Information technology — Programming languages — Guide for the use of the Ada programming language in high integrity systems

1 Scope This Technical Report provides guidance on the use of Ada when producing high integrity systems. In producing such applications it is usually the case that adherence to guidelines or standards has to be demonstrated to independent bodies. These guidelines or standards vary according to the application area, industrial sector or nature of the risk involved. For safety applications, the international generic standard is [IEC 61508] of which part 3 is concerned with software. For security systems, the multi-national generic assessment guide is [ISO CD 15408]. For sector-specific guidance and standards there are: Airborne civil avionics: [DO-178B] Nuclear power plants: [IEC 880] Medical systems: [IEC 601-4] Pharmaceutical: [GAMP] For national/regional guidance and standards there are the following: UK Defence: [DS 00-55] European rail: [EN 50128] European security: [ITSEC] US nuclear: [NRC] UK automotive: [MISRA] US medical: [FDA] US space: [NASA] The above standards and guides are referred to as Standards in this Technical Report. The above list is not exhaustive but indicative of the type of Standard to which this Technical Report provides guidance. The specific Standards above are not addressed individually but this Technical Report is synthesized from an analysis of their requirements and recommendations. 1.1 Within the scope This Technical Report assumes that a system is being developed in Ada to meet a standard listed above or one of a similar nature. The primary goal of this Technical Report is to translate general requirements into Ada specific ones. For example, a general standard might require that dynamic testing provides evidence of the execution of all the statements in the code of the application. In the case of generics, this is interpreted by this Technical Report to mean all instantiations of the generic should be executed. ISO/IEC TR 15942:2000 (E) 2 © ISO/IEC 2000 - All rights reserved This Technical Report is intended to provide guidance only, and hence there are no ?shalls'. However, this Technical Report identifies verification and validation issues which should be resolved and documented according to the sector-specific standards being employed. The following topics are within the scope of this Technical Report: _ the choice of features of the language which aid verification and compliance to the standards, _ identification of language features requiring additional verification steps, _ the use of tools to aid design and verification, _ issues concerning qualification of compilers for use on high integrity applications, _ tools, such as graphic design tools, which generate Ada source code which is accessible to users. Tools which generate Ada source code require special consideration. Where generated code may be modified or extended, verification of the extensions and overall system will be assisted if the guidelines have been taken into account. Even where modification is not planned, inspection and analysis of the generated code may be unavoidable unless the generator is trusted or ?qualified' according to an applicable standard. Finally, even if generated code is neither modified nor inspected, the overall verification process may be made more complicated if the code deviates from guidelines intended to facilitate testing and analysis. Potential users of such tools should evaluate their code generation against the guidance provided in this Technical Report. 1.2 Out of scope The following topics are considered to be out of scope with respect to this Technical Report: _ Domain-specific standards, _ Application-specific issues, _ Hardware and system-specific issues, _ Human factor

Technologies de l'information — Langages de programmation — Guide pour l'emploi du langage de programmation Ada dans les systèmes de haute intégrité

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Status
Published
Publication Date
15-Mar-2000
Current Stage
9093 - International Standard confirmed
Start Date
30-Oct-2013
Completion Date
30-Oct-2013
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TECHNICAL ISO/IEC
REPORT TR
15942
First edition
2000-03-01
Information technology — Programming
languages — Guide for the use of the Ada
programming language in high integrity
systems
Technologies de l'information — Langages de programmation — Guide
pour l'emploi du langage de programmation Ada dans les systèmes de
haute intégrité
Reference number
ISO/IEC TR 15942:2000(E)
ISO/IEC 2000
---------------------- Page: 1 ----------------------
ISO/IEC TR 15942:2000(E)
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ii © ISO/IEC 2000 – All rights reserved
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ISO/IEC TR 15942:2000 (E)
Contents

1 Scope.................................................................................................................................................................1

1.1 Within the scope....................................................................................................................................1

1.2 Out of scope ..........................................................................................................................................2

2 Verification Techniques ...................................................................................................................................2

2.1 Traceability ............................................................................................................................................2

2.2 Reviews ..................................................................................................................................................3

2.3 Analysis..................................................................................................................................................3

2.3.1 Control Flow analysis ...........................................................................................................................4

2.3.2 Data Flow analysis ................................................................................................................................4

2.3.3 Information Flow analysis ....................................................................................................................4

2.3.4 Symbolic Execution ..............................................................................................................................4

2.3.5 Formal Code Verification......................................................................................................................5

2.3.6 Range Checking ....................................................................................................................................6

2.3.7 Stack Usage analysis............................................................................................................................6

2.3.8 Timing Analysis.....................................................................................................................................6

2.3.9 Other Memory Usage analysis.............................................................................................................6

2.3.10 Object Code Analysis ...........................................................................................................................7

2.4 Testing....................................................................................................................................................7

2.4.1 Principles ...............................................................................................................................................7

2.4.2 Requirements-based Testing...............................................................................................................7

2.4.3 Structure-based Testing.......................................................................................................................8

2.5 Use of Verification Techniques in this Technical Report..................................................................8

3 General Language Issues ................................................................................................................................9

3.1 Writing Verifiable Programs .................................................................................................................9

3.1.1 Language Rules to Achieve Predictability........................................................................................10

3.1.2 Language Rules to Allow Modelling..................................................................................................10

3.1.3 Language Rules to Facilitate Testing................................................................................................11

3.1.4 Pragmatic Considerations..................................................................................................................12

3.1.5 Language Enhancements...................................................................................................................12

3.2 The Choice of Language.....................................................................................................................13

4 Significance of Language Features for High Integrity................................................................................14

4.1 Criteria for Assessment of Language Features ...............................................................................14

4.2 How to use this Technical Report .....................................................................................................14

5 Assessment of Language Features..............................................................................................................15

5.1 Types with Static Attributes...............................................................................................................16

5.1.1 Evaluation ............................................................................................................................................17

5.1.2 Notes ....................................................................................................................................................17

5.1.3 Guidance ..............................................................................................................................................17

5.2 Declarations.........................................................................................................................................17

5.2.1 Evaluation ............................................................................................................................................18

5.2.2 Notes ....................................................................................................................................................18

5.2.3 Guidance ..............................................................................................................................................18

5.3 Names, including Scope and Visibility..............................................................................................19

5.3.1 Evaluation ............................................................................................................................................19

5.3.2 Notes ....................................................................................................................................................19

5.3.3 Guidance ..............................................................................................................................................20

5.4 Expressions .........................................................................................................................................20

5.4.1 Evaluation ............................................................................................................................................21

5.4.2 Notes ....................................................................................................................................................21

5.4.3 Guidance ..............................................................................................................................................22

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ISO/IEC TR 15942:2000 (E)

5.5 Statements...........................................................................................................................................22

5.5.1 Evaluation ............................................................................................................................................23

5.5.2 Notes ....................................................................................................................................................23

5.5.3 Guidance..............................................................................................................................................23

5.6 Subprograms.......................................................................................................................................24

5.6.1 Evaluation ............................................................................................................................................24

5.6.2 Notes ....................................................................................................................................................24

5.6.3 Guidance..............................................................................................................................................25

5.7 Packages (child and library) ..............................................................................................................25

5.7.1 Evaluation ............................................................................................................................................26

5.7.2 Notes ....................................................................................................................................................26

5.7.3 Guidance..............................................................................................................................................26

5.8 Arithmetic Types .................................................................................................................................27

5.8.1 Evaluation ............................................................................................................................................27

5.8.2 Notes ....................................................................................................................................................27

5.8.3 Guidance..............................................................................................................................................28

5.9 Low Level and Interfacing..................................................................................................................29

5.9.1 Evaluation ............................................................................................................................................30

5.9.2 Notes ....................................................................................................................................................30

5.9.3 Guidance..............................................................................................................................................31

5.10 Generics...............................................................................................................................................31

5.10.1 Evaluation ............................................................................................................................................32

5.10.2 Notes ....................................................................................................................................................32

5.10.3 Guidance..............................................................................................................................................33

5.11 Access Types and Types with Dynamic Attributes .........................................................................34

5.11.1 Evaluation ............................................................................................................................................34

5.11.2 Notes ....................................................................................................................................................34

5.11.3 Guidance..............................................................................................................................................35

5.12 Exceptions ...........................................................................................................................................35

5.12.1 Evaluation ............................................................................................................................................36

5.12.2 Notes ....................................................................................................................................................36

5.12.3 Guidance..............................................................................................................................................36

5.13 Tasking.................................................................................................................................................37

5.13.1 Evaluation ............................................................................................................................................39

5.13.2 Notes ....................................................................................................................................................39

5.13.3 Guidance..............................................................................................................................................39

5.14 Distribution ..........................................................................................................................................40

5.14.1 Evaluation ............................................................................................................................................40

5.14.2 Notes ....................................................................................................................................................40

5.14.3 Guidance..............................................................................................................................................40

6 Compilers and Run-time Systems ................................................................................................................40

6.1 Language issues .................................................................................................................................41

6.2 Compiler Qualification........................................................................................................................41

6.3 Run-Time System................................................................................................................................42

7 References ......................................................................................................................................................43

7.1 Applicable Documents .......................................................................................................................43

7.2 Referenced Documents ......................................................................................................................44

iv © ISO/IEC 2000 - All rights reserved
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ISO/IEC TR 15942:2000 (E)
Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the

specialized system for worldwide standardization. National bodies that are members of ISO or IEC participate in the

development of International Standards through technical committees established by the respective organization to deal with

particular fields of technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other

international organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the work.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.

In the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1. Draft

International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an

International Standard requires approval by at least 75 % of the member bodies casting a vote.

In exceptional circumstances, when a technical committee has collected data of a different kind from that which is normally

published as an International Standard ("state of the art", for example), it may decide by a simple majority vote of its

participating members to publish a Technical Report. A Technical Report is entirely informative in nature and does not have to

be reviewed until the data it provides are considered to be no longer valid or useful.

Attention is drawn to the possibility that some of the elements of this Technical Report may be the subject of patent rights. ISO

and IEC shall not be held responsible for identifying any or all such patent rights.

ISO/IEC TR 15942 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee

SC 22, Programming languages, their environments and system software interfaces.
© ISO/IEC 2000 - All rights reserved
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ISO/IEC TR 15942:2000 (E)
Introduction

As a society, we are increasingly reliant upon high integrity systems: for safety systems (such as fly-by-wire aircraft), for

security systems (to protect digital information) or for financial systems (e.g., cash dispensers). As the complexity of these

systems grows, so do the demands for improved techniques for the production of the software components of the system.

These high integrity systems must be shown to be fully predictable in operation and have all the properties required of them.

This can only be achieved by analysing the software, in addition to the use of conventional dynamic testing.

There is, currently, no mainstream high level language where all programs in that language are guaranteed to be predictable and

analysable. Therefore for any choice of implementation language it is essential to control the ways that the language is used by

the application.

The Ada language [ARM] is designed with specific mechanisms for controlling the use of certain aspects of the language.

Furthermore,

1. The semantics of Ada programs are well-defined, even in error situations. Specifically, the effect of a program

can be predicted from the language definition with few implementation dependencies or interactions between

language features.

2. The strong typing within the language can be used to reduce the scope (and cost) of analysis to verify key

properties.

3. The Ada language has been successfully used on many high integrity applications. This demonstrates that

validated Ada compilers have the quality required for such applications.

4. Guidance can be provided to facilitate the use of the language and to encourage the development of tools for

further verification.

Ada is therefore ideally suited for implementing high integrity software and this document provides guidance in the controls

that are required on the use of Ada to ensure that programs are predictable and analysable.

All language design balances functionality against integrity. For instance, the ability to control storage allocation directly will

impact the need to ensure the integrity of data. An aspect of the integrity of Ada programs is the possibility of avoiding access

types (references) completely, whereas in other languages references are linked to array accessing and/or parameter passing,

and therefore cannot be excluded.

There are a number of different analysis techniques in use for high integrity software and this document is not prescriptive

about which techniques to use. Furthermore, each analysis technique requires different controls on the use of the language. Ada

assists analysis: for instance, the modes of Ada parameters, suitably used, provide information for data flow analysis which

other languages cannot always provide. This Technical Report, therefore, catalogues specific verification techniques (see 2.5),

and classifies the impact that language features have on the use of these techniques (in the tables in Section 5).

It is the user's responsibility to select the analysis techniques for a particular application; this document can then be used to

define the full set of controls necessary for using that set of techniques.

The guidance given here first specifies its scope, by reference to the safety and security standards to which high integrity

applications may be written.

Section 2 then analyses the verification techniques that are applied in the development of high integrity systems. By this

means, the regulatory rules of the standards for safety and security are abstracted to avoid the need to consider each such

standard separately.

Section 3 addresses general issues concerning how computer languages must be constructed if programs written in that

language are to be fully predictable. These issues are relevant to any restricted language defined through the application of this

guidance.
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ISO/IEC TR 15942:2000 (E)

Section 4 provides identification of a three-way classification system used for Ada language features. This classification is

based upon the ease with which verification techniques can be applied to a program containing the feature. This classification

is needed since while the majority of the core features in Ada assists verification, the use of certain features makes the resulting

code difficult or impossible to analyse with the currently available program analysis tools and techniques.

Section 5 provides the main technical material of this Technical Report by classifying Ada language features. Users of this

Technical Report can then determine which features of Ada are appropriate to use from the verification techniques that are to

be employed. The assessment has shown that the vast majority of the Ada features lend themselves to effective use in the

construction of high integrity systems.

The Technical Report concludes, in Section 6, by providing information to aid the choice of a suitable Ada compiler together

with its associated run-time system.

References to relevant standards and guides are provided. A detailed analysis of Ada95 for high integrity systems is available

in References [CAT1, CAT2] and [CAT3] .
A comprehensive index is provided to ease the use of the Technical Report.
Levels of criticality

Many of the Standards to which high integrity software is written use multiple levels to classify the criticality of the software

components which make up the system. While the number and nature of the levels vary, the general approach is always the

same: the higher the criticality of the system, the more verification techniques need to be used for its assurance. Table 1 relates

the various levels of classification used in some well known International Standards.

Table 1: Levels of criticality in some Standards
Standard Number of levels Lowest Level Highest Level
[DO-178B] 4 D A
[IEC-61508] 4 Safety Integrity Level 1 Safety Integrity Level 4
[ITSEC] 7 E0 E6

This Technical Report emphasizes the higher levels of criticality, for which the more demanding verification techniques are

employed and for which Ada provides major benefits.

This Technical Report, however, does not directly use any such levels but focuses on the correlation between the features of the

language and the verification techniques to be employed at the higher levels of criticality. The material in [ISO/IEC 15026],

[DS 00-56], [ARP 4754] and [ARP 4761] may be useful in determining the criticality of a system if this is not covered by

application-specific standards.
Readership
This Technical Report has been written for:

1. Those responsible for coding standards applicable to high integrity Ada software.

2. Those developing high integrity systems in Ada.

3. Vendors marketing Ada compilers, source code generators, and verification tools for use

in the development of high integrity systems.

4. Regulators who need to approve high integrity systems containing software written in

Ada.

5. Those concerned with high integrity systems who wish to consider the advantages of

using the Ada language.

This Technical Report is not a tutorial on the use of Ada or on the development of high integrity software. Developers using

this report are assumed to have a working knowledge of the language and an understanding of good Ada style, as in [AQS].

vii
© ISO/IEC 2000 - All rights reserved
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ISO/IEC TR 15942:2000 (E)
History

When proposals were made that a subset of Ada should be specified for high integrity applications, it was realized that the

provision of the Safety and Security Annex in the Ada standard did not satisfy all the requirements of the developers of high

integrity systems. In consequence, a group was formed under WG9 to consider what action was needed. This group, called the

HRG, proposed and drafted this Technical Report over a three year period.
Conventions

In line with the Ada standard, the main text is in a Roman font. Ada identifiers are set in a sans-serif font, and the Ada

keywords in a bold sans-serif font.
Postscript

The guidance provided here reflects the understanding of the issues based mainly on using the previous Ada standard in

developing high integrity applications. Over the next few years, the current Ada standard will be used for further high integrity

applications which will no doubt need to be reflected in a revision of this guidance. Specifically, further detail can be produced

based upon the experience gained.
Instructions for comment submission

Informal comments on this Technical Report may be sent by e-mail to hrg@cise.npl.co.uk. If appropriate, the project editor will

document the issue for corrective action.
Comments should use the following format:
!topic: Title which is a summary in one sentence
!reference TR 15942- ss.ss.ss
!from Author, Name, yy-mm-dd
!keywords keywords related to topic
!discussion
text of discussion

where ss.ss.ss is the section number, yy-mm-dd is the date. If the comment requests a change, a rationale for this and the

substance of the actual change proposed would facilitate the processing of the comment.

viii © ISO/IEC 2000 - All rights reserved
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TECHNICAL REPORT
ISO/IEC TR 15942:2000 (E)
Information technology - Programming languages - Guide for the
use of the Ada programming language in high integrity systems
1 Scope

This Technical Report provides guidance on the use of Ada when producing high integrity systems. In producing such

applications it is usually the case that adherence to guidelines or standards has to be demonstrated to independent bodies.

These guidelines or standards vary according to the application area, industrial sector or nature of the risk involved.

For safety applications, the international generic standard is [IEC 61508] of which part 3 is concerned with software.

For security systems, the multi-national generic assessment guide is [ISO CD 15408].

For sector-specific guidance and standards there are:
Airborne civil avionics: [DO-178B]
Nuclear power plants: [IEC 880]
Medical systems: [IEC 601-4]
Pharmaceutical:[GAMP]
For national/regional guidance and standards there are the following:
UK Defence: [DS 00-55]
European rail: [EN 50128]
European security:[ITSEC]
US nuclear:[NRC]
UK automotive:[MISRA]
US medical:[FDA]
US space:[NASA]

The above standards and guides are referred to as Standards in this Technical Report. The above list is not exhaustive but

indicative of the type of Standard to which this Technical Report provides guidance.

The specific Standards above are not addressed individually but this Technical Report

...

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