H.4 High Integrity Restrictions

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.

Ramification: Thus allocators are permitted only in expressions whose evaluation can only be performed before the main subprogram is invoked. 

This paragraph was deleted.{8652/0042} {AI95-00130}

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. 

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.

Discussion: {AI95-00114-01} 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 restriction could be used to inform a compiler that a variant of the architecture is being used which does not have floating point instructions.

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.

Discussion: Most critical applications would require some restrictions or additional validation checks on uses of access-to-subprogram types. 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, and to restriction No_Dependence => Ada.Unchecked_Conversion.

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.

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.

Discussion: {AI95-00347-01} The reference to pragma Profile(Ravenscar) is intended to show that properly restricted tasking is appropriate for use in high integrity systems. The Ada 95 Annex seemed to suggest that tasking was inappropriate for such systems. 

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 Requirement: If a pragma Restrictions(No_Exceptions) is specified, the effects of all constructs where language-defined checks are still performed.

Discussion: {AI95-00114-01} 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 dereferencing 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.

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!

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.

Discussion: The specific mention of these two uses is meant to replace the identifiers now banished to J.13, “Dependence Restriction Identifiers”.

Restriction No_Dependence => Ada.Unchecked_Deallocation 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. 

{8652/0042} {AI95-00130-01} {extensions to Ada 95} No_Local_Allocators no longer prohibits generic instantiations. 

{AI95-00285-01} Wide_Wide_Text_IO (which is new) is added to the No_IO restriction.

{AI95-00347-01} The title of this clause was changed to match the change to the Annex title. Pragma Profile(Ravenscar) is part of this annex.

{AI95-00394-01} Restriction No_Dependence is used instead of special restriction_identifiers. The old names are banished to Obsolescent Features (see J.13).

{AI95-00394-01} The bizarre wording “apply in this Annex” (which no one quite can explain the meaning of) is banished.