9 Tasks and Synchronization 1/3 The execution of an Ada program consists of the execution of one or more 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 clause, and include: 2 * the activation and termination of a task; 3 * 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 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. 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. While ready, a task competes for the available execution resources that it requires to run. 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. 9.1 Task Units and Task Objects 1 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 2/3 task_type_declaration ::= task type defining_identifier [known_discriminant_part] [aspect_specification] [is [new interface_list with] task_definition]; 3/3 single_task_declaration ::= task defining_identifier [aspect_specification][is [new interface_list with] task_definition]; 4 task_definition ::= {task_item} [ private {task_item}] end [task_identifier] 5/1 task_item ::= entry_declaration | aspect_clause 6/3 task_body ::= task body defining_identifier [aspect_specification] 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. Paragraph 8 was deleted. Static Semantics 9 A task_definition defines a task type and its first subtype. The first list of task_items of a task_definition, together with the known_discriminant_- part, if any, is called the visible part of the task unit. The optional list of task_items after the reserved word private is called the private part of the task unit. 9.1/1 For a task declaration without a task_definition, a task_definition without task_items is assumed. 9.2/3 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 using an implicitly declared nonabstract subprogram which has the same profile as the inherited subprogram and which overrides it. Legality Rules 9.3/2 A task declaration requires a completion, which shall be a task_body, and every task_body shall be the completion of some task declaration. 9.4/2 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. 9.5/3 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 subprogram has the same defining name as the entry and the first parameter of the subprogram is of the task type or is an access parameter designating the task type. 9.6/2 For each primitive subprogram inherited by the type declared by a task declaration, at most one of the following shall apply: 9.7/2 * 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 9.8/2 * 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. 9.9/2 If neither applies, the inherited subprogram shall be a null procedure. 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. Dynamic Semantics 10 The elaboration of a task declaration elaborates the task_definition. The elaboration of a single_task_declaration also creates an object of an (anonymous) task type. 11 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 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. 13 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; 18 * A representation of the state of the associated task. NOTES 19/2 2 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). 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 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 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); 9.2 Task Execution - Task Activation Dynamic Semantics 1 The execution of a task of a given task type consists of the execution of the corresponding task_body. 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. 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 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. 3/2 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. 4/2 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. 5 The task that created the new tasks and initiated their activations (the activator) is blocked until all of these activations complete (successfully or not). 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. 6/3 If the master that directly encloses the point where the activation of a task T would be initiated, completes before the activation of T is initiated, T becomes terminated and is never activated. Furthermore, if a return statement is left such that the return object is not returned to the caller, any task that was created as a part of the return object or one of its coextensions immediately becomes terminated and is 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; 9.3 Task Dependence - Termination of Tasks Dynamic Semantics 1 Each task (other than an environment task - see 10.2) depends on one or more masters (see 7.6.1), as follows: 2/4 * If the task is created by the evaluation of an allocator for a given named 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 * 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. 4 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. 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. 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 * 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. 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 9.4 Protected Units and Protected Objects 1 A protected object provides coordinated access to shared data, through calls on its visible protected operations, which can be protected subprograms or protected entries. 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. Syntax 2/3 protected_type_declaration ::= protected type defining_identifier [known_discriminant_part] [aspect_specification] is [new interface_list with] protected_definition; 3/3 single_protected_declaration ::= protected defining_identifier [aspect_specification] is [new interface_list with] protected_definition; 4 protected_definition ::= { protected_operation_declaration } [ private { protected_element_declaration } ] end [protected_identifier] 5/1 protected_operation_declaration ::= subprogram_declaration | entry_declaration | aspect_clause 6 protected_element_declaration ::= protected_operation_declaration | component_declaration 7/3 protected_body ::= protected body defining_identifier [aspect_specification] is { protected_operation_item } end [protected_identifier]; 8/4 protected_operation_item ::= subprogram_declaration | subprogram_body | null_procedure_declaration | expression_function_declaration | 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. Paragraph 10 was deleted. Static Semantics 11/2 A protected_definition defines a protected type and its first subtype. 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. The optional list of protected_element_declarations after the reserved word private is called the private part of the protected unit. 11.1/3 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 using an implicitly declared nonabstract subprogram which has the same profile as the inherited subprogram and which overrides it. Legality Rules 11.2/2 A protected declaration requires a completion, which shall be a protected_body, and every protected_body shall be the completion of some protected declaration. 11.3/2 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. 11.4/3 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 subprogram has the same defining name as the protected operation and the first parameter of the subprogram is of the protected type or is an access parameter designating the protected type. 11.5/2 For each primitive subprogram inherited by the type declared by a protected declaration, at most one of the following shall apply: 11.6/2 * 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 11.7/2 * 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. 11.8/2 If neither applies, the inherited subprogram shall be a null procedure. 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. 11.9/3 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. If an inherited subprogram is implemented by a protected function, then the first parameter of the inherited subprogram shall be of mode in, but not an access-to-variable parameter. 11.10/2 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 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. Dynamic Semantics 12 The elaboration of a protected declaration elaborates the protected_definition. The elaboration of a single_protected_declaration also creates an object of an (anonymous) protected type. 13 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 As part of the initialization of a protected object, any per-object constraints (see 3.8) are elaborated. 15 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; 18 * 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 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. Bounded (Run-Time) Errors 20.1/2 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. NOTES 21/2 13 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). 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 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. 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; 9.5 Intertask Communication 1 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 2/3 When a name or prefix denotes an entry, protected subprogram, or a prefixed view of a primitive subprogram of a limited interface whose first parameter is a controlling parameter, the name or prefix determines a target object, as follows: 3/3 * 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; a call using such a name is defined to be an internal call; 4/3 * If it is a selected_component that is not an expanded name, then the target object is explicitly specified to be the object denoted by the prefix of the name; a call using such a name is defined to be an external call; 5/3 * 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; a call using such a name 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. 6.1/3 A call on an entry or a protected subprogram either uses a name or prefix that determines a target object implicitly, as above, or is a call on (a non-prefixed view of) a primitive subprogram of a limited interface whose first parameter is a controlling parameter, in which case the target object is identified explicitly by the first parameter. This latter case is an external call. 7 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 7.1/3 If a name or prefix determines a target object, and the name denotes a protected entry or procedure, then the target object shall be a variable, unless the prefix is for an attribute_reference to the Count attribute (see 9.9). 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. 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. Syntax 10/3 synchronization_kind ::= By_Entry | By_Protected_Procedure | Optional Static Semantics 11/3 For the declaration of a primitive procedure of a synchronized tagged type the following language-defined representation aspect may be specified with an aspect_specification (see 13.1.1): 12/3 Synchronization If specified, the aspect definition shall be a synchronization_kind. 13/3 Inherited subprograms inherit the Synchronization aspect, if any, from the corresponding subprogram of the parent or progenitor type. If an overriding operation does not have a directly specified Synchronization aspect then the Synchronization aspect of the inherited operation is inherited by the overriding operation. Legality Rules 14/3 The synchronization_kind By_Protected_Procedure shall not be applied to a primitive procedure of a task interface. 15/3 A procedure for which the specified synchronization_kind is By_Entry shall be implemented by an entry. A procedure for which the specified synchronization_kind is By_Protected_Procedure shall be implemented by a protected procedure. A procedure for which the specified synchronization_kind is Optional may be implemented by an entry or by a procedure (including a protected procedure). 16/3 If a primitive procedure overrides an inherited operation for which the Synchronization aspect has been specified to be By_Entry or By_Protected_Procedure, then any specification of the aspect Synchronization applied to the overriding operation shall have the same synchronization_kind. 17/3 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. NOTES 18/3 18 The synchronization_kind By_Protected_Procedure implies that the operation will not block. 9.5.1 Protected Subprograms and Protected Actions 1 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. 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). 2.1/4 For a type declared by a protected_type_declaration or for the anonymous type of an object declared by a single_protected_declaration, the following language-defined type-related representation aspect may be specified: 2.2/4 Exclusive_Functions The type of aspect Exclusive_Functions is Boolean. If not specified (including by inheritance), the aspect is False. 2.3/4 A value of True for this aspect indicates that protected functions behave in the same way as protected procedures with respect to mutual exclusion and queue servicing (see below). 2.4/4 A protected procedure or entry is an exclusive protected operation. A protected function of a protected type P is an exclusive protected operation if the Exclusive_Functions aspect of P is True. Dynamic Semantics 3 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). 4/4 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 nonexclusive protected function. This rule is expressible in terms of the execution resource associated with the protected object: 5/4 * Starting a protected action on a protected object corresponds to acquiring the execution resource associated with the protected object, either for exclusive read-write access if the protected action is for a call on an exclusive protected operation, or for concurrent read-only access otherwise; 6 * Completing the protected action corresponds to releasing the associated execution resource. 7/4 After performing an exclusive protected operation on a protected object, 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 During a protected action, it is a bounded error to invoke an operation that is potentially blocking. The following are defined to be potentially blocking operations: 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; 16 * a call on a subprogram whose body contains a potentially blocking operation. 17 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. 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. NOTES 19 19 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". 20 20 The body of a protected unit may contain declarations and bodies for local subprograms. These are not visible outside the protected unit. 21 21 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 22 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. 22.1/2 23 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; 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 2/3 entry_declaration ::= [overriding_indicator] entry defining_identifier [(discrete_subtype_definition )] parameter_profile [aspect_specification]; 3 accept_statement ::= accept entry_direct_name [(entry_index)] parameter_profile [do handled_sequence_of_statements end [entry_identifier]]; 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]; 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. 10.1/2 An overriding_indicator is not allowed in an entry_declaration that includes a discrete_subtype_definition. Name Resolution Rules 11 In an accept_statement, the expected profile for the entry_direct_name is that of the entry_declaration; 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. Legality Rules 13 An entry_declaration in a task declaration shall not contain a specification for an access parameter (see 3.10). 13.1/2 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 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. 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. 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. 16 An entry_declaration of a protected unit requires a completion, which shall be an entry_body, and every entry_body shall be the completion of an entry_declaration of a protected unit. The profile of the entry_body shall conform fully to that of the corresponding declaration. 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). 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). 20 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. 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; the value of the named entry index identifies which entry of the family was called. Dynamic Semantics 22/1 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. 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 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. 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. 25 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). 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). NOTES 27 24 A task entry has corresponding accept_statements (zero or more), whereas a protected entry has a corresponding entry_body (exactly one). 28 25 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 26 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. 30 27 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; 9.5.3 Entry Calls 1 An entry_call_statement (an entry call) can appear in various contexts. 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 Under certain circumstances (detailed below), an entry of a task or protected object is checked to see whether it is open or closed: 6/3 * 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. 7/3 * 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. 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. 8 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). 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; 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. 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; 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 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; 15/4 * If after performing, as part of a protected action on the associated protected object, an exclusive protected operation on the object, the entry is checked and found to be open. 16 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 The entry queuing policy controls selection among queued calls both for task and protected entry queues. 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. 19 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. 20 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). 21 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. 23/4 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 call to an exclusive protected operation of the object since the condition was last evaluated. 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. 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. NOTES 26 28 If an exception is raised during the execution of an entry_body, it is propagated to the corresponding caller (see 11.4). 27 29 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. 28 30 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). 29 31 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. 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. Syntax 2/3 requeue_statement ::= requeue procedure_or_entry_name [with abort]; Name Resolution Rules 3/3 The procedure_or_entry_name of a requeue_statement shall resolve to denote a procedure or an entry (the requeue target). The profile of the entry, or the profile or prefixed profile of the procedure, shall either have no parameters, or be type conformant (see 6.3.1) with the profile of the innermost enclosing entry_body or accept_statement. 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. 5/3 If the requeue target has parameters, then its (prefixed) profile shall be subtype conformant with the profile of the innermost enclosing callable construct. 5.1/4 Given a requeue_statement where the innermost enclosing callable construct is for an entry E1, for every specific or class-wide postcondition expression P1 that applies to E1, there shall exist a postcondition expression P2 that applies to the requeue target E2 such that 5.2/4 * P1 is fully conformant with the expression produced by replacing each reference in P2 to a formal parameter of E2 with a reference to the corresponding formal paramter of E1; and 5.3/4 * if P1 is enabled, then P2 is also enabled. 5.4/4 The requeue target shall not have an applicable specific or class-wide postcondition which includes an Old attribute_reference. 5.5/4 If the requeue target is declared immediately within the task_definition of a named task type or the protected_definition of a named protected type, and if the requeue statement occurs within the body of that type, and if the requeue is an external requeue, then the requeue target shall not have a specific or class-wide postcondition which includes a name denoting either the current instance of that type or any entity declared within the declaration of that type. 5.6/4 If the target is a procedure, the name shall denote a renaming of an entry, or shall denote a view or a prefixed view of a primitive subprogram of a synchronized interface, where the first parameter of the unprefixed view of the primitive subprogram shall be a controlling parameter, and the Synchronization aspect shall be specified with synchronization_kind By_Entry for the primitive subprogram. 6/3 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 for the entry_body. Dynamic Semantics 7/4 The execution of a requeue_statement proceeds by first evaluating the procedure_or_entry_name, including the prefix identifying the target task or protected object and the expression identifying the entry within an entry family, if any. Precondition checks are then performed as for a call to the requeue target entry or subprogram. The entry_body or accept_statement enclosing the requeue_statement is then completed, finalized, and left (see 7.6.1). 8 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 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); 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). 12/4 If the requeue target named in the requeue_statement has formal parameters, then during the execution of the accept_statement or entry_body corresponding to the new entry and during the checking of any preconditions of 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 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. NOTES 17 32 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. 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 9.6 Delay Statements, Duration, and Time 1 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. 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 The expected type for the delay_expression in a delay_relative_statement is the predefined type Duration. The delay_expression in a delay_until_statement is expected to be of any nonlimited type. Legality Rules 6/3 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), the type Time in the package Real_Time (see D.8), or some other implementation-defined time type. 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. 8/3 A value of the type Time in package Calendar, or of some other 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 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; 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 For the execution of a delay_statement, the delay_expression is first evaluated. 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. 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. The time base associated with relative delays is as defined in D.9, "Delay Accuracy" or is implementation defined. 21 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. 22/3 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. 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. 24/2 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. 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 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. 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 28/3 An implementation may define additional time types. 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. Implementation Advice 30 Whenever possible in an implementation, 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. NOTES 32 33 A delay_relative_statement with a negative value of the delay_expression is equivalent to one with a zero value. 33 34 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 35 A delay_statement is an abort completion point and a potentially blocking operation, even if the task is not actually blocked. 35 36 There is no necessary relationship between System.Tick (the resolution of the clock of package Calendar) and Duration'Small (the small of type Duration). 36 37 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 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; 9.6.1 Formatting, Time Zones, and other operations for Time Static Semantics 1/2 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; 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; 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 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; 42/3 Returns, as a number of minutes, the result of subtracting the implementation-defined time zone of Calendar from UTC time, at the time Date. If the time zone of the Calendar implementation is unknown, then Unknown_Zone_Error is raised. 43/2 procedure Difference (Left, Right : in Time; Days : out Day_Count; Seconds : out Duration; Leap_Seconds : out Leap_Seconds_Count); 44/2 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). 45/2 function "+" (Left : Time; Right : Day_Count) return Time; function "+" (Left : Day_Count; Right : Time) return Time; 46/2 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 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 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 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 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 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 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 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 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 Returns the second within the hour and minute for Date. 65/2 function Sub_Second (Date : Time) return Second_Duration; 66/2 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 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); 70/3 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. If Seconds = 86400.0, Split propagates Time_Error. 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 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. 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 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 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 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 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 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. 83/2 function Value (Date : String; Time_Zone : Time_Zones.Time_Offset := 0) return Time; 84/2 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. 85/2 function Image (Elapsed_Time : Duration; Include_Time_Fraction : Boolean := False) return String; 86/2 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. 87/2 function Value (Elapsed_Time : String) return Duration; 88/2 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. Implementation Advice 89/2 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. NOTES 90/2 38 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). 91/2 39 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. 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; 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. 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 * 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. Dynamic Semantics 14 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 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. 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. 20/3 * An open terminate_alternative is selected if the conditions stated at the end of subclause 9.3 are satisfied. 21 The exception Program_Error is raised if all alternatives are closed and there is no else part. NOTES 22 40 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; 9.7.2 Timed Entry Calls 1/2 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. Syntax 2 timed_entry_call ::= select entry_call_alternative or delay_alternative end select; 3/2 entry_call_alternative ::= procedure_or_entry_call [sequence_of_statements] 3.1/2 procedure_or_entry_call ::= procedure_call_statement | entry_call_statement Legality Rules 3.2/2 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). Dynamic Semantics 4/2 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. 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; 9.7.3 Conditional Entry Calls 1/2 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. Syntax 2 conditional_entry_call ::= select entry_call_alternative else sequence_of_statements end select; Dynamic Semantics 3 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 41 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; 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 triggering_statement ::= procedure_or_entry_call | delay_statement 5 abortable_part ::= sequence_of_statements Dynamic Semantics 6/2 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 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. Examples 10 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: 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; 14/4 Note that these examples presume that there are abort completion points within the execution of the abortable_part. 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 Each task_name is expected to be of any task type; they need not all be of the same task type. Dynamic Semantics 4 For the execution of an abort_statement, the given task_names are evaluated in an arbitrary order. 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. 5 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; 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. 13 When a master is aborted, all tasks that depend on that master are aborted. 14 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; 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; 19 * the start of the execution of a select_statement, or of the sequence_of_statements of an exception_handler. Bounded (Run-Time) Errors 20/3 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. 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. Erroneous Execution 21 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 42 An abort_statement should be used only in situations requiring unconditional termination. 23 43 A task is allowed to abort any task it can name, including itself. 24 44 Additional requirements associated with abort are given in D.6 , "Preemptive Abort". 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; 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 45 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 46 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 47 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 1/3 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. Any two nonoverlapping objects are independently addressable if either object is specified as independently addressable (see C.6). Otherwise, two nonoverlapping objects are independently addressable except when they are both parts of a composite object for which a nonconfirming value is specified for any of the following representation aspects: (record) Layout, Component_Size, Pack, Atomic, or Convention; in this case it is unspecified whether the parts are independently addressable. 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; 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; 6.1/3 * If A1 is the termination of a task T, and A2 is either an evaluation of the expression T'Terminated that results in True, or a call to Ada.Task_Identification.Is_Terminated with an actual parameter that identifies T and a result of True (see C.7.1); 7/3 * 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; 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; 10 * If A1 signals some action that in turn signals A2. Erroneous Execution 11 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. 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; 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. 15/3 Aspect Atomic or aspect Atomic_Components may also be specified to ensure that certain reads and updates are sequential - see C.6. 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 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 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 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 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 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 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 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 entry Remove_First_Wait(Person : out Person_Name) when Count > 0 is begin Remove_First(Person); end Remove_First_Wait; 11/2 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 function Cur_Count return Natural is begin return Buffer.Count; end Cur_Count; 13/2 function Max_Count return Natural is begin return Pool'Length; end Max_Count; end Buffer;