H.4 Safety and Security Restrictions

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This clause 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

The following restrictions, the same as in D.7, apply in this Annex: No_Task_Hierarchy, No_Abort_Statement, No_Implicit_Heap_Allocation, Max_Task_Entries is 0, Max_Asynchronous_Select_Nesting is 0, and Max_Tasks is 0. The last three restrictions are checked prior to program execution.

The following additional restrictions apply in this Annex.

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.
Semantic dependence on Unchecked_Deallocation is not allowed.
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 run-time checks are generated; however, a run-time 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.
Semantic dependence on the predefined generic Unchecked_Conversion is not allowed.
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, or Stream_IO 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.

Implementation Requirements

If an implementation supports pragma Restrictions for a particular argument, then except for the restrictions No_Unchecked_Deallocation, No_Unchecked_Conversion, No_Access_Subprograms, and No_Unchecked_Access, 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 run-time 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.

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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual