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E.2.3 Remote Call Interface Library Units

[A remote call interface library unit can be used as an interface for remote procedure calls (RPCs) (or remote function calls) between active partitions.] 

Language Design Principles

The restrictions governing a remote call interface library unit are intended to ensure that the values of the actual parameters in a remote call can be meaningfully sent between two active partitions.


{AI12-0417-1} The form of a pragma Remote_Call_Interface is as follows: 
{AI12-0417-1}   pragma Remote_Call_Interface[(library_unit_name)];
{AI12-0417-1} The form of a pragma All_Calls_Remote is as follows: 
{AI12-0417-1}   pragma All_Calls_Remote[(library_unit_name)];
{AI12-0417-1} A pragma All_Calls_Remote is a library unit pragma. 
Paragraphs 2 through 6 were moved to Annex J, “Obsolescent Features”. 

Legality Rules

{8652/0078} {AI95-00048-01} {AI05-0243-1} {AI12-0417-1} When the library unit aspect (see 13.1.1) A pragma Remote_Call_Interface of is used to specify that a library unit is True, the library unit is a remote call interface (RCI), namely that the Remote_Call_Interface aspect of the library unit is True. A subprogram declared in the visible part of such a library unit, or declared by such a library unit, is called a remote subprogram.
Aspect Description for Remote_Call_Interface: Subprograms in a given package may be used in remote procedure calls.
{AI05-0206-1} {AI05-0243-1} The declaration of an RCI library unit shall be preelaborable (see 10.2.1), and shall depend semantically only upon declared pure library_items, shared passive library units, remote types library units, other remote call interface library units, or preelaborated normal library units that are mentioned only in private with clauses.
Ramification: {AI05-0243-1} We say declared pure library_item here, so that (all) limited views are allowed; those are not library units, but they are declared pure library_items.
{8652/0078} {AI95-00048-01} In addition, the following restrictions apply to an RCI library unit:
{8652/0078} {AI95-00048-01} its visible part shall not contain the declaration of a variable; 
Reason: {8652/0078} {AI95-00048-01} Remote call interface units do not provide remote data access. A shared passive package has to be used for that. 
{8652/0078} {AI95-00048-01} its visible part shall not contain the declaration of a limited type;
Reason: {AI95-00240-01} {AI95-00366-01} We disallow the declaration of task and protected types, since calling an entry or a protected subprogram implicitly passes an object of a limited type (the target task or protected object). We disallow other limited types since we require that such types have available Read and Write attributes, but we certainly don't want the Read and Write attributes themselves to involve remote calls (thereby defeating their purpose of marshalling the value for remote calls). 
{8652/0078} {AI95-00048-01} its visible part shall not contain a nested generic_declaration;
Reason: This is disallowed because the body of the nested generic would presumably have access to data inside the body of the RCI package, and if instantiated in a different partition, remote data access might result, which is not supported. 
{8652/0078} {AI95-00048-01} {AI05-0229-1} it shall not be, nor shall its visible part contain, the declaration of a subprogram for which aspect Inline is True;
{AI12-0283-1} it shall not be, nor shall its visible part contain, the declaration of a subprogram that is nonblocking (see 9.5);
Reason: All such subprograms are remote subprograms, and all calls on remote subprograms are considered potentially blocking, so they cannot statically be allowed in nonblocking code. 
Ramification: The type declaration of a remote subprogram is illegal if the Nonblocking aspect is True, either implicitly by inheritance or by explicit specification.
{8652/0078} {AI95-00048-01} {AI95-00240-01} {AI95-00366-01} {AI05-0101-1} it shall not be, nor shall its visible part contain, a subprogram (or access-to-subprogram) declaration whose profile has a parameter or result of a type that does not support external streaming (see 13.13.2);
Ramification: {AI05-0101-1} No anonymous access types support external streaming, so they are never allowed as parameters or results of RCI subprograms. 
any public child of the library unit shall be a remote call interface library unit. 
Reason: {AI12-0417-1} No restrictions apply to the private part of an RCI package, and since a public child can “see” the private part of its parent, such a child must itself have a Remote_Call_Interface aspect pragma, and be assigned to the same partition (see below). 
Discussion: {AI12-0417-1} We considered making the public child of an RCI package implicitly RCI, but it seemed better to require an explicit aspect pragma to avoid any confusion.
Note that there is no need for a private child to be an RCI package, since it can only be seen from the body of its parent or its siblings, all of which are required to be in the same active partition. 
  {AI12-0002-1} Specification of a stream-oriented attribute is illegal in the specification of a remote call interface library unit. In addition to the places where Legality Rules normally apply (see 12.3), this rule applies also in the private part of an instance of a generic unit.
Ramification: This applies whether the stream-oriented attribute is specified with an attribute_definition_clause or with an aspect_specification.
Reason: Stream-oriented attributes are intended to be used locally (to marshall values), while subprograms in a remote call interface are intended to be used remotely. We resolve the conflict by banning the local use routines. The type should be included in a remote types package and imported into the remote call interface package. 
{AI05-0229-1} {AI12-0417-1} A pragma All_Calls_Remotesets the All_Calls_Remote is a library unit representation aspect of the library unit to which the pragma applies to the value True. If the All_Calls_Remote aspect of a library unit is True, the library unit shall be a remote call interface.
Aspect Description for All_Calls_Remote: All indirect or dispatching remote subprogram calls, and all direct remote subprogram procedure calls, should use the Partition Communication Subsystem, even if they are local.

Post-Compilation Rules

A remote call interface library unit shall be assigned to at most one partition of a given program. A remote call interface library unit whose parent is also an RCI library unit shall be assigned only to the same partition as its parent. 
Implementation Note: {8652/0078} {AI95-00048-01} The declaration of an RCI unit, with a calling-stub body, is automatically included in all active partitions with compilation units that depend on it. However the whole RCI library unit, including its (non-stub) body, will only be in one of the active partitions. 
Notwithstanding the rule given in 10.2, a compilation unit in a given partition that semantically depends on the declaration of an RCI library unit, needs (in the sense of 10.2) only the declaration of the RCI library unit, not the body, to be included in that same partition. [Therefore, the body of an RCI library unit is included only in the partition to which the RCI library unit is explicitly assigned.]

Implementation Requirements

{8652/0078} {AI95-00048-01} {AI05-0229-1} {AI12-0031-1} If aspect All_Calls_Remote is True for a given RCI library unit, then the implementation shall route any of the following calls call to a subprogram of the RCI unit from outside the declarative region of the unit through the Partition Communication Subsystem (PCS); see E.5:. Calls to such subprograms from within the declarative region of the unit are defined to be local and shall not go through the PCS. 
{AI12-0031-1} A direct call to a subprogram of the RCI unit from outside the declarative region of the unit;
{AI12-0031-1} An indirect call through a remote access-to-subprogram value that designates a subprogram of the RCI unit;
{AI12-0031-1} A dispatching call with a controlling operand designated by a remote access-to-class-wide value whose tag identifies a type declared in the RCI unit. 
Discussion: {8652/0078} {AI95-00048-01} {AI05-0229-1} {AI12-0031-1} {AI12-0005-1} When this aspect is False (or not used), it is presumed that most implementations will not make direct calls through the PCS if the call originates in the same partition as that of the RCI unit. When this aspect is True, all indirect or dispatching remote subprogram calls to the RCI unit, and all direct calls from outside the subsystem rooted at the RCI unit, are treated like calls from outside the partition, ensuring that the PCS is involved in all such calls (for debugging, redundancy, etc.). 
Reason: {AI12-0031-1} {AI12-0005-1} There is no point in forcing to force local direct calls (including or calls from children) to go through the PCS, since on the target system, these calls are always local, and all the units are in the same active partition. 

Implementation Permissions

{AI05-0243-1} {AI12-0417-1} {AI12-0444-1} An implementation may omit need not support for the Remote_Call_Interface pragma or aspect or nor the All_Calls_Remote aspect pragma. [Explicit message-based communication between active partitions can be supported as an alternative to RPC.] 
Ramification: {AI12-0417-1} Of course, it is pointless to support the All_Calls_Remote aspect pragma if the Remote_Call_Interface aspect pragma (or some approximate equivalent) is not supported. 

Incompatibilities With Ada 95

{AI95-00240-01} {AI05-0248-1} Amendment Correction: The wording was changed from “user-specified” to “available” read and write attributes. (This was then further changed, see below.) This means that a type with the attributes specified in the private part would originally have been allowed as a formal parameter of an RCI subprogram, but that is no longer allowed. This change was made so that the rules were consistent with the rules introduced for the Corrigendum for stream attributes; moreover, legality should not depend on the contents of the private part.

Wording Changes from Ada 95

{8652/0078} {AI95-00048-01} Corrigendum: Changed the wording to allow a library subprogram to be a remote call interface unit.
{AI95-00366-01} Changed the wording to use the newly defined term type that supports external streaming, so that various issues with access types in pure units and implicitly declared attributes for type extensions are properly handled. 

Incompatibilities With Ada 2005

{AI05-0101-1} Correction: Added a rule to ensure that function results are streamable; this was missing in previous versions of Ada. While programs that returned unstreamable types from RCI functions were legal, it is not clear what they could have done (as the results could not be marshalled). Thus, it seems unlikely that any real programs will be impacted by this change. 

Extensions to Ada 2005

{AI05-0206-1} Added wording to allow private withs of preelaborated normal units in the specification of a remote call interface unit.
{AI05-0229-1} {AI12-0417-1} All_Calls_Remote is now a representation aspect, so it can be specified by an aspect_specification — although the pragma is still preferred by the Reference Manual.
{AI05-0243-1} {AI12-0417-1} Remote_Call_Interface is now a categorization aspect, so it can be specified by an aspect_specification — although the pragma is still preferred by the Reference Manual.

Inconsistencies With Ada 2012

{AI12-0031-1} Corrigendum: Redefined when indirect and dispatching remote calls have to be remote for a unit for which the aspect All_Calls_Remote is True. With the new rules, a local target called indirectly or via dispatching will be routed through the PCS, while that was not necessarily true in earlier Ada. If a program depended on local targets not being routed through the PCS even when All_Calls_Remote is used, then it might behave differently or fail in corrected Ada 2012 as corrected with this refinement. This is highly unlikely as the PCS is going to be able to communicate with any partition of the program, including the local partition.

Incompatibilities With Ada 2012

{AI12-0002-1} Correction: Specifying user-defined stream-oriented attributes is now illegal in an RCI unit. If a program contains such attributes, they and their associated type will need to be moved to a remote types package (which is then imported into the RCI unit). 

Wording Changes from Ada 2012

{AI12-0283-1} Added a rule to ensure that potentially blocking remote calls are not considered nonblocking.
{AI12-0417-1} The pragmas Remote_Call_Interface and All_Calls_Remote are now obsolescent. 

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