E.2.3 Remote Call Interface Library Units
1
[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
1.a
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.
Syntax
2
The
form of a
pragma
Remote_Call_Interface is as follows:
3
pragma Remote_Call_Interface[(
library_unit_name)];
4
The form of a
pragma
All_Calls_Remote is as follows:
5
pragma All_Calls_Remote[(
library_unit_name)];
6
A
pragma
All_Calls_Remote is a library unit pragma.
Legality Rules
7/3
{
8652/0078}
{
AI95-00048-01}
{
AI05-0243-1}
A
pragma Remote_Call_Interface
is used to specify that a library unit is a remote call interface
(RCI), namely that the is a library unit to which the pragma Remote_Call_Interface
aspect
of the library unit is True applies.
A subprogram declared in the visible part of such a library unit
,
or declared by such a library unit, is called a
remote subprogram.
7.a/3
Aspect Description
for Remote_Call_Interface: Subprograms
in a given package may be used in remote procedure calls.
8/3
{
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,
or other remote call
interface library units
, or preelaborated normal
library units that are mentioned only in private with clauses.
8.a/3
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.
9/1
{
8652/0078}
{
AI95-00048-01}
In addition, the following restrictions apply to
the
visible part of an RCI library unit:
10/1
10.a/1
Reason: {
8652/0078}
{
AI95-00048-01}
Remote call interface
units packages
do not provide remote data access. A shared passive package has to be
used for that.
11/1
11.a/2
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 user-defined
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).
12/1
12.a
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.
13/3
{
8652/0078}
{
AI95-00048-01}
{
AI05-0229-1}
it shall not
be, nor shall its visible part
contain
, the declaration of a subprogram
for to which
aspect a pragma
Inline
is True applies;
14/3
{
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
[an
access parameter or] a parameter
or result of
a type that does not support external streaming (see 13.13.2) an
access parameter, or a formal parameter of a limited type unless that
limited type has user-specified Read and Write attributes;
14.a/3
Ramification: {
AI05-0101-1}
No anonymous access types support external streaming,
so they are never allowed as parameters or results of RCI subprograms.
15
any public child of the library unit shall be a
remote call interface library unit.
15.a
Reason: 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
pragma, and be assigned to the same partition (see below).
15.b
Discussion: We considered making the
public child of an RCI package implicitly RCI, but it seemed better to
require an explicit pragma to avoid any confusion.
15.c
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.
16/3
{
AI05-0229-1}
A If a pragma
All_Calls_Remote
sets the All_Calls_Remote representation
aspect of the applies
to a 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.
16.a/3
Aspect Description
for All_Calls_Remote: All
remote procedure calls should use the Partition Communication Subsystem,
even if they are local.
Post-Compilation Rules
17
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.
17.a/1
Implementation Note: {
8652/0078}
{
AI95-00048-01}
The declaration of an RCI
unit package,
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.
18
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
19/3
{
8652/0078}
{
AI95-00048-01}
{
AI05-0229-1}
If
aspect a pragma
All_Calls_Remote
is True for applies
to a given RCI library
unit package,
then the implementation shall route any call to a subprogram of the RCI
unit package
from outside the declarative region of the
unit package
through the Partition Communication Subsystem (PCS); see
E.5.
Calls to such subprograms from within the declarative region of the
unit package
are defined to be local and shall not go through the PCS.
19.a/3
Discussion: {
8652/0078}
{
AI95-00048-01}
{
AI05-0229-1}
When this aspect is False (or not used) Without
this pragma, it is presumed that most implementations will make
direct calls if the call originates in the same partition as that of
the RCI
unit package.
When this aspect is True With
this pragma, all calls from outside the subsystem rooted at the
RCI
unit package
are treated like calls from outside the partition, ensuring that the
PCS is involved in all such calls (for debugging, redundancy, etc.).
19.b
Reason: There is no point to force local
calls (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
20/3
{
AI05-0243-1}
An implementation need not support the Remote_Call_Interface pragma
or
aspect nor the All_Calls_Remote pragma. [Explicit message-based
communication between active partitions can be supported as an alternative
to RPC.]
20.a
Ramification: Of course, it is pointless
to support the All_Calls_Remote pragma if the Remote_Call_Interface pragma
(or some approximate equivalent) is not supported.
Incompatibilities With Ada 95
20.b/3
{
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
20.c/2
{
8652/0078}
{
AI95-00048-01}
Corrigendum: Changed the wording to allow
a library subprogram to be a remote call interface unit.20.d/2
{
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
20.e/3
{
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
20.f/3
{
AI05-0206-1}
Added wording to allow private
withs of preelaborated normal units in the specification of a remote
call interface unit.20.g/3
{
AI05-0229-1}
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 Standard.20.h/3
{
AI05-0243-1}
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 Standard.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
