E.2.1 Shared Passive Library Units
[A shared passive library unit is used for managing
global data shared between active partitions. The restrictions on shared
passive library units prevent the data or tasks of one active partition
from being accessible to another active partition through references
implicit in objects declared in the shared passive library unit.]
Language Design Principles
The restrictions governing a shared passive
library unit are designed to ensure that objects and subprograms declared
in the package can be used safely from multiple active partitions, even
though the active partitions live in different address spaces, and have
separate run-time systems.
Legality Rules
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When the library unit aspect (see
13.1.1)
Shared_Passive
of a library
unit is True, the library unit is a
shared passive library unit.
The following restrictions apply to such a library unit:
Aspect Description for Shared_Passive:
A given package is used to represent shared memory in a distributed
system.
[it shall be preelaborable (see
10.2.1);]
Ramification: It cannot contain library-level
declarations of protected objects with entries, nor of task objects.
Task objects are disallowed because passive partitions don't have any
threads of control of their own, nor any run-time system of their own.
Protected objects with entries are disallowed because an entry queue
contains references to calling tasks, and that would require in effect
a pointer from a passive partition back to a task in some active partition.
Reason: Shared passive packages cannot
depend semantically upon remote types packages because the values of
an access type declared in a remote types package refer to the local
heap of the active partition including the remote types package.
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of an access type that designates a class-wide type;
Reason: These kinds of access types are
disallowed because the object designated by an access value of such a
type could contain an implicit reference back to the active partition
on whose behalf the designated object was created.
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of a type with a part that is of a task type;
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that contains a name that denotes a type declared within a declared-pure
package, if that type has a part that is of an access type; for the purposes
of this rule, the parts considered include those of the full views of
any private types or private extensions.
Reason: This rule breaks privacy by looking
into the full views of private types. Avoiding privacy breakage here
would have required disallowing the use in a shared passive package of
any private type declared in a declared-pure package, which would have
been severely incompatible.
Notwithstanding
the definition of accessibility given in
3.10.2,
the declaration of a library unit P1 is not accessible from within the
declarative region of a shared passive library unit P2, unless the shared
passive library unit P2 depends semantically on P1.
Discussion: We considered a more complex
rule, but dropped it. This is the simplest rule that recognizes that
a shared passive package may outlive some other library package, unless
it depends semantically on that package. In a nondistributed program,
all library packages are presumed to have the same lifetime.
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Implementations may define additional aspects or pragmas that force two
library packages to be in the same partition, or to have the same lifetime,
which would allow this rule to be relaxed in the presence of such aspects
or pragmas.
Static Semantics
A shared passive library unit
is preelaborated.
Post-Compilation Rules
A shared passive library unit shall be assigned to
at most one partition within a given program.
Notwithstanding
the rule given in
10.2, a compilation unit
in a given partition does not
need (in the sense of
10.2)
the shared passive library units on which it depends semantically to
be included in that same partition; they will typically reside in separate
passive partitions.
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This rule is necessary so that the shared data of a shared passive partition
is not duplicated in each partition. It does not imply a particular implementation;
in particular, code can be replicated in each active partition.
Implementation Note: {
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One possible implementation for a shared passive partition is as a shared
library that is mapped into the address space of of each active partition.
In such case, both the code and the data of the passive partition would
be mapped into the active partitions and directly called/accessed from
each active partition. For instance, on Microsoft Windows a DLL has the
correct semantics.
Alternatively, the shared data can be represented
as a file or persistent memory, with the shared code being replicated
in each active partition. Code replication is an as-if optimization;
it should be impossible to tell where the code lives since no elaboration
is necessary.
Wording Changes from Ada 95
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Corrigendum: Corrected the wording to allow access types in blocks
in shared passive generic packages.
Extensions to Ada 2005
Incompatibilities With Ada 2012
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Corrigendum: Uses of access types declared
in declared-pure units are not allowed in library-level shared passive
packages. These were allowed by Ada 2005 and Ada 2012, but it is unlikely
that they work properly, as active partitions could disappear before
the shared-passive partition. As such, the new errors are more likely
to catch bugs than to cause them.
Wording Changes from Ada 2012
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The pragma Shared_Passive is now obsolescent.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
