H.4 High Integrity Restrictions
This subclause defines restrictions that can be used
with pragma Restrictions (see
13.12); these
facilitate the demonstration of program correctness by allowing tailored
versions of the run-time system.
Static Semantics
This paragraph was
deleted.
The following
restriction_identifiers
are language defined:
Tasking-related
restriction:
There are no declarations of protected types or protected objects.
Memory-management
related restrictions:
There are no occurrences of an
allocator.
Allocators
are prohibited in subprograms, generic subprograms, tasks, and entry
bodies.
There are no
allocators
of anonymous access types.
There are no coextensions. See
3.10.2.
No_Access_Parameter_Allocators
Allocators
are not permitted as the actual parameter to an access parameter. See
6.1.
This paragraph was deleted.
Immediate_Reclamation
Except for storage occupied by objects created by
allocators
and not deallocated via unchecked deallocation, any storage reserved
at run time for an object is immediately reclaimed when the object no
longer exists.
Exception-related
restriction:
Raise_statements
and
exception_handlers
are not allowed. No language-defined runtime checks are generated; however,
a runtime check performed automatically by the hardware is permitted.
Other restrictions:
Uses of predefined floating point types and operations, and declarations
of new floating point types, are not allowed.
Uses of predefined fixed point types and operations, and declarations
of new fixed point types, are not allowed.
This paragraph was deleted.
No_Access_Subprograms
The declaration of access-to-subprogram types is not allowed.
The
Unchecked_Access attribute
is not allowed.
Occurrences of T'Class are not allowed, for any (tagged) subtype T.
Semantic dependence on any of the library units Sequential_IO, Direct_IO,
Text_IO, Wide_Text_IO, Wide_Wide_Text_IO, Stream_IO, or Directories is
not allowed.
Delay_Statements
and semantic dependence on package Calendar are not allowed.
As part of the execution of a subprogram, the same subprogram is not
invoked.
During the execution of a subprogram by a task, no other task invokes
the same subprogram.
No library-level entity shall have a Global aspect of Unspecified, either
explicitly or by default. No library-level entity shall have a Global'Class
aspect of Unspecified, explicitly or by default, if it is used as part
of a dispatching call.
No_Hidden_Indirect_Globals
When within a context where the applicable Global aspect is neither Unspecified
nor in out all, any execution within such a context does neither
of the following:
Update a variable that is reachable via
a sequence of zero or more dereferences of access-to-object values from
a formal parameter of mode
in (after any
overriding –
see
H.7), or from a global that is not within
the applicable global variable set, or has mode
in;
Read a variable that is updatable via
a sequence of zero or more dereferences of access-to-object values from
a global that is not within the applicable global variable set.
For the purposes of the above rules, if
an applicable global variable set includes a package name, and the collection
of some pool-specific access type (see
7.6.1)
is implicitly declared in a part of the declarative region of the package
included within the global variable set, then all objects allocated from
that collection are considered included within the global variable set.
The consequences of violating the No_Hidden_Indirect_Globals
restriction is implementation-defined. Any aspects or other means for
identifying such violations prior to or during execution are implementation-defined.
Dynamic Semantics
The following
restriction_parameter_identifier
is language defined:
Specifies the maximum length for the result of an Image, Wide_Image,
or Wide_Wide_Image attribute. Violation of this restriction results in
the raising of Program_Error at the point of the invocation of an image
attribute.
Implementation Requirements
An implementation
of this Annex shall support:
the restrictions defined in this subclause; and
the following restrictions defined in
D.7:
No_Task_Hierarchy, No_Abort_Statement, No_Implicit_Heap_Allocation, No_Standard_Allocators_After_Elaboration;
and
the pragma Profile(Ravenscar); and
the following uses
of
restriction_parameter_identifiers
defined in
D.7, which are checked prior to
program execution:
Max_Task_Entries => 0,
Max_Asynchronous_Select_Nesting
=> 0, and
Max_Tasks => 0.
If a Max_Image_Length restriction applies to any
compilation unit in the partition, then for any subtype S, S'Image, S'Wide_Image,
and S'Wide_Wide_Image shall be implemented within that partition without
any dynamic allocation.
If an implementation supports
pragma
Restrictions for a particular argument, then except for the restrictions
No_Access_Subprograms, No_Unchecked_Access, No_Specification_of_Aspect,
No_Use_of_Attribute, No_Use_of_Pragma, No_Dependence => Ada.Unchecked_Conversion,
and No_Dependence => Ada.Unchecked_Deallocation, the associated restriction
applies to the run-time system.
Documentation Requirements
If a pragma Restrictions(No_Exceptions) is specified,
the implementation shall document the effects of all constructs where
language-defined checks are still performed automatically (for example,
an overflow check performed by the processor).
Erroneous Execution
Program execution is erroneous
if pragma Restrictions(No_Exceptions) has been specified and the conditions
arise under which a generated language-defined runtime check would fail.
Program execution is erroneous
if pragma Restrictions(No_Recursion) has been specified and a subprogram
is invoked as part of its own execution, or if pragma Restrictions(No_Reentrancy)
has been specified and during the execution of a subprogram by a task,
another task invokes the same subprogram.
10 Uses of
restriction_parameter_identifier
No_Dependence defined in
13.12.1: No_Dependence
=> Ada.Unchecked_Deallocation and No_Dependence => Ada.Unchecked_Conversion
may be appropriate for high-integrity systems. Other uses of No_Dependence
can also be appropriate for high-integrity systems.
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