are still treated as bounded errors during a
protected action, because allowing them might impose an
undesirable implementation burden.
9 * a select_statement;
10 * an accept_statement;
11 * an entry_call_statement;
12 * a delay_statement;
13 * an abort_statement;
14 * task creation or activation;
15 * an external call on a protected subprogram (or an external requeue)
with the same target object as that of the protected action;
15.a Reason: This is really a deadlocking call, rather than a blocking
call, but we include it in this list for simplicity.
16 * a call on a subprogram whose body contains a potentially blocking
operation.
16.a Reason: This allows an implementation to check and raise
Program_Error as soon as a subprogram is called, rather than
waiting to find out whether it actually reaches the potentially
blocking operation. This in turn allows the potentially blocking
operation check to be performed prior to run time in some
environments.
17 {Program_Error (raised by failure of run-time check)} If the bounded error
is detected, Program_Error is raised. If not detected, the bounded error might
result in deadlock or a (nested) protected action on the same target object.
17.a/2 Discussion: {AI95-00305-01} By "nested protected action", we mean
that an additional protected action can be started by another task
on the same protected object. This means that mutual exclusion may
be broken in this bounded error case. A way to ensure that this
does not happen is to use pragma Detect_Blocking (see H.5).
18 Certain language-defined subprograms are potentially blocking. In
particular, the subprograms of the language-defined input-output packages that
manipulate files (implicitly or explicitly) are potentially blocking. Other
potentially blocking subprograms are identified where they are defined. When
not specified as potentially blocking, a language-defined subprogram is
nonblocking.
18.a/2 Discussion: {AI95-00178-01} Any subprogram in a language-defined
input-output package that has a file parameter or result or
operates on a default file is considered to manipulate a file. An
instance of a language-defined input-output generic package
provides subprograms that are covered by this rule. The only
subprograms in language-defined input-output packages not covered
by this rule (and thus not potentially blocking) are the Get and
Put routines that take string parameters defined in the packages
nested in Text_IO.
NOTES
19 18 If two tasks both try to start a protected action on a protected
object, and at most one is calling a protected function, then only one
of the tasks can proceed. Although the other task cannot proceed, it
is not considered blocked, and it might be consuming processing
resources while it awaits its turn. There is no language-defined
ordering or queuing presumed for tasks competing to start a protected
action - on a multiprocessor such tasks might use busy-waiting; for
monoprocessor considerations, see D.3, "Priority Ceiling Locking".
19.a Discussion: The intended implementation on a multi-processor is in
terms of "spin locks" - the waiting task will spin.
20 19 The body of a protected unit may contain declarations and bodies
for local subprograms. These are not visible outside the protected
unit.
21 20 The body of a protected function can contain internal calls on
other protected functions, but not protected procedures, because the
current instance is a constant. On the other hand, the body of a
protected procedure can contain internal calls on both protected
functions and procedures.
22 21 From within a protected action, an internal call on a protected
subprogram, or an external call on a protected subprogram with a
different target object is not considered a potentially blocking
operation.
22.a Reason: This is because a task is not considered blocked while
attempting to acquire the execution resource associated with a
protected object. The acquisition of such a resource is rather
considered part of the normal competition for execution resources
between the various tasks that are ready. External calls that turn
out to be on the same target object are considered potentially
blocking, since they can deadlock the task indefinitely.
22.1/2 22 {AI95-00305-01} The pragma Detect_Blocking may be used to ensure
that all executions of potentially blocking operations during a
protected action raise Program_Error. See H.5.
Examples
23 Examples of protected subprogram calls (see 9.4):
24 Shared_Array.Set_Component(N, E);
E := Shared_Array.Component(M);
Control.Release;
Wording Changes from Ada 95
24.a/2 {AI95-00305-01} Added a note pointing out the existence of
pragma Detect_Blocking. This pragma can be used to ensure portable
(somewhat pessimistic) behavior of protected actions by converting
the Bounded Error into a required check.
9.5.2 Entries and Accept Statements
1 Entry_declarations, with the corresponding entry_bodies or
accept_statements, are used to define potentially queued operations on tasks
and protected objects.
Syntax
2/2 {AI95-00397-01} entry_declaration ::=
[overriding_indicator]
entry defining_identifier [(discrete_subtype_definition
)] parameter_profile;
3 accept_statement ::=
accept entry_direct_name [(entry_index)] parameter_profile [do
handled_sequence_of_statements
end [entry_identifier]];
3.a Reason: We cannot use defining_identifier for accept_statements.
Although an accept_statement is sort of like a body, it can appear
nested within a block_statement, and therefore be hidden from its
own entry by an outer homograph.
4 entry_index ::= expression
5 entry_body ::=
entry defining_identifier entry_body_formal_part entry_barrier is
declarative_part
begin
handled_sequence_of_statements
end [entry_identifier];
5.a/2 Discussion: {AI95-00397-01} We don't allow an
overriding_indicator on an entry_body because entries always
implement procedures at the point of the type declaration; there
is no late implementation. And we don't want to have to think
about overriding_indicators on accept_statements.
6 entry_body_formal_part ::= [(entry_index_specification
)] parameter_profile
7 entry_barrier ::= when condition
8 entry_index_specification ::= for defining_identifier
in discrete_subtype_definition
9 If an entry_identifier appears at the end of an accept_statement, it
shall repeat the entry_direct_name. If an entry_identifier appears at
the end of an entry_body, it shall repeat the defining_identifier.
10 [An entry_declaration is allowed only in a protected or task
declaration.]
10.a Proof: This follows from the BNF.
10.1/2 {AI95-00397-01} An overriding_indicator is not allowed in an
entry_declaration that includes a discrete_subtype_definition.
10.a.1/2 Reason: An entry family can never implement something, so allowing
an indicator is felt by the majority of the ARG to be redundant.
Name Resolution Rules
11 {expected profile (accept_statement entry_direct_name) [partial]} In an
accept_statement, the expected profile for the entry_direct_name is that of
the entry_declaration; {expected type (entry_index) [partial]} the expected
type for an entry_index is that of the subtype defined by the discrete_-
subtype_definition of the corresponding entry_declaration.
12 Within the handled_sequence_of_statements of an accept_statement, if a
selected_component has a prefix that denotes the corresponding
entry_declaration, then the entity denoted by the prefix is the accept_-
statement, and the selected_component is interpreted as an expanded name (see
4.1.3)[; the selector_name of the selected_component has to be the
identifier for some formal parameter of the accept_statement].
12.a Proof: The only declarations that occur immediately within the
declarative region of an accept_statement are those for its formal
parameters.
Legality Rules
13 An entry_declaration in a task declaration shall not contain a
specification for an access parameter (see 3.10).
13.a Reason: Access parameters for task entries would require a complex
implementation. For example:
13.b task T is
entry E(Z : access Integer); -- Illegal!
end T;
13.c task body T is
begin
declare
type A is access all Integer;
X : A;
Int : aliased Integer;
task Inner;
task body Inner is
begin
T.E(Int'Access);
end Inner;
begin
accept E(Z : access Integer) do
X := A(Z); -- Accessibility_Check
end E;
end;
end T;
13.d Implementing the Accessibility_Check inside the accept_statement
for E is difficult, since one does not know whether the entry
caller is calling from inside the immediately enclosing declare
block or from outside it. This means that the lexical nesting
level associated with the designated object is not sufficient to
determine whether the Accessibility_Check should pass or fail.
13.e Note that such problems do not arise with protected entries,
because entry_bodies are always nested immediately within the
protected_body; they cannot be further nested as can
accept_statements, nor can they be called from within the
protected_body (since no entry calls are permitted inside a
protected_body).
13.1/2 {AI95-00397-01} If an entry_declaration has an overriding_indicator,
then at the point of the declaration:
13.2/2 * if the overriding_indicator is overriding, then the entry shall
implement an inherited subprogram;
13.3/2 * if the overriding_indicator is not overriding, then the entry shall
not implement any inherited subprogram.
13.4/2 {generic contract issue [partial]} In addition to the places where
Legality Rules normally apply (see 12.3), these rules also apply in the
private part of an instance of a generic unit.
13.f/2 Discussion: These rules are subtly different than those for
subprograms (see 8.3.1) because there cannot be "late" inheritance
of primitives from interfaces. Hidden (that is, private)
interfaces are prohibited explicitly (see 7.3), as are hidden
primitive operations (as private operations of public abstract
types are prohibited - see 3.9.3).
14 For an accept_statement, the innermost enclosing body shall be a
task_body, and the entry_direct_name shall denote an entry_declaration in the
corresponding task declaration; the profile of the accept_statement shall
conform fully to that of the corresponding entry_declaration.
{full conformance (required)} An accept_statement shall have a parenthesized
entry_index if and only if the corresponding entry_declaration has a discrete_-
subtype_definition.
15 An accept_statement shall not be within another accept_statement that
corresponds to the same entry_declaration, nor within an asynchronous_select
inner to the enclosing task_body.
15.a Reason: Accept_statements are required to be immediately within
the enclosing task_body (as opposed to being in a nested
subprogram) to ensure that a nested task does not attempt to
accept the entry of its enclosing task. We considered relaxing
this restriction, either by making the check a run-time check, or
by allowing a nested task to accept an entry of its enclosing
task. However, neither change seemed to provide sufficient benefit
to justify the additional implementation burden.
15.b Nested accept_statements for the same entry (or entry family) are
prohibited to ensure that there is no ambiguity in the resolution
of an expanded name for a formal parameter of the entry. This
could be relaxed by allowing the inner one to hide the outer one
from all visibility, but again the small added benefit didn't seem
to justify making the change for Ada 95.
15.c Accept_statements are not permitted within asynchronous_select
statements to simplify the semantics and implementation: an
accept_statement in an abortable_part could result in
Tasking_Error being propagated from an entry call even though the
target task was still callable; implementations that use multiple
tasks implicitly to implement an asynchronous_select might have
trouble supporting "up-level" accepts. Furthermore, if
accept_statements were permitted in the abortable_part, a task
could call its own entry and then accept it in the
abortable_part, leading to rather unusual and possibly
difficult-to-specify semantics.
16 {requires a completion (protected entry_declaration) [partial]} An
entry_declaration of a protected unit requires a completion[, which shall be
an entry_body,] {only as a completion (entry_body) [partial]} and every entry_-
body shall be the completion of an entry_declaration of a protected unit.
{completion legality (entry_body) [partial]} The profile of the entry_body
shall conform fully to that of the corresponding declaration.
{full conformance (required)}
16.a Ramification: An entry_declaration, unlike a
subprogram_declaration, cannot be completed with a renaming_-
declaration.
16.b To be honest: The completion can be a pragma Import, if the
implementation supports it.
16.c Discussion: The above applies only to protected entries, which are
the only ones completed with entry_bodies. Task entries have
corresponding accept_statements instead of having entry_bodies,
and we do not consider an accept_statement to be a "
completion," because a task entry_declaration is allowed to have zero, one,
or more than one corresponding accept_statements.
17 An entry_body_formal_part shall have an entry_index_specification if and
only if the corresponding entry_declaration has a discrete_subtype_definition.
In this case, the discrete_subtype_definitions of the entry_declaration and
the entry_index_specification shall fully conform to one another (see 6.3.1).
{full conformance (required)}
18 A name that denotes a formal parameter of an entry_body is not allowed
within the entry_barrier of the entry_body.
Static Semantics
19 The parameter modes defined for parameters in the parameter_profile of an
entry_declaration are the same as for a subprogram_declaration and have the
same meaning (see 6.2).
19.a Discussion: Note that access parameters are not allowed for task
entries (see above).
20 {family (entry)} {entry family} {entry index subtype} An
entry_declaration with a discrete_subtype_definition (see 3.6) declares a
family of distinct entries having the same profile, with one such entry for
each value of the entry index subtype defined by the
discrete_subtype_definition. [A name for an entry of a family takes the form
of an indexed_component, where the prefix denotes the entry_declaration for
the family, and the index value identifies the entry within the family.]
{single entry} {entry (single)} The term single entry is used to refer to any
entry other than an entry of an entry family.
21 In the entry_body for an entry family, the entry_index_specification
declares a named constant whose subtype is the entry index subtype defined by
the corresponding entry_declaration; {named entry index} the value of the
named entry index identifies which entry of the family was called.
21.a Ramification: The discrete_subtype_definition of the
entry_index_specification is not elaborated; the subtype of the
named constant declared is defined by the
discrete_subtype_definition of the corresponding
entry_declaration, which is elaborated, either when the type is
declared, or when the object is created, if its constraint is
per-object.
Dynamic Semantics
22/1 {8652/0002} {AI95-00171-01} {elaboration (entry_declaration) [partial]}
The elaboration of an entry_declaration for an entry family consists of the
elaboration of the discrete_subtype_definition, as described in 3.8. The
elaboration of an entry_declaration for a single entry has no effect.
22.a Discussion: The elaboration of the declaration of a protected
subprogram has no effect, as specified in clause 6.1. The default
initialization of an object of a task or protected type is covered
in 3.3.1.
23 [The actions to be performed when an entry is called are specified by the
corresponding accept_statements (if any) for an entry of a task unit, and by
the corresponding entry_body for an entry of a protected unit.]
24 {execution (accept_statement) [partial]} For the execution of an
accept_statement, the entry_index, if any, is first evaluated and converted to
the entry index subtype; this index value identifies which entry of the family
is to be accepted. {implicit subtype conversion (entry index) [partial]}
{blocked (on an accept_statement) [partial]} {selection (of an entry caller)}
Further execution of the accept_statement is then blocked until a caller of
the corresponding entry is selected (see 9.5.3), whereupon the
handled_sequence_of_statements, if any, of the accept_statement is executed,
with the formal parameters associated with the corresponding actual parameters
of the selected entry call. Upon completion of the
handled_sequence_of_statements, the accept_statement completes and is left.
When an exception is propagated from the handled_sequence_of_statements of an
accept_statement, the same exception is also raised by the execution of the
corresponding entry_call_statement.
24.a Ramification: This is in addition to propagating it to the
construct containing the accept_statement. In other words, for a
rendezvous, the raising splits in two, and continues concurrently
in both tasks.
24.b The caller gets a new occurrence; this isn't considered
propagation.
24.c Note that we say "propagated from the
handled_sequence_of_statements of an accept_statement", not "
propagated from an accept_statement." The latter would be wrong -
we don't want exceptions propagated by the entry_index to be sent
to the caller (there is none yet!).
25 {rendezvous} The above interaction between a calling task and an accepting
task is called a rendezvous. [After a rendezvous, the two tasks continue their
execution independently.]
26 [An entry_body is executed when the condition of the entry_barrier
evaluates to True and a caller of the corresponding single entry, or entry of
the corresponding entry family, has been selected (see 9.5.3).]
{execution (entry_body) [partial]} For the execution of the entry_body, the
declarative_part of the entry_body is elaborated, and the handled_sequence_of_-
statements of the body is executed, as for the execution of a
subprogram_body. The value of the named entry index, if any, is determined by
the value of the entry index specified in the entry_name of the selected entry
call (or intermediate requeue_statement - see 9.5.4).
26.a To be honest: If the entry had been renamed as a subprogram, and
the call was a procedure_call_statement using the name declared by
the renaming, the entry index (if any) comes from the entry name
specified in the subprogram_renaming_declaration.
NOTES
27 23 A task entry has corresponding accept_statements (zero or more),
whereas a protected entry has a corresponding entry_body (exactly one).
28 24 A consequence of the rule regarding the allowed placements of
accept_statements is that a task can execute accept_statements only
for its own entries.
29/2 25 {AI95-00318-02} A return statement (see 6.5) or a
requeue_statement (see 9.5.4) may be used to complete the execution of
an accept_statement or an entry_body.
29.a Ramification: An accept_statement need not have a
handled_sequence_of_statements even if the corresponding entry has
parameters. Equally, it can have a
handled_sequence_of_statements even if the corresponding entry has
no parameters.
29.b Ramification: A single entry overloads a subprogram, an
enumeration literal, or another single entry if they have the same
defining_identifier. Overloading is not allowed for entry family
names. A single entry or an entry of an entry family can be
renamed as a procedure as explained in 8.5.4.
30 26 The condition in the entry_barrier may reference anything visible
except the formal parameters of the entry. This includes the entry
index (if any), the components (including discriminants) of the
protected object, the Count attribute of an entry of that protected
object, and data global to the protected unit.
31 The restriction against referencing the formal parameters within an
entry_barrier ensures that all calls of the same entry see the same
barrier value. If it is necessary to look at the parameters of an
entry call before deciding whether to handle it, the entry_barrier can
be "when True" and the caller can be requeued (on some private entry)
when its parameters indicate that it cannot be handled immediately.
Examples
32 Examples of entry declarations:
33 entry Read(V : out Item);
entry Seize;
entry Request(Level)(D : Item); -- a family of entries
34 Examples of accept statements:
35 accept Shut_Down;
36 accept Read(V : out Item) do
V := Local_Item;
end Read;
37 accept Request(Low)(D : Item) do
...
end Request;
Extensions to Ada 83
37.a {extensions to Ada 83} The syntax rule for entry_body is new.
37.b Accept_statements can now have exception_handlers.
Wording Changes from Ada 95
37.c/2 {8652/0002} {AI95-00171-01} Corrigendum: Clarified the elaboration
of per-object constraints.
37.d/2 {AI95-00397-01} Overriding_indicators can be used on entries; this
is only useful when a task or protected type inherits from an
interface.
9.5.3 Entry Calls
1 {entry call} [An entry_call_statement (an entry call) can appear in
various contexts.] {simple entry call} {entry call (simple)} A simple entry
call is a stand-alone statement that represents an unconditional call on an
entry of a target task or a protected object. [Entry calls can also appear as
part of select_statements (see 9.7).]
Syntax
2 entry_call_statement ::= entry_name [actual_parameter_part];
Name Resolution Rules
3 The entry_name given in an entry_call_statement shall resolve to denote an
entry. The rules for parameter associations are the same as for subprogram
calls (see 6.4 and 6.4.1).
Static Semantics
4 [The entry_name of an entry_call_statement specifies (explicitly or
implicitly) the target object of the call, the entry or entry family, and the
entry index, if any (see 9.5).]
Dynamic Semantics
5 {open entry} {entry (open)} {closed entry} {entry (closed)} Under certain
circumstances (detailed below), an entry of a task or protected object is
checked to see whether it is open or closed:
6 * {open entry (of a task)} {closed entry (of a task)} An entry of a task
is open if the task is blocked on an accept_statement that corresponds
to the entry (see 9.5.2), or on a selective_accept (see 9.7.1) with an
open accept_alternative that corresponds to the entry; otherwise it is
closed.
7 * {open entry (of a protected object)}
{closed entry (of a protected object)} An entry of a protected object
is open if the condition of the entry_barrier of the corresponding
entry_body evaluates to True; otherwise it is closed.
{Program_Error (raised by failure of run-time check)} If the evaluation
of the condition propagates an exception, the exception Program_Error
is propagated to all current callers of all entries of the protected
object.
7.a Reason: An exception during barrier evaluation is considered
essentially a fatal error. All current entry callers are notified
with a Program_Error. In a fault-tolerant system, a protected
object might provide a Reset protected procedure, or equivalent,
to support attempts to restore such a "broken" protected object to
a reasonable state.
7.b Discussion: Note that the definition of when a task entry is open
is based on the state of the (accepting) task, whereas the
"openness" of a protected entry is defined only when it is
explicitly checked, since the barrier expression needs to be
evaluated. Implementation permissions are given (below) to allow
implementations to evaluate the barrier expression more or less
often than it is checked, but the basic semantic model presumes it
is evaluated at the times when it is checked.
8 {execution (entry_call_statement) [partial]} For the execution of an
entry_call_statement, evaluation of the name and of the parameter associations
is as for a subprogram call (see 6.4). {issue (an entry call)} The entry call
is then issued: For a call on an entry of a protected object, a new protected
action is started on the object (see 9.5.1). The named entry is checked to see
if it is open; {select an entry call (immediately)} if open, the entry call is
said to be selected immediately, and the execution of the call proceeds as
follows:
9 * For a call on an open entry of a task, the accepting task becomes
ready and continues the execution of the corresponding
accept_statement (see 9.5.2).
10 * For a call on an open entry of a protected object, the corresponding
entry_body is executed (see 9.5.2) as part of the protected action.
11 If the accept_statement or entry_body completes other than by a requeue
(see 9.5.4), return is made to the caller (after servicing the entry queues -
see below); any necessary assigning back of formal to actual parameters
occurs, as for a subprogram call (see 6.4.1); such assignments take place
outside of any protected action.
11.a Ramification: The return to the caller will generally not occur
until the protected action completes, unless some other thread of
control is given the job of completing the protected action and
releasing the associated execution resource.
12 If the named entry is closed, the entry call is added to an entry queue
(as part of the protected action, for a call on a protected entry), and the
call remains queued until it is selected or cancelled; {entry queue} there is
a separate (logical) entry queue for each entry of a given task or protected
object [(including each entry of an entry family)].
13 {service (an entry queue)} {select an entry call (from an entry queue)}
When a queued call is selected, it is removed from its entry queue. Selecting
a queued call from a particular entry queue is called servicing the entry
queue. An entry with queued calls can be serviced under the following
circumstances:
14 * When the associated task reaches a corresponding accept_statement, or
a selective_accept with a corresponding open accept_alternative;
15 * If after performing, as part of a protected action on the associated
protected object, an operation on the object other than a call on a
protected function, the entry is checked and found to be open.
16 {select an entry call (from an entry queue)} If there is at least one call
on a queue corresponding to an open entry, then one such call is selected
according to the entry queuing policy in effect (see below), and the
corresponding accept_statement or entry_body is executed as above for an entry
call that is selected immediately.
17 {entry queuing policy} The entry queuing policy controls selection among
queued calls both for task and protected entry queues.
{default entry queuing policy} {entry queuing policy (default policy)} The
default entry queuing policy is to select calls on a given entry queue in
order of arrival. If calls from two or more queues are simultaneously eligible
for selection, the default entry queuing policy does not specify which queue
is serviced first. Other entry queuing policies can be specified by pragmas
(see D.4).
18 For a protected object, the above servicing of entry queues continues
until there are no open entries with queued calls, at which point the
protected action completes.
18.a Discussion: While servicing the entry queues of a protected
object, no new calls can be added to any entry queue of the
object, except due to an internal requeue (see 9.5.4). This is
because the first step of a call on a protected entry is to start
a new protected action, which implies acquiring (for exclusive
read-write access) the execution resource associated with the
protected object, which cannot be done while another protected
action is already in progress.
19 {blocked (during an entry call) [partial]} For an entry call that is added
to a queue, and that is not the triggering_statement of an asynchronous_-
select (see 9.7.4), the calling task is blocked until the call is cancelled,
or the call is selected and a corresponding accept_statement or entry_body
completes without requeuing. In addition, the calling task is blocked during a
rendezvous.
19.a Ramification: For a call on a protected entry, the caller is not
blocked if the call is selected immediately, unless a requeue
causes the call to be queued.
20 {cancellation (of an entry call)} An attempt can be made to cancel an
entry call upon an abort (see 9.8) and as part of certain forms of
select_statement (see 9.7.2, 9.7.3, and 9.7.4). The cancellation does not take
place until a point (if any) when the call is on some entry queue, and not
protected from cancellation as part of a requeue (see 9.5.4); at such a point,
the call is removed from the entry queue and the call completes due to the
cancellation. The cancellation of a call on an entry of a protected object is
a protected action[, and as such cannot take place while any other protected
action is occurring on the protected object. Like any protected action, it
includes servicing of the entry queues (in case some entry barrier depends on
a Count attribute).]
20.a/2 Implementation Note: {AI95-00114-01} In the case of an attempted
cancellation due to abort, this removal might have to be performed
by the calling task itself if the ceiling priority of the
protected object is lower than the priority of the task initiating
the abort.
21 {Tasking_Error (raised by failure of run-time check)} A call on an entry
of a task that has already completed its execution raises the exception
Tasking_Error at the point of the call; similarly, this exception is raised at
the point of the call if the called task completes its execution or becomes
abnormal before accepting the call or completing the rendezvous (see 9.8).
This applies equally to a simple entry call and to an entry call as part of a
select_statement.
Implementation Permissions
22 An implementation may perform the sequence of steps of a protected action
using any thread of control; it need not be that of the task that started the
protected action. If an entry_body completes without requeuing, then the
corresponding calling task may be made ready without waiting for the entire
protected action to complete.
22.a Reason: These permissions are intended to allow flexibility for
implementations on multiprocessors. On a monoprocessor, which
thread of control executes the protected action is essentially
invisible, since the thread is not abortable in any case, and the
"current_task" function is not guaranteed to work during a
protected action (see C.7.1).
23 When the entry of a protected object is checked to see whether it is open,
the implementation need not reevaluate the condition of the corresponding
entry_barrier if no variable or attribute referenced by the condition
(directly or indirectly) has been altered by the execution (or cancellation)
of a protected procedure or entry call on the object since the condition was
last evaluated.
23.a Ramification: Changes to variables referenced by an entry barrier
that result from actions outside of a protected procedure or entry
call on the protected object need not be "noticed." For example,
if a global variable is referenced by an entry barrier, it should
not be altered (except as part of a protected action on the
object) any time after the barrier is first evaluated. In other
words, globals can be used to "parameterize" a protected object,
but they cannot reliably be used to control it after the first use
of the protected object.
23.b Implementation Note: Note that even if a global variable is
volatile, the implementation need only reevaluate a barrier if the
global is updated during a protected action on the protected
object. This ensures that an entry-open bit-vector implementation
approach is possible, where the bit-vector is computed at the end
of a protected action, rather than upon each entry call.
24 An implementation may evaluate the conditions of all entry_barriers of a
given protected object any time any entry of the object is checked to see if
it is open.
24.a Ramification: In other words, any side-effects of evaluating an
entry barrier should be innocuous, since an entry barrier might be
evaluated more or less often than is implied by the "official"
dynamic semantics.
24.b Implementation Note: It is anticipated that when the number of
entries is known to be small, all barriers will be evaluated any
time one of them needs to be, to produce an "entry-open
bit-vector." The appropriate bit will be tested when the entry is
called, and only if the bit is false will a check be made to see
whether the bit-vector might need to be recomputed. This should
allow an implementation to maximize the performance of a call on
an open entry, which seems like the most important case.
24.c In addition to the entry-open bit-vector, an "is-valid" bit is
needed per object, which indicates whether the current bit-vector
setting is valid. A "depends-on-Count-attribute" bit is needed per
type. The "is-valid" bit is set to false (as are all the bits of
the bit-vector) when the protected object is first created, as
well as any time an exception is propagated from computing the
bit-vector. Is-valid would also be set false any time the Count is
changed and "depends-on-Count-attribute" is true for the type, or
a protected procedure or entry returns indicating it might have
updated a variable referenced in some barrier.
24.d A single procedure can be compiled to evaluate all of the
barriers, set the entry-open bit-vector accordingly, and set the
is-valid bit to true. It could have a "when others" handler to set
them all false, and call a routine to propagate Program_Error to
all queued callers.
24.e For protected types where the number of entries is not known to be
small, it makes more sense to evaluate a barrier only when the
corresponding entry is checked to see if it is open. It isn't
worth saving the state of the entry between checks, because of the
space that would be required. Furthermore, the entry queues
probably want to take up space only when there is actually a
caller on them, so rather than an array of all entry queues, a
linked list of nonempty entry queues make the most sense in this
case, with the first caller on each entry queue acting as the
queue header.
25 When an attempt is made to cancel an entry call, the implementation need
not make the attempt using the thread of control of the task (or interrupt)
that initiated the cancellation; in particular, it may use the thread of
control of the caller itself to attempt the cancellation, even if this might
allow the entry call to be selected in the interim.
25.a Reason: Because cancellation of a protected entry call is a
protected action (which helps make the Count attribute of a
protected entry meaningful), it might not be practical to attempt
the cancellation from the thread of control that initiated the
cancellation. For example, if the cancellation is due to the
expiration of a delay, it is unlikely that the handler of the
timer interrupt could perform the necessary protected action
itself (due to being on the interrupt level). Similarly, if the
cancellation is due to an abort, it is possible that the task
initiating the abort has a priority higher than the ceiling
priority of the protected object (for implementations that support
ceiling priorities). Similar considerations could apply in a
multiprocessor situation.
NOTES
26 27 If an exception is raised during the execution of an entry_body,
it is propagated to the corresponding caller (see 11.4).
27 28 For a call on a protected entry, the entry is checked to see if it
is open prior to queuing the call, and again thereafter if its Count
attribute (see 9.9) is referenced in some entry barrier.
27.a Ramification: Given this, extra care is required if a reference to
the Count attribute of an entry appears in the entry's own
barrier.
27.b Reason: An entry is checked to see if it is open prior to queuing
to maximize the performance of a call on an open entry.
28 29 In addition to simple entry calls, the language permits timed,
conditional, and asynchronous entry calls (see 9.7.2, 9.7.3, and see
9.7.4).
28.a Ramification: A task can call its own entries, but the task will
deadlock if the call is a simple entry call.
29 30 The condition of an entry_barrier is allowed to be evaluated by an
implementation more often than strictly necessary, even if the
evaluation might have side effects. On the other hand, an
implementation need not reevaluate the condition if nothing it
references was updated by an intervening protected action on the
protected object, even if the condition references some global
variable that might have been updated by an action performed from
outside of a protected action.
Examples
30 Examples of entry calls:
31 Agent.Shut_Down; -- see 9.1
Parser.Next_Lexeme(E); -- see 9.1
Pool(5).Read(Next_Char); -- see 9.1
Controller.Request(Low)(Some_Item); -- see 9.1
Flags(3).Seize; -- see 9.4
9.5.4 Requeue Statements
1 [A requeue_statement can be used to complete an accept_statement or
entry_body, while redirecting the corresponding entry call to a new (or the
same) entry queue. {requeue} Such a requeue can be performed with or without
allowing an intermediate cancellation of the call, due to an abort or the
expiration of a delay. {preference control: See requeue}
{broadcast signal: See requeue} ]
Syntax
2 requeue_statement ::= requeue entry_name [with abort];
Name Resolution Rules
3 {target entry (of a requeue_statement)} The entry_name of a
requeue_statement shall resolve to denote an entry (the target entry) that
either has no parameters, or that has a profile that is type conformant (see
6.3.1) with the profile of the innermost enclosing entry_body or accept_-
statement. {type conformance (required)}
Legality Rules
4 A requeue_statement shall be within a callable construct that is either an
entry_body or an accept_statement, and this construct shall be the innermost
enclosing body or callable construct.
5 If the target entry has parameters, then its profile shall be subtype
conformant with the profile of the innermost enclosing callable construct.
{subtype conformance (required)}
6 {accessibility rule (requeue statement) [partial]} In a
requeue_statement of an accept_statement of some task unit, either the target
object shall be a part of a formal parameter of the accept_statement, or the
accessibility level of the target object shall not be equal to or statically
deeper than any enclosing accept_statement of the task unit. In a requeue_-
statement of an entry_body of some protected unit, either the target object
shall be a part of a formal parameter of the entry_body, or the accessibility
level of the target object shall not be statically deeper than that of the
entry_declaration.
6.a Ramification: In the entry_body case, the intent is that the
target object can be global, or can be a component of the
protected unit, but cannot be a local variable of the entry_body.
6.b Reason: These restrictions ensure that the target object of the
requeue outlives the completion and finalization of the enclosing
callable construct. They also prevent requeuing from a nested
accept_statement on a parameter of an outer accept_statement,
which could create some strange "long-distance" connections
between an entry caller and its server.
6.c Note that in the strange case where a task_body is nested inside
an accept_statement, it is permissible to requeue from an
accept_statement of the inner task_body on parameters of the outer
accept_statement. This is not a problem because all calls on the
inner task have to complete before returning from the outer
accept_statement, meaning no "dangling calls" will be created.
6.d Implementation Note: By disallowing certain requeues, we ensure
that the normal terminate_alternative rules remain sensible, and
that explicit clearing of the entry queues of a protected object
during finalization is rarely necessary. In particular, such
clearing of the entry queues is necessary only (ignoring premature
Unchecked_Deallocation) for protected objects declared in a
task_body (or created by an allocator for an access type declared
in such a body) containing one or more requeue_statements.
Protected objects declared in subprograms, or at the library
level, will never need to have their entry queues explicitly
cleared during finalization.
Dynamic Semantics
7 {execution (requeue_statement) [partial]} The execution of a
requeue_statement proceeds by first evaluating the entry_name[, including the
prefix identifying the target task or protected object and the expression
identifying the entry within an entry family, if any]. The entry_body or
accept_statement enclosing the requeue_statement is then completed[,
finalized, and left (see 7.6.1)].
8 {execution (requeue task entry) [partial]} For the execution of a requeue
on an entry of a target task, after leaving the enclosing callable construct,
the named entry is checked to see if it is open and the requeued call is
either selected immediately or queued, as for a normal entry call (see 9.5.3).
9 {execution (requeue protected entry) [partial]} For the execution of a
requeue on an entry of a target protected object, after leaving the enclosing
callable construct:
10 * if the requeue is an internal requeue (that is, the requeue is back on
an entry of the same protected object - see 9.5), the call is added to
the queue of the named entry and the ongoing protected action
continues (see 9.5.1);
10.a Ramification: Note that for an internal requeue, the call is
queued without checking whether the target entry is open. This is
because the entry queues will be serviced before the current
protected action completes anyway, and considering the requeued
call immediately might allow it to "jump" ahead of existing
callers on the same queue.
11 * if the requeue is an external requeue (that is, the target protected
object is not implicitly the same as the current object - see 9.5), a
protected action is started on the target object and proceeds as for a
normal entry call (see 9.5.3).
12 If the new entry named in the requeue_statement has formal parameters,
then during the execution of the accept_statement or entry_body corresponding
to the new entry, the formal parameters denote the same objects as did the
corresponding formal parameters of the callable construct completed by the
requeue. [In any case, no parameters are specified in a requeue_statement; any
parameter passing is implicit.]
13 {requeue-with-abort} If the requeue_statement includes the reserved words
with abort (it is a requeue-with-abort), then:
14 * if the original entry call has been aborted (see 9.8), then the
requeue acts as an abort completion point for the call, and the call
is cancelled and no requeue is performed;
15 * if the original entry call was timed (or conditional), then the
original expiration time is the expiration time for the requeued call.
16 If the reserved words with abort do not appear, then the call remains
protected against cancellation while queued as the result of the
requeue_statement.
16.a Ramification: This protection against cancellation lasts only
until the call completes or a subsequent requeue-with-abort is
performed on the call.
16.b Reason: We chose to protect a requeue, by default, against abort
or cancellation. This seemed safer, since it is likely that extra
steps need to be taken to allow for possible cancellation once the
servicing of an entry call has begun. This also means that in the
absence of with abort the usual Ada 83 behavior is preserved,
namely that once an entry call is accepted, it cannot be cancelled
until it completes.
NOTES
17 31 A requeue is permitted from a single entry to an entry of an entry
family, or vice-versa. The entry index, if any, plays no part in the
subtype conformance check between the profiles of the two entries; an
entry index is part of the entry_name for an entry of a family.
{subtype conformance [partial]}
Examples
18 Examples of requeue statements:
19 requeue Request(Medium) with abort;
-- requeue on a member of an entry family of the current task, see 9.1
20 requeue Flags(I).Seize;
-- requeue on an entry of an array component, see 9.4
Extensions to Ada 83
20.a {extensions to Ada 83} The requeue_statement is new.
9.6 Delay Statements, Duration, and Time
1 [{expiration time [partial]} A delay_statement is used to block further
execution until a specified expiration time is reached. The expiration time
can be specified either as a particular point in time (in a delay_until_-
statement), or in seconds from the current time (in a
delay_relative_statement). The language-defined package Calendar provides
definitions for a type Time and associated operations, including a function
Clock that returns the current time. {timing: See delay_statement} ]
Syntax
2 delay_statement ::= delay_until_statement
| delay_relative_statement
3 delay_until_statement ::= delay until delay_expression;
4 delay_relative_statement ::= delay delay_expression;
Name Resolution Rules
5 {expected type (delay_relative_statement expression) [partial]} The
expected type for the delay_expression in a delay_relative_statement is the
predefined type Duration. {expected type (delay_until_statement expression)
[partial]} The delay_expression in a delay_until_statement is expected to be
of any nonlimited type.
Legality Rules
6 {time type} {time base} {clock} There can be multiple time bases, each
with a corresponding clock, and a corresponding time type. The type of the
delay_expression in a delay_until_statement shall be a time type - either the
type Time defined in the language-defined package Calendar (see below), or
some other implementation-defined time type (see D.8).
6.a Implementation defined: Any implementation-defined time types.
Static Semantics
7 [There is a predefined fixed point type named Duration, declared in the
visible part of package Standard;] a value of type Duration is used to
represent the length of an interval of time, expressed in seconds. [The type
Duration is not specific to a particular time base, but can be used with any
time base.]
8 A value of the type Time in package Calendar, or of some other
implementation-defined time type, represents a time as reported by a
corresponding clock.
9 The following language-defined library package exists:
10
package Ada.Calendar is
type Time is private;
11/2 {AI95-00351-01} subtype Year_Number is Integer range 1901 .. 2399;
subtype Month_Number is Integer range 1 .. 12;
subtype Day_Number is Integer range 1 .. 31;
subtype Day_Duration is Duration range 0.0 .. 86_400.0;
11.a/2 Reason: {AI95-00351-01} A range of 500 years was chosen, as that
only requires one extra bit for the year as compared to Ada 95.
This was done to minimize disruptions with existing
implementations. (One implementor reports that their time values
represent nanoseconds, and this year range requires 63.77 bits to
represent.)
12 function Clock return Time;
13 function Year (Date : Time) return Year_Number;
function Month (Date : Time) return Month_Number;
function Day (Date : Time) return Day_Number;
function Seconds(Date : Time) return Day_Duration;
14 procedure Split (Date : in Time;
Year : out Year_Number;
Month : out Month_Number;
Day : out Day_Number;
Seconds : out Day_Duration);
15 function Time_Of(Year : Year_Number;
Month : Month_Number;
Day : Day_Number;
Seconds : Day_Duration := 0.0)
return Time;
16 function "+" (Left : Time; Right : Duration) return Time;
function "+" (Left : Duration; Right : Time) return Time;
function "-" (Left : Time; Right : Duration) return Time;
function "-" (Left : Time; Right : Time) return Duration;
17 function "<" (Left, Right : Time) return Boolean;
function "<="(Left, Right : Time) return Boolean;
function ">" (Left, Right : Time) return Boolean;
function ">="(Left, Right : Time) return Boolean;
18 Time_Error : exception;
19 private
... -- not specified by the language
end Ada.Calendar;
Dynamic Semantics
20 {execution (delay_statement) [partial]} For the execution of a
delay_statement, the delay_expression is first evaluated.
{expiration time (for a delay_until_statement)} For a delay_until_statement,
the expiration time for the delay is the value of the delay_expression, in the
time base associated with the type of the expression.
{expiration time (for a delay_relative_statement)} For a
delay_relative_statement, the expiration time is defined as the current time,
in the time base associated with relative delays, plus the value of the
delay_expression converted to the type Duration, and then rounded up to the next
clock tick. {implicit subtype conversion (delay expression) [partial]} The
time base associated with relative delays is as defined in D.9, "
Delay Accuracy" or is implementation defined.
20.a Implementation defined: The time base associated with relative
delays.
20.b Ramification: Rounding up to the next clock tick means that the
reading of the delay-relative clock when the delay expires should
be no less than the current reading of the delay-relative clock
plus the specified duration.
21 {blocked (on a delay_statement) [partial]} The task executing a
delay_statement is blocked until the expiration time is reached, at which
point it becomes ready again. If the expiration time has already passed, the
task is not blocked.
21.a Discussion: For a delay_relative_statement, this case corresponds
to when the value of the delay_expression is zero or negative.
21.b Even though the task is not blocked, it might be put back on the
end of its ready queue. See D.2, "Priority Scheduling".
22 {cancellation (of a delay_statement)} If an attempt is made to cancel the
delay_statement [(as part of an asynchronous_select or abort - see 9.7.4 and
9.8)], the _statement is cancelled if the expiration time has not yet passed,
thereby completing the delay_statement.
22.a Reason: This is worded this way so that in an
asynchronous_select where the triggering_statement is a
delay_statement, an attempt to cancel the delay when the
abortable_part completes is ignored if the expiration time has
already passed, in which case the optional statements of the
triggering_alternative are executed.
23 The time base associated with the type Time of package Calendar is
implementation defined. The function Clock of package Calendar returns a value
representing the current time for this time base. [The implementation-defined
value of the named number System.Tick (see 13.7) is an approximation of the
length of the real-time interval during which the value of Calendar.Clock
remains constant.]
23.a Implementation defined: The time base of the type Calendar.Time.
24/2 {AI95-00351-01} The functions Year, Month, Day, and Seconds return the
corresponding values for a given value of the type Time, as appropriate to an
implementation-defined time zone; the procedure Split returns all four
corresponding values. Conversely, the function Time_Of combines a year number,
a month number, a day number, and a duration, into a value of type Time. The
operators "+" and "-" for addition and subtraction of times and durations, and
the relational operators for times, have the conventional meaning.
24.a/2 Implementation defined: The time zone used for package Calendar
operations.
25 If Time_Of is called with a seconds value of 86_400.0, the value returned
is equal to the value of Time_Of for the next day with a seconds value of 0.0.
The value returned by the function Seconds or through the Seconds parameter of
the procedure Split is always less than 86_400.0.
26/1 {8652/0030} {AI95-00113-01} The exception Time_Error is raised by the
function Time_Of if the actual parameters do not form a proper date. This
exception is also raised by the operators "+" and "-" if the result is not
representable in the type Time or Duration, as appropriate. This exception is
also raised by the functions Year, Month, Day, and Seconds and the procedure
Split if the year number of the given date is outside of the range of the
subtype Year_Number.
26.a/1 To be honest: {8652/0106} {AI95-00160-01} By "proper date" above
we mean that the given year has a month with the given day. For
example, February 29th is a proper date only for a leap year. We
do not mean to include the Seconds in this notion; in particular,
we do not mean to require implementations to check for the "
missing hour" that occurs when Daylight Savings Time starts in the
spring.
26.b/2 Reason: {8652/0030} {AI95-00113-01} {AI95-00351-01} We allow Year
and Split to raise Time_Error because the arithmetic operators are
allowed (but not required) to produce times that are outside the
range of years from 1901 to 2399. This is similar to the way
integer operators may return values outside the base range of
their type so long as the value is mathematically correct. We
allow the functions Month, Day and Seconds to raise Time_Error so
that they can be implemented in terms of Split.
Implementation Requirements
27 The implementation of the type Duration shall allow representation of time
intervals (both positive and negative) up to at least 86400 seconds (one day);
Duration'Small shall not be greater than twenty milliseconds. The
implementation of the type Time shall allow representation of all dates with
year numbers in the range of Year_Number[; it may allow representation of
other dates as well (both earlier and later).]
Implementation Permissions
28 An implementation may define additional time types (see D.8).
29 An implementation may raise Time_Error if the value of a
delay_expression in a delay_until_statement of a select_statement represents a time
more than 90 days past the current time. The actual limit, if any, is
implementation-defined.
29.a Implementation defined: Any limit on delay_until_statements of
select_statements.
29.b Implementation Note: This allows an implementation to implement
select_statement timeouts using a representation that does not
support the full range of a time type. In particular 90 days of
seconds can be represented in 23 bits, allowing a signed 24-bit
representation for the seconds part of a timeout. There is no
similar restriction allowed for stand-alone
delay_until_statements, as these can be implemented internally
using a loop if necessary to accommodate a long delay.
Implementation Advice
30 Whenever possible in an implementation, the value of Duration'Small should
be no greater than 100 microseconds.
30.a Implementation Note: This can be satisfied using a 32-bit 2's
complement representation with a small of 2.0**(-14) - that is, 61
microseconds - and a range of ± 2.0**17 - that is, 131_072.0.
30.b/2 Implementation Advice: The value of Duration'Small should be no
greater than 100 microseconds.
31 The time base for delay_relative_statements should be monotonic; it need
not be the same time base as used for Calendar.Clock.
31.a/2 Implementation Advice: The time base for
delay_relative_statements should be monotonic.
NOTES
32 32 A delay_relative_statement with a negative value of the
delay_expression is equivalent to one with a zero value.
33 33 A delay_statement may be executed by the environment task;
consequently delay_statements may be executed as part of the
elaboration of a library_item or the execution of the main subprogram.
Such statements delay the environment task (see 10.2).
34 34 {potentially blocking operation (delay_statement) [partial]}
{blocking, potentially (delay_statement) [partial]} A delay_statement
is an abort completion point and a potentially blocking operation,
even if the task is not actually blocked.
35 35 There is no necessary relationship between System.Tick (the
resolution of the clock of package Calendar) and Duration'Small (the
small of type Duration).
35.a Ramification: The inaccuracy of the delay_statement has no
relation to System.Tick. In particular, it is possible that the
clock used for the delay_statement is less accurate than
Calendar.Clock.
35.b We considered making Tick a run-time-determined quantity, to allow
for easier configurability. However, this would not be upward
compatible, and the desired configurability can be achieved using
functionality defined in Annex D, "Real-Time Systems".
36 36 Additional requirements associated with delay_statements are given
in D.9, "Delay Accuracy".
Examples
37 Example of a relative delay statement:
38 delay 3.0; -- delay 3.0 seconds
39 {periodic task (example)} {periodic task: See delay_until_statement}
Example of a periodic task:
40 declare
use Ada.Calendar;
Next_Time : Time := Clock + Period;
-- Period is a global constant of type Duration
begin
loop -- repeated every Period seconds
delay until Next_Time;
... -- perform some actions
Next_Time := Next_Time + Period;
end loop;
end;
Inconsistencies With Ada 83
40.a {inconsistencies with Ada 83} For programs that raise Time_Error
on "+" or "-" in Ada 83,the exception might be deferred until a
call on Split or Year_Number, or might not be raised at all (if
the offending time is never Split after being calculated). This
should not affect typical programs, since they deal only with
times corresponding to the relatively recent past or near future.
Extensions to Ada 83
40.b {extensions to Ada 83} The syntax rule for delay_statement is
modified to allow delay_until_statements.
40.c/2 {AI95-00351-01} The type Time may represent dates with year
numbers outside of Year_Number. Therefore, the operations "+" and
"-" need only raise Time_Error if the result is not representable
in Time (or Duration); also, Split or Year will now raise
Time_Error if the year number is outside of Year_Number. This
change is intended to simplify the implementation of "+" and "-"
(allowing them to depend on overflow for detecting when to raise
Time_Error) and to allow local time zone information to be
considered at the time of Split rather than Clock (depending on
the implementation approach). For example, in a POSIX environment,
it is natural for the type Time to be based on GMT, and the
results of procedure Split (and the functions Year, Month, Day,
and Seconds) to depend on local time zone information. In other
environments, it is more natural for the type Time to be based on
the local time zone, with the results of Year, Month, Day, and
Seconds being pure functions of their input.
40.d/2 This paragraph was deleted.{AI95-00351-01}
Inconsistencies With Ada 95
40.e/2 {AI95-00351-01} {inconsistencies with Ada 95} The upper bound of
Year_Number has been changed to avoid a year 2100 problem. A
program which expects years past 2099 to raise Constraint_Error
will fail in Ada 2005. We don't expect there to be many programs
which are depending on an exception to be raised. A program that
uses Year_Number'Last as a magic number may also fail if values of
Time are stored outside of the program. Note that the lower bound
of Year_Number wasn't changed, because it is not unusual to use
that value in a constant to represent an unknown time.
Wording Changes from Ada 95
40.f/2 {8652/0002} {AI95-00171-01} Corrigendum: Clarified that Month,
Day, and Seconds can raise Time_Error.
9.6.1 Formatting, Time Zones, and other operations for Time
Static Semantics
1/2 {AI95-00351-01} {AI95-00427-01} The following language-defined library
packages exist:
2/2 package Ada.Calendar.Time_Zones is
3/2 -- Time zone manipulation:
4/2 type Time_Offset is range -28*60 .. 28*60;
4.a/2 Reason: We want to be able to specify the difference between any
two arbitrary time zones. You might think that 1440 (24 hours)
would be enough, but there are places (like Tonga, which is
UTC+13hr) which are more than 12 hours than UTC. Combined with
summer time (known as daylight saving time in some parts of the
world) - which switches opposite in the northern and souther
hemispheres - and even greater differences are possible. We know
of cases of a 26 hours difference, so we err on the safe side by
selecting 28 hours as the limit.
5/2 Unknown_Zone_Error : exception;
6/2 function UTC_Time_Offset (Date : Time := Clock) return Time_Offset;
7/2 end Ada.Calendar.Time_Zones;
8/2
package Ada.Calendar.Arithmetic is
9/2 -- Arithmetic on days:
10/2 type Day_Count is range
-366*(1+Year_Number'Last - Year_Number'First)
..
366*(1+Year_Number'Last - Year_Number'First);
11/2 subtype Leap_Seconds_Count is Integer range -2047 .. 2047;
11.a/2 Reason: The maximum number of leap seconds is likely to be much
less than this, but we don't want to reach the limit too soon if
the earth's behavior suddenly changes. We believe that the maximum
number is 1612, based on the current rules, but that number is too
weird to use here.
12/2 procedure Difference (Left, Right : in Time;
Days : out Day_Count;
Seconds : out Duration;
Leap_Seconds : out Leap_Seconds_Count);
13/2 function "+" (Left : Time; Right : Day_Count) return Time;
function "+" (Left : Day_Count; Right : Time) return Time;
function "-" (Left : Time; Right : Day_Count) return Time;
function "-" (Left, Right : Time) return Day_Count;
14/2 end Ada.Calendar.Arithmetic;
15/2
with Ada.Calendar.Time_Zones;
package Ada.Calendar.Formatting is
16/2 -- Day of the week:
17/2 type Day_Name is (Monday, Tuesday, Wednesday, Thursday,
Friday, Saturday, Sunday);
18/2 function Day_of_Week (Date : Time) return Day_Name;
19/2 -- Hours:Minutes:Seconds access:
20/2 subtype Hour_Number is Natural range 0 .. 23;
subtype Minute_Number is Natural range 0 .. 59;
subtype Second_Number is Natural range 0 .. 59;
subtype Second_Duration is Day_Duration range 0.0 .. 1.0;
21/2 function Year (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Year_Number;
22/2 function Month (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Month_Number;
23/2 function Day (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Day_Number;
24/2 function Hour (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Hour_Number;
25/2 function Minute (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Minute_Number;
26/2 function Second (Date : Time)
return Second_Number;
27/2 function Sub_Second (Date : Time)
return Second_Duration;
28/2 function Seconds_Of (Hour : Hour_Number;
Minute : Minute_Number;
Second : Second_Number := 0;
Sub_Second : Second_Duration := 0.0)
return Day_Duration;
29/2 procedure Split (Seconds : in Day_Duration;
Hour : out Hour_Number;
Minute : out Minute_Number;
Second : out Second_Number;
Sub_Second : out Second_Duration);
30/2 function Time_Of (Year : Year_Number;
Month : Month_Number;
Day : Day_Number;
Hour : Hour_Number;
Minute : Minute_Number;
Second : Second_Number;
Sub_Second : Second_Duration := 0.0;
Leap_Second: Boolean := False;
Time_Zone : Time_Zones.Time_Offset := 0)
return Time;
31/2 function Time_Of (Year : Year_Number;
Month : Month_Number;
Day : Day_Number;
Seconds : Day_Duration := 0.0;
Leap_Second: Boolean := False;
Time_Zone : Time_Zones.Time_Offset := 0)
return Time;
32/2 procedure Split (Date : in Time;
Year : out Year_Number;
Month : out Month_Number;
Day : out Day_Number;
Hour : out Hour_Number;
Minute : out Minute_Number;
Second : out Second_Number;
Sub_Second : out Second_Duration;
Time_Zone : in Time_Zones.Time_Offset := 0);
33/2 procedure Split (Date : in Time;
Year : out Year_Number;
Month : out Month_Number;
Day : out Day_Number;
Hour : out Hour_Number;
Minute : out Minute_Number;
Second : out Second_Number;
Sub_Second : out Second_Duration;
Leap_Second: out Boolean;
Time_Zone : in Time_Zones.Time_Offset := 0);
34/2 procedure Split (Date : in Time;
Year : out Year_Number;
Month : out Month_Number;
Day : out Day_Number;
Seconds : out Day_Duration;
Leap_Second: out Boolean;
Time_Zone : in Time_Zones.Time_Offset := 0);
35/2 -- Simple image and value:
function Image (Date : Time;
Include_Time_Fraction : Boolean := False;
Time_Zone : Time_Zones.Time_Offset := 0) return String;
36/2 function Value (Date : String;
Time_Zone : Time_Zones.Time_Offset := 0) return Time;
37/2 function Image (Elapsed_Time : Duration;
Include_Time_Fraction : Boolean := False) return String;
38/2 function Value (Elapsed_Time : String) return Duration;
39/2 end Ada.Calendar.Formatting;
40/2 {AI95-00351-01} Type Time_Offset represents the number of minutes
difference between the implementation-defined time zone used by Calendar and
another time zone.
41/2 function UTC_Time_Offset (Date : Time := Clock) return Time_Offset;
42/2 {AI95-00351-01} Returns, as a number of minutes, the difference
between the implementation-defined time zone of Calendar, and UTC
time, at the time Date. If the time zone of the Calendar
implementation is unknown, then Unknown_Zone_Error is raised.
42.a/2 Discussion: The Date parameter is needed to take into account time
differences caused by daylight-savings time and other time
changes. This parameter is measured in the time zone of Calendar,
if any, not necessarily the UTC time zone.
42.b/2 Other time zones can be supported with a child package. We don't
define one because of the lack of agreement on the definition of a
time zone.
42.c/2 The accuracy of this routine is not specified; the intent is that
the facilities of the underlying target operating system are used
to implement it.
43/2 procedure Difference (Left, Right : in Time;
Days : out Day_Count;
Seconds : out Duration;
Leap_Seconds : out Leap_Seconds_Count);
44/2 {AI95-00351-01} {AI95-00427-01} Returns the difference between
Left and Right. Days is the number of days of difference, Seconds
is the remainder seconds of difference excluding leap seconds, and
Leap_Seconds is the number of leap seconds. If Left < Right, then
Seconds <= 0.0, Days <= 0, and Leap_Seconds <= 0. Otherwise, all
values are nonnegative. The absolute value of Seconds is always
less than 86_400.0. For the returned values, if Days = 0, then
Seconds + Duration(Leap_Seconds) = Calendar."-" (Left, Right).
44.a/2 Discussion: Leap_Seconds, if any, are not included in Seconds.
However, Leap_Seconds should be included in calculations using the
operators defined in Calendar, as is specified for "-" above.
45/2 function "+" (Left : Time; Right : Day_Count) return Time;
function "+" (Left : Day_Count; Right : Time) return Time;
46/2 {AI95-00351-01} Adds a number of days to a time value. Time_Error
is raised if the result is not representable as a value of type
Time.
47/2 function "-" (Left : Time; Right : Day_Count) return Time;
48/2 {AI95-00351-01} Subtracts a number of days from a time value.
Time_Error is raised if the result is not representable as a value
of type Time.
49/2 function "-" (Left, Right : Time) return Day_Count;
50/2 {AI95-00351-01} Subtracts two time values, and returns the number
of days between them. This is the same value that Difference would
return in Days.
51/2 function Day_of_Week (Date : Time) return Day_Name;
52/2 {AI95-00351-01} Returns the day of the week for Time. This is
based on the Year, Month, and Day values of Time.
53/2 function Year (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Year_Number;
54/2 {AI95-00427-01} Returns the year for Date, as appropriate for the
specified time zone offset.
55/2 function Month (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Month_Number;
56/2 {AI95-00427-01} Returns the month for Date, as appropriate for the
specified time zone offset.
57/2 function Day (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Day_Number;
58/2 {AI95-00427-01} Returns the day number for Date, as appropriate
for the specified time zone offset.
59/2 function Hour (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Hour_Number;
60/2 {AI95-00351-01} Returns the hour for Date, as appropriate for the
specified time zone offset.
61/2 function Minute (Date : Time;
Time_Zone : Time_Zones.Time_Offset := 0)
return Minute_Number;
62/2 {AI95-00351-01} Returns the minute within the hour for Date, as
appropriate for the specified time zone offset.
63/2 function Second (Date : Time)
return Second_Number;
64/2 {AI95-00351-01} {AI95-00427-01} Returns the second within the hour
and minute for Date.
65/2 function Sub_Second (Date : Time)
return Second_Duration;
66/2 {AI95-00351-01} {AI95-00427-01} Returns the fraction of second for
Date (this has the same accuracy as Day_Duration). The value
returned is always less than 1.0.
67/2 function Seconds_Of (Hour : Hour_Number;
Minute : Minute_Number;
Second : Second_Number := 0;
Sub_Second : Second_Duration := 0.0)
return Day_Duration;
68/2 {AI95-00351-01} {AI95-00427-01} Returns a Day_Duration value for
the combination of the given Hour, Minute, Second, and Sub_Second.
This value can be used in Calendar.Time_Of as well as the argument
to Calendar."+" and Calendar."-". If Seconds_Of is called with a
Sub_Second value of 1.0, the value returned is equal to the value
of Seconds_Of for the next second with a Sub_Second value of 0.0.
69/2 procedure Split (Seconds : in Day_Duration;
Hour : out Hour_Number;
Minute : out Minute_Number;
Second : out Second_Number;
Sub_Second : out Second_Duration);
70/2 {AI95-00351-01} {AI95-00427-01} Splits Seconds into Hour, Minute,
Second and Sub_Second in such a way that the resulting values all
belong to their respective subtypes. The value returned in the
Sub_Second parameter is always less than 1.0.
70.a/2 Ramification: There is only one way to do the split which meets
all of the requirements.
71/2 function Time_Of (Year : Year_Number;
Month : Month_Number;
Day : Day_Number;
Hour : Hour_Number;
Minute : Minute_Number;
Second : Second_Number;
Sub_Second : Second_Duration := 0.0;
Leap_Second: Boolean := False;
Time_Zone : Time_Zones.Time_Offset := 0)
return Time;
72/2 {AI95-00351-01} {AI95-00427-01} If Leap_Second is False, returns a
Time built from the date and time values, relative to the
specified time zone offset. If Leap_Second is True, returns the
Time that represents the time within the leap second that is one
second later than the time specified by the other parameters.
Time_Error is raised if the parameters do not form a proper date
or time. If Time_Of is called with a Sub_Second value of 1.0, the
value returned is equal to the value of Time_Of for the next
second with a Sub_Second value of 0.0.
72.a/2 Discussion: Time_Error should be raised if Leap_Second is True,
and the date and time values do not represent the second before a
leap second. A leap second always occurs at midnight UTC, and is
23:59:60 UTC in ISO notation. So, if the time zone is UTC and
Leap_Second is True, if any of Hour /= 23, Minute /= 59, or Second
/= 59, then Time_Error should be raised. However, we do not say
that, because other time zones will have different values where a
leap second is allowed.
73/2 function Time_Of (Year : Year_Number;
Month : Month_Number;
Day : Day_Number;
Seconds : Day_Duration := 0.0;
Leap_Second: Boolean := False;
Time_Zone : Time_Zones.Time_Offset := 0)
return Time;
74/2 {AI95-00351-01} {AI95-00427-01} If Leap_Second is False, returns a
Time built from the date and time values, relative to the
specified time zone offset. If Leap_Second is True, returns the
Time that represents the time within the leap second that is one
second later than the time specified by the other parameters.
Time_Error is raised if the parameters do not form a proper date
or time. If Time_Of is called with a Seconds value of 86_400.0,
the value returned is equal to the value of Time_Of for the next
day with a Seconds value of 0.0.
75/2 procedure Split (Date : in Time;
Year : out Year_Number;
Month : out Month_Number;
Day : out Day_Number;
Hour : out Hour_Number;
Minute : out Minute_Number;
Second : out Second_Number;
Sub_Second : out Second_Duration;
Leap_Second: out Boolean;
Time_Zone : in Time_Zones.Time_Offset := 0);
76/2 {AI95-00351-01} {AI95-00427-01} If Date does not represent a time
within a leap second, splits Date into its constituent parts
(Year, Month, Day, Hour, Minute, Second, Sub_Second), relative to
the specified time zone offset, and sets Leap_Second to False. If
Date represents a time within a leap second, set the constituent
parts to values corresponding to a time one second earlier than
that given by Date, relative to the specified time zone offset,
and sets Leap_Seconds to True. The value returned in the
Sub_Second parameter is always less than 1.0.
77/2 procedure Split (Date : in Time;
Year : out Year_Number;
Month : out Month_Number;
Day : out Day_Number;
Hour : out Hour_Number;
Minute : out Minute_Number;
Second : out Second_Number;
Sub_Second : out Second_Duration;
Time_Zone : in Time_Zones.Time_Offset := 0);
78/2 {AI95-00351-01} {AI95-00427-01} Splits Date into its constituent
parts (Year, Month, Day, Hour, Minute, Second, Sub_Second),
relative to the specified time zone offset. The value returned in
the Sub_Second parameter is always less than 1.0.
79/2 procedure Split (Date : in Time;
Year : out Year_Number;
Month : out Month_Number;
Day : out Day_Number;
Seconds : out Day_Duration;
Leap_Second: out Boolean;
Time_Zone : in Time_Zones.Time_Offset := 0);
80/2 {AI95-00351-01} {AI95-00427-01} If Date does not represent a time
within a leap second, splits Date into its constituent parts
(Year, Month, Day, Seconds), relative to the specified time zone
offset, and sets Leap_Second to False. If Date represents a time
within a leap second, set the constituent parts to values
corresponding to a time one second earlier than that given by
Date, relative to the specified time zone offset, and sets
Leap_Seconds to True. The value returned in the Seconds parameter
is always less than 86_400.0.
81/2 function Image (Date : Time;
Include_Time_Fraction : Boolean := False;
Time_Zone : Time_Zones.Time_Offset := 0) return String;
82/2 {AI95-00351-01} Returns a string form of the Date relative to the
given Time_Zone. The format is "Year-Month-Day
Hour:Minute:Second", where the Year is a 4-digit value, and all
others are 2-digit values, of the functions defined in Calendar
and Calendar.Formatting, including a leading zero, if needed. The
separators between the values are a minus, another minus, a colon,
and a single space between the Day and Hour. If
Include_Time_Fraction is True, the integer part of Sub_Seconds*100
is suffixed to the string as a point followed by a 2-digit value.
82.a/2 Discussion: The Image provides a string in ISO 8601 format, the
international standard time format. Alternative representations
allowed in ISO 8601 are not supported here.
82.b/2 ISO 8601 allows 24:00:00 for midnight; and a seconds value of 60
for leap seconds. These are not allowed here (the routines
mentioned above cannot produce those results).
82.c/2 Ramification: The fractional part is truncated, not rounded. It
would be quite hard to define the result with proper rounding, as
it can change all of the values of the image. Values can be
rounded up by adding an appropriate constant (0.5 if
Include_Time_Fraction is False, 0.005 otherwise) to the time
before taking the image.
83/2 function Value (Date : String;
Time_Zone : Time_Zones.Time_Offset := 0) return Time;
84/2 {AI95-00351-01} Returns a Time value for the image given as Date,
relative to the given time zone. Constraint_Error is raised if the
string is not formatted as described for Image, or the function
cannot interpret the given string as a Time value.
84.a/2 Discussion: The intent is that the implementation enforce the same
range rules on the string as the appropriate function Time_Of,
except for the hour, so "cannot interpret the given string as a
Time value" happens when one of the values is out of the required
range. For example, "2005-08-31 24:0:0" should raise
Constraint_Error (the hour is out of range).
85/2 function Image (Elapsed_Time : Duration;
Include_Time_Fraction : Boolean := False) return String;
86/2 {AI95-00351-01} Returns a string form of the Elapsed_Time. The
format is "Hour:Minute:Second", where all values are 2-digit
values, including a leading zero, if needed. The separators
between the values are colons. If Include_Time_Fraction is True,
the integer part of Sub_Seconds*100 is suffixed to the string as a
point followed by a 2-digit value. If Elapsed_Time < 0.0, the
result is Image (abs Elapsed_Time, Include_Time_Fraction) prefixed
with a minus sign. If abs Elapsed_Time represents 100 hours or
more, the result is implementation-defined.
86.a/2 Implementation defined: The result of Calendar.Formating.Image if
its argument represents more than 100 hours.
86.b/2 Implementation Note: This cannot be implemented (directly) by
calling Calendar.Formatting.Split, since it may be out of the
range of Day_Duration, and thus the number of hours may be out of
the range of Hour_Number.
86.c If a Duration value can represent more then 100 hours, the
implementation will need to define a format for the return of
Image.
87/2 function Value (Elapsed_Time : String) return Duration;
88/2 {AI95-00351-01} Returns a Duration value for the image given as
Elapsed_Time. Constraint_Error is raised if the string is not
formatted as described for Image, or the function cannot interpret
the given string as a Duration value.
88.a/2 Discussion: The intent is that the implementation enforce the same
range rules on the string as the appropriate function Time_Of,
except for the hour, so "cannot interpret the given string as a
Time value" happens when one of the values is out of the required
range. For example, "10:23:60" should raise Constraint_Error (the
seconds value is out of range).
Implementation Advice
89/2 {AI95-00351-01} An implementation should support leap seconds if the
target system supports them. If leap seconds are not supported, Difference
should return zero for Leap_Seconds, Split should return False for
Leap_Second, and Time_Of should raise Time_Error if Leap_Second is True.
89.a/2 Implementation Advice: Leap seconds should be supported if the
target system supports them. Otherwise, operations in
Calendar.Formatting should return results consistent with no leap
seconds.
89.b/2 Discussion: An implementation can always support leap seconds when
the target system does not; indeed, this isn't particularly hard
(all that is required is a table of when leap seconds were
inserted). As such, leap second support isn't "impossible or
impractical" in the sense of 1.1.3. However, for some purposes, it
may be important to follow the target system's lack of leap second
support (if the target is a GPS satellite, which does not use leap
seconds, leap second support would be a handicap to work around).
Thus, this Implementation Advice should be read as giving
permission to not support leap seconds on target systems that
don't support leap seconds. Implementers should use the needs of
their customers to determine whether or not support leap seconds
on such targets.
NOTES
90/2 37 {AI95-00351-01} The implementation-defined time zone of package
Calendar may, but need not, be the local time zone. UTC_Time_Offset
always returns the difference relative to the implementation-defined
time zone of package Calendar. If UTC_Time_Offset does not raise
Unknown_Zone_Error, UTC time can be safely calculated (within the
accuracy of the underlying time-base).
90.a/2 Discussion: {AI95-00351-01} The time in the time zone known as
Greenwich Mean Time (GMT) is generally very close to UTC time; for
most purposes they can be treated the same. GMT is the time based
on the rotation of the Earth; UTC is the time based on atomic
clocks, with leap seconds periodically inserted to realign with
GMT (because most human activities depend on the rotation of the
Earth). At any point in time, there will be a sub-second
difference between GMT and UTC.
91/2 38 {AI95-00351-01} Calling Split on the results of subtracting
Duration(UTC_Time_Offset*60) from Clock provides the components
(hours, minutes, and so on) of the UTC time. In the United States, for
example, UTC_Time_Offset will generally be negative.
91.a/2 Discussion: This is an illustration to help specify the value of
UTC_Time_Offset. A user should pass UTC_Time_Offset as the
Time_Zone parameter of Split, rather than trying to make the above
calculation.
Extensions to Ada 95
91.b/2 {AI95-00351-01} {AI95-00428-01} {extensions to Ada 95} Packages
Calendar.Time_Zones, Calendar.Arithmetic, and Calendar.Formatting
are new.
9.7 Select Statements
1 [There are four forms of the select_statement. One form provides a
selective wait for one or more select_alternatives. Two provide timed and
conditional entry calls. The fourth provides asynchronous transfer of
control.]
Syntax
2 select_statement ::=
selective_accept
| timed_entry_call
| conditional_entry_call
| asynchronous_select
Examples
3 Example of a select statement:
4 select
accept Driver_Awake_Signal;
or
delay 30.0*Seconds;
Stop_The_Train;
end select;
Extensions to Ada 83
4.a {extensions to Ada 83} Asynchronous_select is new.
9.7.1 Selective Accept
1 [This form of the select_statement allows a combination of waiting for,
and selecting from, one or more alternatives. The selection may depend on
conditions associated with each alternative of the selective_accept.
{time-out: See selective_accept} ]
Syntax
2 selective_accept ::=
select
[guard]
select_alternative
{ or
[guard]
select_alternative }
[ else
sequence_of_statements ]
end select;
3 guard ::= when condition =>
4 select_alternative ::=
accept_alternative
| delay_alternative
| terminate_alternative
5 accept_alternative ::=
accept_statement [sequence_of_statements]
6 delay_alternative ::=
delay_statement [sequence_of_statements]
7 terminate_alternative ::= terminate;
8 A selective_accept shall contain at least one accept_alternative. In
addition, it can contain:
9 * a terminate_alternative (only one); or
10 * one or more delay_alternatives; or
11 * {else part (of a selective_accept)} an else part (the reserved
word else followed by a sequence_of_statements).
12 These three possibilities are mutually exclusive.
Legality Rules
13 If a selective_accept contains more than one delay_alternative, then all
shall be delay_relative_statements, or all shall be delay_until_statements for
the same time type.
13.a Reason: This simplifies the implementation and the description of
the semantics.
Dynamic Semantics
14 {open alternative} A select_alternative is said to be open if it is not
immediately preceded by a guard, or if the condition of its guard evaluates to
True. It is said to be closed otherwise.
15 {execution (selective_accept) [partial]} For the execution of a
selective_accept, any guard conditions are evaluated; open alternatives are
thus determined. For an open delay_alternative, the delay_expression is also
evaluated. Similarly, for an open accept_alternative for an entry of a family,
the entry_index is also evaluated. These evaluations are performed in an
arbitrary order, except that a delay_expression or entry_index is not
evaluated until after evaluating the corresponding condition, if any.
Selection and execution of one open alternative, or of the else part, then
completes the execution of the selective_accept; the rules for this selection
are described below.
16 Open accept_alternatives are first considered. Selection of one such
alternative takes place immediately if the corresponding entry already has
queued calls. If several alternatives can thus be selected, one of them is
selected according to the entry queuing policy in effect (see 9.5.3 and D.4).
When such an alternative is selected, the selected call is removed from its
entry queue and the handled_sequence_of_statements (if any) of the
corresponding accept_statement is executed; after the rendezvous completes any
subsequent sequence_of_statements of the alternative is executed.
{blocked (execution of a selective_accept) [partial]} If no selection is
immediately possible (in the above sense) and there is no else part, the task
blocks until an open alternative can be selected.
17 Selection of the other forms of alternative or of an else part is
performed as follows:
18 * An open delay_alternative is selected when its expiration time is
reached if no accept_alternative or other delay_alternative can be
selected prior to the expiration time. If several delay_alternatives
have this same expiration time, one of them is selected according to
the queuing policy in effect (see D.4); the default queuing policy
chooses arbitrarily among the delay_alternatives whose expiration time
has passed.
19 * The else part is selected and its sequence_of_statements is executed
if no accept_alternative can immediately be selected; in particular,
if all alternatives are closed.
20 * An open terminate_alternative is selected if the conditions stated at
the end of clause 9.3 are satisfied.
20.a Ramification: In the absence of a requeue_statement, the
conditions stated are such that a terminate_alternative cannot be
selected while there is a queued entry call for any entry of the
task. In the presence of requeues from a task to one of its
subtasks, it is possible that when a terminate_alternative of the
subtask is selected, requeued calls (for closed entries only)
might still be queued on some entry of the subtask. Tasking_Error
will be propagated to such callers, as is usual when a task
completes while queued callers remain.
21 {Program_Error (raised by failure of run-time check)} The exception
Program_Error is raised if all alternatives are closed and there is no else
part.
NOTES
22 39 A selective_accept is allowed to have several open
delay_alternatives. A selective_accept is allowed to have several open
accept_alternatives for the same entry.
Examples
23 Example of a task body with a selective accept:
24 task body Server is
Current_Work_Item : Work_Item;
begin
loop
select
accept Next_Work_Item(WI : in Work_Item) do
Current_Work_Item := WI;
end;
Process_Work_Item(Current_Work_Item);
or
accept Shut_Down;
exit; -- Premature shut down requested
or
terminate; -- Normal shutdown at end of scope
end select;
end loop;
end Server;
Wording Changes from Ada 83
24.a The name of selective_wait was changed to selective_accept to
better describe what is being waited for. We kept
select_alternative as is, because selective_accept_alternative was
too easily confused with accept_alternative.
9.7.2 Timed Entry Calls
1/2 {AI95-00345-01} [A timed_entry_call issues an entry call that is cancelled
if the call (or a requeue-with-abort of the call) is not selected before the
expiration time is reached. A procedure call may appear rather than an entry
call for cases where the procedure might be implemented by an entry.
{time-out: See timed_entry_call} ]
Syntax
2 timed_entry_call ::=
select
entry_call_alternative
or
delay_alternative
end select;
3/2 {AI95-00345-01} entry_call_alternative ::=
procedure_or_entry_call [sequence_of_statements]
3.1/2 {AI95-00345-01} procedure_or_entry_call ::=
procedure_call_statement | entry_call_statement
Legality Rules
3.2/2 {AI95-00345-01} If a procedure_call_statement is used for a
procedure_or_entry_call, the procedure_name or procedure_prefix of the
procedure_call_statement shall statically denote an entry renamed as a
procedure or (a view of) a primitive subprogram of a limited interface whose
first parameter is a controlling parameter (see 3.9.2).
3.a/2 Reason: This would be a confusing way to call a procedure, so we
only allow it when it is possible that the procedure is actually
an entry. We could have allowed formal subprograms here, but we
didn't because we'd have to allow all formal subprograms, and it
would increase the difficulty of generic code sharing.
3.b/2 We say "statically denotes" because an access-to-subprogram cannot
be primitive, and we don't have anything like access-to-entry. So
only names of entries or procedures are possible.
Static Semantics
3.3/2 {AI95-00345-01} If a procedure_call_statement is used for a
procedure_or_entry_call, and the procedure is implemented by an entry, then
the procedure_name, or procedure_prefix and possibly the first parameter of
the procedure_call_statement, determine the target object of the call and the
entry to be called.
3.c/2 Discussion: The above says "possibly the first parameter", because
Ada allows entries to be renamed and passed as formal subprograms.
In those cases, the task or protected object is implicit in the
name of the routine; otherwise the object is an explicit parameter
to the call.
Dynamic Semantics
4/2 {AI95-00345-01} {execution (timed_entry_call) [partial]} For the execution
of a timed_entry_call, the entry_name, procedure_name, or procedure_prefix,
and any actual parameters are evaluated, as for a simple entry call (see
9.5.3) or procedure call (see 6.4). The expiration time (see 9.6) for the call
is determined by evaluating the delay_expression of the delay_alternative. If
the call is an entry call or a call on a procedure implemented by an entry,
the entry call is then issued. Otherwise, the call proceeds as described in
6.4 for a procedure call, followed by the sequence_of_statements of the
entry_call_alternative; the sequence_of_statements of the delay_alternative is
ignored.
5 If the call is queued (including due to a requeue-with-abort), and not
selected before the expiration time is reached, an attempt to cancel the call
is made. If the call completes due to the cancellation, the optional
sequence_of_statements of the delay_alternative is executed; if the entry call
completes normally, the optional sequence_of_statements of the entry_call_-
alternative is executed.
5.a/2 This paragraph was deleted.{AI95-00345-01}
Examples
6 Example of a timed entry call:
7 select
Controller.Request(Medium)(Some_Item);
or
delay 45.0;
-- controller too busy, try something else
end select;
Wording Changes from Ada 83
7.a This clause comes before the one for Conditional Entry Calls, so
we can define conditional entry calls in terms of timed entry
calls.
Incompatibilities With Ada 95
7.b/2 {AI95-00345-01} {incompatibilities with Ada 95} A procedure can be
used as the in a timed or conditional entry call, if the procedure
might actually be an entry. Since the fact that something is an
entry could be used in resolving these calls in Ada 95, it is
possible for timed or conditional entry calls that resolved in Ada
95 to be ambiguous in Ada 2005. That could happen if both an entry
and procedure with the same name and profile exist, which should
be rare.
9.7.3 Conditional Entry Calls
1/2 {AI95-00345-01} [A conditional_entry_call issues an entry call that is
then cancelled if it is not selected immediately (or if a requeue-with-abort
of the call is not selected immediately). A procedure call may appear rather
than an entry call for cases where the procedure might be implemented by an
entry.]
1.a To be honest: In the case of an entry call on a protected object,
it is OK if the entry is closed at the start of the corresponding
protected action, so long as it opens and the call is selected
before the end of that protected action (due to changes in the
Count attribute).
Syntax
2 conditional_entry_call ::=
select
entry_call_alternative
else
sequence_of_statements
end select;
Dynamic Semantics
3 {execution (conditional_entry_call) [partial]} The execution of a
conditional_entry_call is defined to be equivalent to the execution of a timed_-
entry_call with a delay_alternative specifying an immediate expiration time
and the same sequence_of_statements as given after the reserved word else.
NOTES
4 40 A conditional_entry_call may briefly increase the Count attribute
of the entry, even if the conditional call is not selected.
Examples
5 Example of a conditional entry call:
6 procedure Spin(R : in Resource) is
begin
loop
select
R.Seize;
return;
else
null; -- busy waiting
end select;
end loop;
end;
Wording Changes from Ada 83
6.a This clause comes after the one for Timed Entry Calls, so we can
define conditional entry calls in terms of timed entry calls. We
do that so that an "expiration time" is defined for both, thereby
simplifying the definition of what happens on a
requeue-with-abort.
9.7.4 Asynchronous Transfer of Control
1 [An asynchronous select_statement provides asynchronous transfer of
control upon completion of an entry call or the expiration of a delay.]
Syntax
2 asynchronous_select ::=
select
triggering_alternative
then abort
abortable_part
end select;
3 triggering_alternative ::= triggering_statement
[sequence_of_statements]
4/2 {AI95-00345-01} triggering_statement ::= procedure_or_entry_call
| delay_statement
5 abortable_part ::= sequence_of_statements
Dynamic Semantics
6/2 {AI95-00345-01}
{execution (asynchronous_select with an entry call trigger) [partial]}
{execution (asynchronous_select with a procedure call trigger) [partial]} For
the execution of an asynchronous_select whose triggering_statement is a
procedure_or_entry_call, the entry_name, procedure_name, or procedure_prefix,
and actual parameters are evaluated as for a simple entry call (see 9.5.3) or
procedure call (see 6.4). If the call is an entry call or a call on a
procedure implemented by an entry, the entry call is issued. If the entry call
is queued (or requeued-with-abort), then the abortable_part is executed. [If
the entry call is selected immediately, and never requeued-with-abort, then
the abortable_part is never started.] If the call is on a procedure that is
not implemented by an entry, the call proceeds as described in 6.4, followed
by the sequence_of_statements of the triggering_alternative[; the
abortable_part is never started].
7 {execution (asynchronous_select with a delay_statement trigger)
[partial]} For the execution of an asynchronous_select whose triggering_-
statement is a delay_statement, the delay_expression is evaluated and the
expiration time is determined, as for a normal delay_statement. If the
expiration time has not already passed, the abortable_part is executed.
8 If the abortable_part completes and is left prior to completion of the
triggering_statement, an attempt to cancel the triggering_statement is made.
If the attempt to cancel succeeds (see 9.5.3 and 9.6), the
asynchronous_select is complete.
9 If the triggering_statement completes other than due to cancellation, the
abortable_part is aborted (if started but not yet completed - see 9.8). If the
triggering_statement completes normally, the optional sequence_of_statements
of the triggering_alternative is executed after the abortable_part is left.
9.a Discussion: We currently don't specify when the by-copy [in] out
parameters are assigned back into the actuals. We considered
requiring that to happen after the abortable_part is left.
However, that doesn't seem useful enough to justify possibly
overspecifying the implementation approach, since some of the
parameters are passed by reference anyway.
9.b In an earlier description, we required that the sequence_of_-
statements of the triggering_alternative execute after aborting
the abortable_part, but before waiting for it to complete and
finalize, to provide more rapid response to the triggering event
in case the finalization was unbounded. However, various reviewers
felt that this created unnecessary complexity in the description,
and a potential for undesirable concurrency (and nondeterminism)
within a single task. We have now reverted to simpler, more
deterministic semantics, but anticipate that further discussion of
this issue might be appropriate during subsequent reviews. One
possibility is to leave this area implementation defined, so as to
encourage experimentation. The user would then have to assume the
worst about what kinds of actions are appropriate for the
sequence_of_statements of the triggering_alternative to achieve
portability.
Examples
10 {signal handling (example)}
{interrupt (example using asynchronous_select)} {terminal interrupt (example)}
Example of a main command loop for a command interpreter:
11 loop
select
Terminal.Wait_For_Interrupt;
Put_Line("Interrupted");
then abort
-- This will be abandoned upon terminal interrupt
Put_Line("-> ");
Get_Line(Command, Last);
Process_Command(Command(1..Last));
end select;
end loop;
12 Example of a time-limited calculation:
{time-out: See asynchronous_select} {time-out (example)}
{time limit (example)} {interrupt (example using asynchronous_select)}
{timer interrupt (example)}
13 select
delay 5.0;
Put_Line("Calculation does not converge");
then abort
-- This calculation should finish in 5.0 seconds;
-- if not, it is assumed to diverge.
Horribly_Complicated_Recursive_Function(X, Y);
end select;
Extensions to Ada 83
13.a {extensions to Ada 83} Asynchronous_select is new.
Extensions to Ada 95
13.b/2 {AI95-00345-01} {extensions to Ada 95} A procedure can be used as
the triggering_statement of an asynchronous_select, if the
procedure might actually be an entry
9.8 Abort of a Task - Abort of a Sequence of Statements
1 [An abort_statement causes one or more tasks to become abnormal, thus
preventing any further interaction with such tasks. The completion of the
triggering_statement of an asynchronous_select causes a sequence_of_statements
to be aborted.]
Syntax
2 abort_statement ::= abort task_name {, task_name};
Name Resolution Rules
3 {expected type (abort_statement task_name) [partial]} Each task_name is
expected to be of any task type[; they need not all be of the same task type.]
Dynamic Semantics
4 {execution (abort_statement) [partial]} For the execution of an
abort_statement, the given task_names are evaluated in an arbitrary order.
{abort (of a task)} {abnormal task} {task state (abnormal) [partial]} Each
named task is then aborted, which consists of making the task abnormal and
aborting the execution of the corresponding task_body, unless it is already
completed.
4.a/2 Ramification: {AI95-00114-01} Note that aborting those tasks is
not defined to be an abort-deferred operation. Therefore, if one
of the named tasks is the task executing the abort_statement, or
if the task executing the abort_statement depends on one of the
named tasks, then it is possible for the execution of the
abort_statement to be aborted, thus leaving some of the tasks
unaborted. This allows the implementation to use either a sequence
of calls to an "abort task" run-time system primitive, or a single
call to an "abort list of tasks" run-time system primitive.
5 {execution (aborting the execution of a construct) [partial]}
{abort (of the execution of a construct)} When the execution of a construct is
aborted (including that of a task_body or of a sequence_of_statements), the
execution of every construct included within the aborted execution is also
aborted, except for executions included within the execution of an
abort-deferred operation; the execution of an abort-deferred operation
continues to completion without being affected by the abort;
{abort-deferred operation} the following are the abort-deferred operations:
6 * a protected action;
7 * waiting for an entry call to complete (after having initiated the
attempt to cancel it - see below);
8 * waiting for the termination of dependent tasks;
9 * the execution of an Initialize procedure as the last step of the
default initialization of a controlled object;
10 * the execution of a Finalize procedure as part of the finalization of a
controlled object;
11 * an assignment operation to an object with a controlled part.
12 [The last three of these are discussed further in 7.6.]
12.a Reason: Deferring abort during Initialize and finalization allows,
for example, the result of an allocator performed in an Initialize
operation to be assigned into an access object without being
interrupted in the middle, which would cause storage leaks. For an
object with several controlled parts, each individual Initialize
is abort-deferred. Note that there is generally no semantic
difference between making each Finalize abort-deferred, versus
making a group of them abort-deferred, because if the task gets
aborted, the first thing it will do is complete any remaining
finalizations. Individual objects are finalized prior to an
assignment operation (if nonlimited controlled) and as part of
Unchecked_Deallocation.
12.b Ramification: Abort is deferred during the entire assignment
operation to an object with a controlled part, even if only some
subcomponents are controlled. Note that this says "assignment
operation," not "assignment_statement." Explicit calls to
Initialize, Finalize, or Adjust are not abort-deferred.
13 When a master is aborted, all tasks that depend on that master are aborted.
14 {unspecified [partial]} The order in which tasks become abnormal as the
result of an abort_statement or the abort of a sequence_of_statements is not
specified by the language.
15 If the execution of an entry call is aborted, an immediate attempt is made
to cancel the entry call (see 9.5.3). If the execution of a construct is
aborted at a time when the execution is blocked, other than for an entry call,
at a point that is outside the execution of an abort-deferred operation, then
the execution of the construct completes immediately. For an abort due to an
abort_statement, these immediate effects occur before the execution of the
abort_statement completes. Other than for these immediate cases, the execution
of a construct that is aborted does not necessarily complete before the
abort_statement completes. However, the execution of the aborted construct
completes no later than its next abort completion point (if any) that occurs
outside of an abort-deferred operation; {abort completion point} the following
are abort completion points for an execution:
16 * the point where the execution initiates the activation of another task;
17 * the end of the activation of a task;
18 * the start or end of the execution of an entry call, accept_statement,
delay_statement, or abort_statement;
18.a Ramification: Although the abort completion point doesn't occur
until the end of the entry call or delay_statement, these
operations might be cut short because an abort attempts to cancel
them.
19 * the start of the execution of a select_statement, or of the
sequence_of_statements of an exception_handler.
19.a Reason: The start of an exception_handler is considered an abort
completion point simply because it is easy for an implementation
to check at such points.
19.b Implementation Note: Implementations may of course check for abort
more often than at each abort completion point; ideally, a fully
preemptive implementation of abort will be provided. If preemptive
abort is not supported in a given environment, then supporting the
checking for abort as part of subprogram calls and loop iterations
might be a useful option.
Bounded (Run-Time) Errors
20 {bounded error (cause) [partial]} An attempt to execute an
asynchronous_select as part of the execution of an abort-deferred operation is
a bounded error. Similarly, an attempt to create a task that depends on a
master that is included entirely within the execution of an abort-deferred
operation is a bounded error.
{Program_Error (raised by failure of run-time check)} In both cases,
Program_Error is raised if the error is detected by the implementation;
otherwise the operations proceed as they would outside an abort-deferred
operation, except that an abort of the abortable_part or the created task
might or might not have an effect.
20.a Reason: An asynchronous_select relies on an abort of the
abortable_part to effect the asynchronous transfer of control. For
an asynchronous_select within an abort-deferred operation, the
abort might have no effect.
20.b Creating a task dependent on a master included within an
abort-deferred operation is considered an error, because such
tasks could be aborted while the abort-deferred operation was
still progressing, undermining the purpose of abort-deferral.
Alternatively, we could say that such tasks are abort-deferred for
their entire execution, but that seems too easy to abuse. Note
that task creation is already a bounded error in protected
actions, so this additional rule only applies to local task
creation as part of Initialize, Finalize, or Adjust.
Erroneous Execution
21 {normal state of an object [partial]} {abnormal state of an object
[partial]} {disruption of an assignment} {erroneous execution (cause)
[partial]} If an assignment operation completes prematurely due to an abort,
the assignment is said to be disrupted; the target of the assignment or its
parts can become abnormal, and certain subsequent uses of the object can be
erroneous, as explained in 13.9.1.
NOTES
22 41 An abort_statement should be used only in situations requiring
unconditional termination.
23 42 A task is allowed to abort any task it can name, including itself.
24 43 Additional requirements associated with abort are given in D.6
, "Preemptive Abort".
Wording Changes from Ada 83
24.a This clause has been rewritten to accommodate the concept of
aborting the execution of a construct, rather than just of a task.
9.9 Task and Entry Attributes
Dynamic Semantics
1 For a prefix T that is of a task type [(after any implicit dereference)],
the following attributes are defined:
2 T'Callable Yields the value True when the task denoted by T is callable,
and False otherwise; {task state (callable) [partial]}
{callable} a task is callable unless it is completed or
abnormal. The value of this attribute is of the predefined
type Boolean.
3 T'Terminated
Yields the value True if the task denoted by T is terminated,
and False otherwise. The value of this attribute is of the
predefined type Boolean.
4 For a prefix E that denotes an entry of a task or protected unit, the
following attribute is defined. This attribute is only allowed within the body
of the task or protected unit, but excluding, in the case of an entry of a
task unit, within any program unit that is, itself, inner to the body of the
task unit.
5 E'Count Yields the number of calls presently queued on the entry E of
the current instance of the unit. The value of this attribute
is of the type universal_integer.
NOTES
6 44 For the Count attribute, the entry can be either a single entry or
an entry of a family. The name of the entry or entry family can be
either a direct_name or an expanded name.
7 45 Within task units, algorithms interrogating the attribute E'Count
should take precautions to allow for the increase of the value of this
attribute for incoming entry calls, and its decrease, for example with
timed_entry_calls. Also, a conditional_entry_call may briefly increase
this value, even if the conditional call is not accepted.
8 46 Within protected units, algorithms interrogating the attribute
E'Count in the entry_barrier for the entry E should take precautions
to allow for the evaluation of the condition of the barrier both
before and after queuing a given caller.
9.10 Shared Variables
Static Semantics
1 {shared variable (protection of)} {independently addressable} If two
different objects, including nonoverlapping parts of the same object, are
independently addressable, they can be manipulated concurrently by two
different tasks without synchronization. Normally, any two nonoverlapping
objects are independently addressable. However, if packing, record layout, or
Component_Size is specified for a given composite object, then it is
implementation defined whether or not two nonoverlapping parts of that
composite object are independently addressable.
1.a Implementation defined: Whether or not two nonoverlapping parts of
a composite object are independently addressable, in the case
where packing, record layout, or Component_Size is specified for
the object.
1.b Implementation Note: Independent addressability is the only high
level semantic effect of a pragma Pack. If two objects are
independently addressable, the implementation should allocate them
in such a way that each can be written by the hardware without
writing the other. For example, unless the user asks for it, it is
generally not feasible to choose a bit-packed representation on a
machine without an atomic bit field insertion instruction, because
there might be tasks that update neighboring subcomponents
concurrently, and locking operations on all subcomponents is
generally not a good idea.
1.c Even if packing or one of the other above-mentioned aspects is
specified, subcomponents should still be updated independently if
the hardware efficiently supports it.
Dynamic Semantics
2 [Separate tasks normally proceed independently and concurrently with one
another. However, task interactions can be used to synchronize the actions of
two or more tasks to allow, for example, meaningful communication by the
direct updating and reading of variables shared between the tasks.] The
actions of two different tasks are synchronized in this sense when an action
of one task signals an action of the other task;
{signal (as defined between actions)} an action A1 is defined to signal an
action A2 under the following circumstances:
3 * If A1 and A2 are part of the execution of the same task, and the
language rules require A1 to be performed before A2;
4 * If A1 is the action of an activator that initiates the activation of a
task, and A2 is part of the execution of the task that is activated;
5 * If A1 is part of the activation of a task, and A2 is the action of
waiting for completion of the activation;
6 * If A1 is part of the execution of a task, and A2 is the action of
waiting for the termination of the task;
6.1/1 * {8652/0031} {AI95-00118-01} If A1 is the termination of a task T,
and A2 is either the evaluation of the expression T'Terminated or a
call to Ada.Task_Identification.Is_Terminated with an actual parameter
that identifies T (see C.7.1);
7 * If A1 is the action of issuing an entry call, and A2 is part of the
corresponding execution of the appropriate entry_body or
accept_statement.
7.a Ramification: Evaluating the entry_index of an accept_statement is
not synchronized with a corresponding entry call, nor is
evaluating the entry barrier of an entry_body.
8 * If A1 is part of the execution of an accept_statement or entry_body,
and A2 is the action of returning from the corresponding entry call;
9 * If A1 is part of the execution of a protected procedure body or
entry_body for a given protected object, and A2 is part of a later
execution of an entry_body for the same protected object;
9.a Reason: The underlying principle here is that for one action to
"signal" a second, the second action has to follow a potentially
blocking operation, whose blocking is dependent on the first
action in some way. Protected procedures are not potentially
blocking, so they can only be "signalers," they cannot be
signaled.
9.b Ramification: Protected subprogram calls are not defined to signal
one another, which means that such calls alone cannot be used to
synchronize access to shared data outside of a protected object.
9.c Reason: The point of this distinction is so that on
multiprocessors with inconsistent caches, the caches only need to
be refreshed at the beginning of an entry body, and forced out at
the end of an entry body or protected procedure that leaves an
entry open. Protected function calls, and protected subprogram
calls for entryless protected objects do not require full cache
consistency. Entryless protected objects are intended to be
treated roughly like atomic objects - each operation is
indivisible with respect to other operations (unless both are
reads), but such operations cannot be used to synchronize access
to other nonvolatile shared variables.
10 * If A1 signals some action that in turn signals A2.
Erroneous Execution
11 {erroneous execution (cause) [partial]} Given an action of assigning to an
object, and an action of reading or updating a part of the same object (or of
a neighboring object if the two are not independently addressable), then the
execution of the actions is erroneous unless the actions are sequential.
{sequential (actions)} Two actions are sequential if one of the following is
true:
12 * One action signals the other;
13 * Both actions occur as part of the execution of the same task;
13.a Reason: Any two actions of the same task are sequential, even if
one does not signal the other because they can be executed in an
"arbitrary" (but necessarily equivalent to some "sequential")
order.
14 * Both actions occur as part of protected actions on the same protected
object, and at most one of the actions is part of a call on a
protected function of the protected object.
14.a Reason: Because actions within protected actions do not always
imply signaling, we have to mention them here explicitly to make
sure that actions occurring within different protected actions of
the same protected object are sequential with respect to one
another (unless both are part of calls on protected functions).
14.b Ramification: It doesn't matter whether or not the variable being
assigned is actually a subcomponent of the protected object;
globals can be safely updated from within the bodies of protected
procedures or entries.
15 A pragma Atomic or Atomic_Components may also be used to ensure that
certain reads and updates are sequential - see C.6.
15.a Ramification: If two actions are "sequential" it is known that
their executions don't overlap in time, but it is not necessarily
specified which occurs first. For example, all actions of a single
task are sequential, even though the exact order of execution is
not fully specified for all constructs.
15.b Discussion: Note that if two assignments to the same variable are
sequential, but neither signals the other, then the program is not
erroneous, but it is not specified which assignment ultimately
prevails. Such a situation usually corresponds to a programming
mistake, but in some (rare) cases, the order makes no difference,
and for this reason this situation is not considered erroneous nor
even a bounded error. In Ada 83, this was considered an "
incorrect order dependence" if the "effect" of the program was
affected, but "effect" was never fully defined. In Ada 95, this
situation represents a potential nonportability, and a friendly
compiler might want to warn the programmer about the situation,
but it is not considered an error. An example where this would
come up would be in gathering statistics as part of referencing
some information, where the assignments associated with statistics
gathering don't need to be ordered since they are just
accumulating aggregate counts, sums, products, etc.
Wording Changes from Ada 95
15.c/2 {8652/0031} {AI95-00118-01} Corrigendum: Clarified that a task T2
can rely on values of variables that are updated by another task
T1, if task T2 first verifies that T1'Terminated is True.
9.11 Example of Tasking and Synchronization
Examples
1 The following example defines a buffer protected object to smooth
variations between the speed of output of a producing task and the speed of
input of some consuming task. For instance, the producing task might have the
following structure:
2 task Producer;
3/2 {AI95-00433-01} task body Producer is
Person : Person_Name; -- see 3.10.1
begin
loop
... -- simulate arrival of the next customer
Buffer.Append_Wait(Person);
exit when Person = null;
end loop;
end Producer;
4 and the consuming task might have the following structure:
5 task Consumer;
6/2 {AI95-00433-01} task body Consumer is
Person : Person_Name;
begin
loop
Buffer.Remove_First_Wait(Person);
exit when Person = null;
... -- simulate serving a customer
end loop;
end Consumer;
7/2 {AI95-00433-01} The buffer object contains an internal array of person
names managed in a round-robin fashion. The array has two indices, an In_Index
denoting the index for the next input person name and an Out_Index denoting
the index for the next output person name.
7.1/2 {AI95-00433-01} The Buffer is defined as an extension of the
Synchronized_Queue interface (see 3.9.4), and as such promises to implement
the abstraction defined by that interface. By doing so, the Buffer can be
passed to the Transfer class-wide operation defined for objects of a type
covered by Queue'Class.
8/2 {AI95-00433-01}
protected Buffer is new Synchronized_Queue with -- see 3.9.4
entry Append_Wait(Person : in Person_Name);
entry Remove_First_Wait(Person : out Person_Name);
function Cur_Count return Natural;
function Max_Count return Natural;
procedure Append(Person : in Person_Name);
procedure Remove_First(Person : out Person_Name);
private
Pool : Person_Name_Array(1 .. 100);
Count : Natural := 0;
In_Index, Out_Index : Positive := 1;
end Buffer;
9/2 {AI95-00433-01} protected body Buffer is
entry Append_Wait(Person : in Person_Name)
when Count < Pool'Length is
begin
Append(Person);
end Append_Wait;
9.1/2 {AI95-00433-01} procedure Append(Person : in Person_Name) is
begin
if Count = Pool'Length then
raise Queue_Error with "Buffer Full"; -- see 11.3
end if;
Pool(In_Index) := Person;
In_Index := (In_Index mod Pool'Length) + 1;
Count := Count + 1;
end Append;
10/2 {AI95-00433-01} entry Remove_First_Wait(Person : out Person_Name)
when Count > 0 is
begin
Remove_First(Person);
end Remove_First_Wait;
11/2 {AI95-00433-01} procedure Remove_First(Person : out Person_Name) is
begin
if Count = 0 then
raise Queue_Error with "Buffer Empty"; -- see 11.3
end if;
Person := Pool(Out_Index);
Out_Index := (Out_Index mod Pool'Length) + 1;
Count := Count - 1;
end Remove_First;
12/2 {AI95-00433-01} function Cur_Count return Natural is
begin
return Buffer.Count;
end Cur_Count;
13/2 {AI95-00433-01} function Max_Count return Natural is
begin
return Pool'Length;
end Max_Count;
end Buffer;
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