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9.5 Intertask Communication

The primary means for intertask communication is provided by calls on entries and protected subprograms. Calls on protected subprograms allow coordinated access to shared data objects. Entry calls allow for blocking the caller until a given condition is satisfied (namely, that the corresponding entry is open — see 9.5.3), and then communicating data or control information directly with another task or indirectly via a shared protected object.

Static Semantics

{AI05-0225-1} {AI05-0291-1} 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: 
To be honest: {AI05-0291-1} This wording uses "denotes" to mean "denotes a view of an entity" (when the term is used in Legality Rules), and "denotes an entity" (when the term is used in Dynamic Semantics rules). It does not mean "view of a declaration", as that would not include renames (a renames is not an entry or protected subprogram). 
{AI05-0291-1} 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;
{AI05-0291-1} 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
Discussion: For example: 
protected type Pt is
  procedure Op1;
  procedure Op2;
end Pt;
PO : Pt;
Other_Object : Some_Other_Protected_Type;
protected body Pt is
  procedure Op1 is begin ... end Op1;
  procedure Op2 is
    Op1; -- An internal call.
    Pt.Op1; -- Another internal call.
    PO.Op1; -- An external call. It the current instance is PO, then
            -- this is a bounded error (see 9.5.1).
    Other_Object.Some_Op; -- An external call.
  end Op2;
end Pt;
{AI05-0291-1} 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;
If the name or prefix denotes a subprogram_renaming_declaration, then the target object is as determined by the name of the renamed entity.
 {AI05-0291-1} 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.
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

 {AI95-00345-01} {AI05-0225-1} {AI05-0291-1} 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). 
Reason: {AI05-0225-1} The point is to prevent any calls to such a name whose target object is a constant view of a protected object, directly, or via an access value, renames, or generic formal subprogram. It is, however, legal to say P'Count in a protected function body, even though the protected object is a constant view there. 
Ramification: {AI05-0291-1} This rule does not apply to calls that are not to a prefixed view. Specifically a "normal" call to a primitive operation of a limited interface is not covered by this rule. In that case, the normal parameter passing mode checks will prevent passing a constant protected object to an operation implemented by a protected entry or procedure as the mode is required to be in out or out
 {AI12-0166-1} An internal call on a protected function shall not occur within a precondition expression (see 6.1.1) of a protected operation nor within a default_expression of a parameter_specification of a protected operation.
Reason: {AI125-0166-1} These calls will be made before the start of the protected action, and thus would not be subject to the expected mutual exclusion. As such, they would be an automatic race condition (the state of the called object could change before the start of the protected action for the call on the protected entry or subprogram). 
To be honest: {AI125-0166-1} 6.1.1 actually defines "specific precondition expression" and "class-wide precondition expression". This rule is intended to apply to both. 

Dynamic Semantics

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. 
To be honest: The current instance is defined in the same way within the body of a subprogram declared immediately within a protected_body.
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. 
Reason: Read/write access to the components of a protected object is granted while inside the body of a protected procedure or entry. Also, any protected entry call can change the value of the Count attribute, which represents an update. Any protected procedure call can result in servicing the entries, which again might change the value of a Count attribute. 


{AI05-0030-2} {AI05-0215-1} synchronization_kind ::= By_Entry | By_Protected_Procedure | Optional

Static Semantics

{AI05-0215-1} 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):

If specified, the aspect definition shall be a synchronization_kind.
Aspect Description for Synchronization: Defines whether a given primitive operation of a synchronized interface must be implemented by an entry or protected procedure.
{AI05-0030-2} {AI05-0215-1} 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

{AI05-0030-2} {AI05-0215-1} The synchronization_kind By_Protected_Procedure shall not be applied to a primitive procedure of a task interface.
{AI05-0030-2} {AI05-0215-1} 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).
{AI05-0030-2} {AI05-0215-1} 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.
{AI05-0030-2} 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. 

Static Semantics

{AI12-0064-2} {AI12-0374-2} For a program unit, task entry, formal package, formal subprogram, formal object of an anonymous access-to-subprogram type, enumeration literal, and for a subtype (including a formal subtype), the following language-defined operational aspect is defined:
This aspect specifies the blocking restriction for the entity; it shall be specified by a static Boolean expression. [The aspect_definition can be omitted from the specification of this aspect; in that case the nonblocking expression for the entity is the enumeration literal True.]
Aspect Description for Nonblocking: Specifies that an associated subprogram does not block.
Proof: 13.1.1 allows omitting the aspect expression for any aspect with type Boolean; we take advantage of that here.
{AI12-0064-2} The Nonblocking aspect may be specified for all entities for which it is defined, except for protected operations and task entries. In particular, Nonblocking may be specified for generic formal parameters.
Ramification: The Nonblocking aspect cannot be specified for predefined operators or enumeration literals but we don't need to mention that above. One would have to declare a subprogram in order to specify the aspect in those cases, but that defines a user-defined subprogram which is itself not a predefined operator or an enumeration literal. 
{AI12-0064-2} {AI12-0374-2} When aspect Nonblocking is False for an entity, the entity might contain a potentially blocking operation; such an entity allows blocking. If the aspect is True for an entity, the entity is said to be nonblocking.
Ramification: Specifying Nonblocking as False imposes no restrictions. Specifying Nonblocking as True imposes additional compile-time checks to prevent blocking, but does not prevent deadlock. A pragma Detect_Blocking can be used to ensure that Program_Error is raised in a deadlock situation. 
{AI12-0064-2} {AI12-0374-2} For a generic instantiation and entities declared within such an instance, the aspect is determined by the Nonblocking aspect for the corresponding entity of the generic unit, anded with the Nonblocking aspects of the actual generic parameters used by the entity. If the aspect is directly specified for an instance, the specified expression shall have the same value as the Nonblocking aspect of the instance (after anding with the aspects of the used actual parameters). In the absence of a Use_Formal aspect, all actual generic parameters are presumed to be used by an entity (see H.7.1).
Reason: We want to allow confirming aspects for instances, but nothing else. The Legality Rules of the generic body were checked assuming the nonblocking expression of the generic unit, and if that is changed, the instance body might make calls that allow blocking in subprograms that are declared nonblocking.
{AI12-0064-2} {AI12-0374-2} For a (protected or task) entry, the Nonblocking aspect is False.
Reason: An entry can be renamed as a procedure, so the value of the aspect has to be well-defined (as the attribute can be applied to a procedure). We do not want a nonblocking subprogram to be able to call an entry, no matter how it occurs, so the value ought to be False. Moreover, we do not want a subprogram that renames an entry to be able to override a nonblocking subprogram. We could have used individual rules for these cases, but there were already many of them, and this solution avoids the need for extra rules for entries.
{AI12-0064-2} {AI12-0374-2} For an enumeration literal, the Nonblocking aspect is True.
Reason: Enumeration literals can be renamed as functions, and passed to generic formal functions, so we need to define the value of the aspect to ensure the other rules are meaningful. 
{AI12-0064-2} {AI12-0374-2} For a predefined operator of an elementary type, the Nonblocking aspect is True. For a predefined operator of a composite type, the Nonblocking aspect of the operator is the same as the Nonblocking aspect for the type.
Reason: Predefined operators of elementary types can never include any potentially blocking operations, so we want them to declare that. Record equality can be composed of operations including user-defined "=" operators, which might allow blocking. Array equality might use some record equality. So we have to have the possibility of allowing blocking for them. We don't just copy the Nonblocking aspect of the type in every case, as someone could declare an elementary type to allow blocking. 
Ramification: It's not possible to specify the nonblocking expression of a predefined operator; if an operator is declared in order to do that, it is no longer predefined.
{AI12-0064-2} For a dereference of an access-to-subprogram type, the Nonblocking aspect of the designated subprogram is that of the access-to-subprogram type.
{AI12-0374-2} For the base subtype of a scalar (sub)type, the Nonblocking aspect is True.
Reason: The first subtype of a scalar type can allow blocking (which can be useful so a predicate can allow blocking), but the base subtype is always Nonblocking. We need this so the Nonblocking value is well-defined for any subtype that is built from the base subtype (T'Base). T'Base of any scalar type, including a generic formal type, is always nonblocking. 
{AI12-0064-2} For an inherited primitive dispatching subprogram that is null or abstract, the subprogram is nonblocking if and only if a corresponding subprogram of at least one ancestor is nonblocking. For any other inherited subprogram, it is nonblocking if and only if the corresponding subprogram of the parent is nonblocking.
{AI12-0064-2} Unless directly specified, overridings of dispatching operations inherit this aspect.
{AI12-0064-2} {AI12-0374-2} Unless directly specified, for a formal subtype, formal package, or formal subprogram, the Nonblocking aspect is that of the actual subtype, package, or subprogram.
Reason: This means that Nonblocking legality checking for the actual parameters of the instance is only necessary when the aspect is explicitly specified for the formal type. 
{AI12-0064-2} {AI12-0374-2} Unless directly specified, for a non-first subtype S, the Nonblocking aspect is that of the subtype identified in the subtype_indication defining S; unless directly specified for the first subtype of a derived type, the Nonblocking aspect is that of the ancestor subtype.
Discussion: The expressions that can be specified for a such a subtype are limited by a Legality Rule, see below. 
{AI12-0064-2} Unless directly specified, for any other program unit, first subtype, or formal object, the Nonblocking aspect of the entity is determined by the Nonblocking aspect for the innermost program unit enclosing the entity.
{AI12-0064-2} {AI12-0374-2} If not specified for a library unit, the nonblocking expression is True if the library unit is declared pure, or False otherwise.
{AI12-0064-2} {AI12-0247-1} The following are defined to be potentially blocking operations:
Reason: The primary purpose of these rules is to define what operations are not allowed in a protected operation (blocking is not allowed). Some of these operations are not directly blocking. However, they are still treated as potentially blocking, because allowing them in a protected action might impose an undesirable implementation burden. 
a select_statement;
an accept_statement;
an entry_call_statement, or a call on a procedure that renames or is implemented by an entry;
a delay_statement;
an abort_statement;
task creation or activation;
during a protected action, an external call on a protected subprogram (or an external requeue) with the same target object as that of the protected action.
Reason: This is really a deadlocking call, rather than a blocking call, but we include it in this list for simplicity. 
{AI12-0064-2} If a language-defined subprogram allows blocking, then a call on the subprogram is a potentially blocking operation.
Ramification: Calls on other subprograms that allow blocking are not themselves potentially blocking; the execution of the body could execute a potentially blocking operation.
A user-defined instance of a language-defined generic creates user-defined subprograms for the purpose of this rule. A dispatching call to a language-defined abstract subprogram always calls a user-defined concrete subprogram, so that too is not potentially blocking for the purposes of this rule. 

Legality Rules

{AI12-0064-2} {AI12-0267-1} {AI12-0374-2} A portion of program text is called a nonblocking region if it is anywhere within a parallel construct, or if the innermost enclosing program unit is nonblocking. A nonblocking region shall not contain any of the following:
a select_statement;
an accept_statement;
a delay_statement;
an abort_statement;
task creation or activation.
{AI12-0374-2} Futhermore, a parallel construct shall neither contain a call on a callable entity for which the Nonblocking aspect is False, nor shall it contain a call on a callable entity declared within a generic unit that uses a generic formal parameter with Nonblocking aspect False (see Use_Formal aspect in H.7.1). Finally, a nonblocking region that is outside of a parallel construct shall not contain a call on a callable entity for which the Nonblocking aspect is False, unless the region is within a generic unit and the callable entity is associated with a generic formal parameter of the generic unit, or the call is within the aspect_definition of an assertion aspect for an entity that allows blocking.
Reason: {AI12-0374-2} A generic unit or entity declared within one is presumed to use its "used" generic formal parameters at least once each time it is invoked, and this passes through to the parallel construct check. 
Ramification: Implicit calls for finalization, storage pools, and the like are covered by the above prohibition. The rules above say “a call”, not “an explicit call”. Such calls are considered statically bound when that is possible, that is, when the controlling object has a known specific type (even if the actual implementation uses dispatching). 
Discussion: We don't need to worry specially about entry calls (even if the entry has been renamed as a procedure), as they will be detected by the prohibition against calls to entities with the Nonblocking aspect False.
Similarly, we don't need to specially worry about subprograms of limited interfaces that are implemented by entries, as any such subprogram necessarily has the value statically False for the Nonblocking aspect, and thus is already covered by the prohibition against calling such subprograms. 
{AI12-0064-2} {AI12-0374-2} For the purposes of the above rules, an entry_body is considered nonblocking if the immediately enclosing protected unit is nonblocking.
Reason: An entry always allows blocking (by rule); but we want to be able to compile-time check for most violations of prohibition against potentially blocking operations in a protected action (see 9.5.1). We do that by using the nonblocking status of the protected unit as the controlling factor, and enforce that by not allowing the specification of the Nonblocking aspect for any protected operation. 
{AI12-0374-2} For a subtype for which aspect Nonblocking is True, any predicate expression that applies to the subtype shall only contain constructs that are allowed immediately within a nonblocking program unit.
{AI12-0064-2} A subprogram shall be nonblocking if it overrides a nonblocking dispatching operation. An entry shall not implement a nonblocking procedure. If an inherited dispatching subprogram allows blocking, then the corresponding subprogram of each ancestor shall allow blocking.
Discussion: Rules elsewhere in the standard (4.6 and 3.10.2) ensure that access-to-subprogram conversion and the Access attribute enforce nonblocking. 
Ramification: A nonblocking subprogram can override one that allows blocking, but the reverse is illegal. Thus one can declare a Finalize subprogram to be nonblocking, even though it overrides a routine that allows blocking. (This works because a nonblocking subprogram allows a strict subset of the operations allowed in allows blocking subprograms, so calling such a subprogram as if it allows blocking — as is necessary in a dispatching call — is harmless.) 
{AI12-0064-2} {AI12-0374-2} AIt is illegal to specify aspect Nonblocking for the first subtype of the full view of a type that has a partial or incomplete view.
Reason: We need completions to agree on nonblocking with the original view. One reason this is necessary to prevent the predefined equality operator from being nonblocking in the partial view and allowing blocking in the full view.
{AI12-0064-2} {AI12-0374-2} Aspect Nonblocking shall be directly specified for the first subtype of a derived type only if it has the same value as the Nonblocking aspect of the ancestor subtype or if it is specified True. Aspect Nonblocking shall be directly specified for a nonfirst subtype S only if it has the same value as the Nonblocking aspect of the subtype identified in the subtype_indication defining S or if it is specified True.
Reason: Boolean-valued aspects have a similar rule to the first rule here (see 13.1.1), we want this one to work similarly. We need non-first subtypes to allow blocking only if the original first subtype allows blocking, as that allows the programmer to know that any operation on any subtype of a type are nonblocking if the first subtype is nonblocking. 
{AI12-0319-1} For a composite type that is nonblocking:
All component subtypes shall be nonblocking;
For a record type or extension, every call in the default_expression of a component (including discriminants) shall call an operation that is nonblocking;
For a controlled type, the Initialize, Finalize, and Adjust (if any) subprograms shall be nonblocking.
Reason: These rules ensure that if a type is nonblocking, the default initialization, finalization, and assignment of the type are also nonblocking. This allows the use of the nonblocking attribute of a generic formal type to describe whether these operations of the type allow blocking.
Default initialization, finalization, and assignment of elementary types are always nonblocking, so we don't need any rules for those. 
{AI12-0064-2} {AI12-0374-2} The predefined equality operator for a composite type, unless it is for a record type or record extension and the operator is overridden by a primitive equality operator, is illegal if it is nonblocking and:
for a record type or record extension, the parent primitive "=" allows blocking; or
some component is of a record type or record extension that has a primitive "=" that allows blocking; or
some component is of a type, other than a record type or a record extension, that has a predefined "=" that allows blocking. 
Ramification: This applies to both record and array "=".
This check occurs when the equality operator is declared, so this rule effectively makes the type illegal if the rule is violated. 
Reason: We don't need to check this when the operator is overridden for a record type, as the body of the new definition of equality will enforce the rules, and there is no case where the predefined operator will re-emerge. We do have to check this for array types even if the operator is overridden, as the predefined operator will re-emerge in generics and record equality.
{AI12-0064-2} {AI12-0374-2} In a generic instantiation:
the actual subprogram corresponding to a nonblocking formal subprogram shall be nonblocking [(an actual that is an entry is not permitted in this case)];
the actual subtype corresponding to a nonblocking formal subtype shall be nonblocking;
Ramification: We require matching for formal scalar or access-to-object types, even though their predefined operators are always nonblocking (and they re-emerge in the generic unit) - because a "blocking" predicate might apply to the actual subtype, which will also be enforced on operations on the formal type. 
the actual object corresponding to a formal object of a nonblocking access-to-subprogram type shall be of a nonblocking access-to-subprogram type;
the actual instance corresponding to a nonblocking formal package shall be nonblocking.
{AI12-0064-2} In addition to the places where Legality Rules normally apply (see 12.3), the above rules also apply in the private part of an instance of a generic unit.
Ramification: For a generic formal parameter to be nonblocking (thus, for these rules to apply), it has to explicitly specify aspect Nonblocking to be True. However, if not specified True, these rules do apply in the instance of the specification of the generic unit (the normal re-checking is needed). For instance, the body of an expression function might make a prohibited call.
18  {AI05-0030-2} {AI05-0215-1} The synchronization_kind By_Protected_Procedure implies that the operation will not block. 

Wording Changes from Ada 95

{AI95-00345-01} Added a Legality Rule to make it crystal-clear that the protected object of an entry or procedure call must be a variable. This rule was implied by the Dynamic Semantics here, along with the Static Semantics of 3.3, but it is much better to explicitly say it. While many implementations have gotten this wrong, this is not an incompatibility — allowing updates of protected constants has always been wrong. 

Extensions to Ada 2005

{AI05-0030-2} {AI05-0215-1} Added the Synchronization aspect to allow specifying that an interface procedure is really an entry or a protected procedure.

Wording Changes from Ada 2005

{AI05-0225-1} Correction: Clarified that the target object of any name denoted a protected procedure or entry can never be a constant (other than for the 'Count attribute). This closes holes involving calls to access-to-protected, renaming as a procedure, and generic formal subprograms. 

Inconsistencies With Ada 2012

{AI12-0064-2} Calls on procedures that rename an entry or are implemented by an entry are now defined to be potentially blocking. This means that such a call now might raise Program_Error. However, it never made sense for some entry calls to be excluded from being potentially blocking, and we expect that most implementations already treated all entry calls the same way. Thus do not expect this wording change to actually change the behavior of any implementation, and thus no program will change. 

Incompatibilities With Ada 2012

{AI12-0166-1} Correction: Internal protected calls are now prohibited in preconditions and default expressions of protected operations. These were allowed in Ada 2012, but as they cause race conditions and as most existing Ada 95 compilers crash when given such a default parameter, we expect such code to be extremely rare.

Extensions to Ada 2012

{AI12-0064-2} {AI12-0319-1} {AI12-0374-2} Aspect Nonblocking is new; it allows compile-time checks to prevent using potentially blocking operations in contexts where that is not allowed. 

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