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H.4 Safety and Security Restrictions

   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.
Discussion: Note that the restrictions are absolute. If a partition has 100 library units and just one needs Unchecked_Conversion, then the pragma cannot be used to ensure the other 99 units do not use Unchecked_Conversion. Note also that these are restrictions on all Ada code within a partition, and therefore it may not be evident from the specification of a package whether a restriction can be imposed.

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:
   {Restrictions (No_Protected_Types)} No_Protected_Types
There are no declarations of protected types or protected objects.
   Memory-management related restrictions:
   {Restrictions (No_Allocators)} No_Allocators
There are no occurrences of an allocator.
     {8652/0042} {Restrictions (No_Local_Allocators)} No_Local_Allocators
Allocators are prohibited in subprograms, generic subprograms, tasks, and entry bodies; instantiations of generic packages are also prohibited in these contexts.
Ramification: Thus allocators are permitted only in expressions whose evaluation can only be performed before the main subprogram is invoked.
This paragraph was deleted.Reason: {8652/0042} The reason for the prohibition against instantiations of generic packages is to avoid contract model violations. An alternative would be to prohibit allocators from generic packages, but it seems preferable to allow generality on the defining side and then place the restrictions on the usage (instantiation), rather than inhibiting what can be in the generic while liberalizing where they can be instantiated.
   {Restrictions (No_Unchecked_Deallocation)} No_Unchecked_Deallocation
Semantic dependence on Unchecked_Deallocation is not allowed.
Discussion: This restriction would be useful in those contexts in which heap storage is needed on program start-up, but need not be increased subsequently. The danger of a dangling pointer can therefore be avoided.
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. {Restrictions (Immediate_Reclamation)}
Discussion: Immediate reclamation would apply to storage created by the compiler, such as for a return value from a function whose size is not known at the call site.
    Exception-related restriction:
    {Restrictions (No_Exceptions)} No_Exceptions
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.
Discussion: This restriction mirrors a method of working that is quite common in the safety area. The programmer is required to show that exceptions cannot be raised. Then a simplified run-time system is used without exception handling. However, some hardware checks may still be enforced. If the software check would have failed, or if the hardware check actually fails, then the execution of the program is unpredictable. There are obvious dangers in this approach, but it is similar to programming at the assembler level.
    Other restrictions:
    {Restrictions (No_Floating_Point)} No_Floating_Point
Uses of predefined floating point types and operations, and declarations of new floating point types, are not allowed.
Discussion: The intention is to avoid the use of floating point hardware at run time, but this is expressed in language terms. It is conceivable that floating point is used implicitly in some contexts, say fixed point type conversions of high accuracy. However, the Implementation Requirements below make it clear that the restriction would apply to the ``run-time system'' and hence not be allowed. This parameter could be used to inform a compiler that a variant of the architecture is being used which does not have floating point instructions.
    {Restrictions (No_Fixed_Point)} No_Fixed_Point
Uses of predefined fixed point types and operations, and declarations of new fixed point types, are not allowed.
Discussion: This restriction would have the side-effect of prohibiting the delay_relative_statement. As with the No_Floating_Point restriction, this might be used to avoid any question of rounding errors. Unless an Ada run-time is written in Ada, it seems hard to rule out implicit use of fixed point, since at the machine level, fixed point is virtually the same as integer arithmetic.
    {Restrictions (No_Unchecked_Conversion)} No_Unchecked_Conversion
Semantic dependence on the predefined generic Unchecked_Conversion is not allowed.
Discussion: Most critical applications would require some restrictions or additional validation checks on uses of unchecked conversion. If the application does not require the functionality, then this restriction provides a means of ensuring the design requirement has been satisfied. The same applies to several of the following restrictions.
The declaration of access-to-subprogram types is not allowed. {Restrictions (No_Access_Subprograms)}
    {Restrictions (No_Unchecked_Access)} No_Unchecked_Access
The Unchecked_Access attribute is not allowed.
    {Restrictions (No_Dispatch)} No_Dispatch
Occurrences of T'Class are not allowed, for any (tagged) subtype T.
    {Restrictions (No_IO)} No_IO
Semantic dependence on any of the library units Sequential_IO, Direct_IO, Text_IO, Wide_Text_IO, or Stream_IO is not allowed.
Discussion: Excluding the input-output facilities of an implementation may be needed in those environments which cannot support the supplied functionality. A program in such an environment is likely to require some low level facilities or a call on a non-Ada feature.
    {Restrictions (No_Delay)} No_Delay
Delay_Statements and semantic dependence on package Calendar are not allowed.
Ramification: This implies that delay_alternatives in a select_statement are prohibited.
The purpose of this restriction is to avoid the need for timing facilities within the run-time system.
    {Restrictions (No_Recursion)} No_Recursion
As part of the execution of a subprogram, the same subprogram is not invoked.
    {Restrictions (No_Reentrancy)} No_Reentrancy
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.
Reason: Permission is granted for the run-time system to use the specified otherwise-restricted features, since the use of these features may simplify the run-time system by allowing more of it to be written in Ada.
Discussion: The restrictions that are applied to the partition are also applied to the run-time system. For example, if No_Floating_Point is specified, then an implementation that uses floating point for implementing the delay statement (say) would require that No_Floating_Point is only used in conjunction with No_Delay. It is clearly important that restrictions are effective so that Max_Tasks=0 does imply that tasking is not used, even implicitly (for input-output, say).
An implementation of tasking could be produced based upon a run-time system written in Ada in which the rendezvous was controlled by protected types. In this case, No_Protected_Types could only be used in conjunction with Max_Task_Entries=0. Other implementation dependencies could be envisaged.
If the run-time system is not written in Ada, then the wording needs to be applied in an appropriate fashion.

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).
Implementation defined: Implementation-defined aspects of pragma Restrictions.
Discussion: The documentation requirements here are quite difficult to satisfy. One method is to review the object code generated and determine the checks that are still present, either explicitly, or implicitly within the architecture. As another example from that of overflow, consider the question of deferencing a null pointer. This could be undertaken by a memory access trap when checks are performed. When checks are suppressed via the argument No_Exceptions, it would not be necessary to have the memory access trap mechanism enabled.

Erroneous Execution

    {erroneous execution (cause) [partial]} 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.
Discussion: The situation here is very similar to the application of pragma Suppress. Since users are removing some of the protection the language provides, they had better be careful!
    {erroneous execution (cause) [partial]} 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.
Discussion: In practice, many implementations may not exploit the absence of recursion or need for reentrancy, in which case the program execution would be unaffected by the use of recursion or reentrancy, even though the program is still formally erroneous.
Implementation defined: Any restrictions on pragma Restrictions.

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