Verification and Validation

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 22 Slide 1

Objectives

● To introduce software verification and validation andto discuss the distinction between them

● To describe the program inspection process and itsrole in V & V

● To explain static analysis as a verification technique

● To describe the Cleanroom software developmentprocess

Topics covered

● Verification and validation planning

● Software inspections

● Automated static analysis

● Cleanroom software development

● Verification:"Are we building the product right”.

● The software should conform to itsspecification.

● Validation: "Are we building the right product”.

● The software should do what the user reallyrequires.

Verification vs validation

● Is a whole life-cycle process - V & V must beapplied at each stage in the softwareprocess.

● Has two principal objectives• The discovery of defects in a system;

• The assessment of whether or not the system isuseful and useable in an operational situation.

The V & V process

V& V goals

● Verification and validation should establishconfidence that the software is fit forpurpose.

● This does NOT mean completely free ofdefects.

● Rather, it must be good enough for itsintended use and the type of use willdetermine the degree of confidence that isneeded.

V & V confidence

● Depends on system’s purpose, userexpectations and marketing environment• Software function

• The level of confidence depends on how critical thesoftware is to an organisation.

• User expectations• Users may have low expectations of certain kinds of

software.

• Marketing environment• Getting a product to market early may be more

important than finding defects in the program.

● Software inspections. Concerned with analysis ofthe static system representation to discoverproblems (static verification)• May be supplement by tool-based document and code

analysis

● Software testing. Concerned with exercising andobserving product behaviour (dynamic verification)• The system is executed with test data and its operational

behaviour is observed

Static and dynamic verification

Static and dynamic V&V

● Can reveal the presence of errors NOT theirabsence.

● The only validation technique for non-functional requirements as the software hasto be executed to see how it behaves.

● Should be used in conjunction with staticverification to provide full V&V coverage.

Program testing

● Defect testing• Tests designed to discover system defects.

• A successful defect test is one which reveals thepresence of defects in a system.

• Covered in Chapter 23

● Validation testing• Intended to show that the software meets its

requirements.

• A successful test is one that shows that a requirementshas been properly implemented.

Types of testing

● Defect testing and debugging are distinctprocesses.

● Verification and validation is concerned withestablishing the existence of defects in a program.

● Debugging is concerned with locating andrepairing these errors.

● Debugging involves formulating a hypothesisabout program behaviour then testing thesehypotheses to find the system error.

Testing and debugging

The debugging process

● Careful planning is required to get the mostout of testing and inspection processes.

● Planning should start early in thedevelopment process.

● The plan should identify the balancebetween static verification and testing.

● Test planning is about defining standards forthe testing process rather than describingproduct tests.

V & V planning

The V-model of development

The structure of a software test plan

● The testing process.

● Requirements traceability.

● Tested items.

● Testing schedule.

● Test recording procedures.

● Hardware and software requirements.

● Constraints.

The software test plan

The testing processA description of the major phases of the testing process. These might beas described earlier in this chapter.

Requirements traceabilityUsers are most interested in the system meeting its requirements andtesting should be planned so that all requirements are individually tested.

Tested itemsThe products of the software process that are to be tested should bespecified.

Testing scheduleAn overall testing schedule and resource allocation for this schedule.This, obviously, is linked to the more general project developmentschedule.

Test recording proceduresIt is not enough simply to run tests. The results of the tests must besystematically recorded. It must be possible to audit the testing processto check that it been carried out correctly.

Hardware and software requirementsThis section should set out software tools required and estimatedhardware utilisation.

ConstraintsConstraints affecting the testing process such as staff shortages shouldbe anticipated in this section.

Software inspections

● These involve people examining the sourcerepresentation with the aim of discovering anomaliesand defects.

● Inspections not require execution of a system somay be used before implementation.

● They may be applied to any representation of thesystem (requirements, design,configuration data,test data, etc.).

● They have been shown to be an effective techniquefor discovering program errors.

Inspection success

● Many different defects may be discovered ina single inspection. In testing, one defect,may mask another so several executionsare required.

● The reuse domain and programmingknowledge so reviewers are likely to haveseen the types of error that commonly arise.

Inspections and testing

● Inspections and testing are complementary and notopposing verification techniques.

● Both should be used during the V & V process.

● Inspections can check conformance with aspecification but not conformance with thecustomer’s real requirements.

● Inspections cannot check non-functionalcharacteristics such as performance, usability, etc.

Program inspections

● Formalised approach to document reviews

● Intended explicitly for defect detection (notcorrection).

● Defects may be logical errors, anomalies inthe code that might indicate an erroneouscondition (e.g. an uninitialised variable) ornon-compliance with standards.

Inspection pre-conditions

● A precise specification must be available.

● Team members must be familiar with theorganisation standards.

● Syntactically correct code or other systemrepresentations must be available.

● An error checklist should be prepared.

● Management must accept that inspection willincrease costs early in the software process.

● Management should not use inspections for staffappraisal ie finding out who makes mistakes.

The inspection process

Inspection procedure

● System overview presented to inspectionteam.

● Code and associated documents aredistributed to inspection team in advance.

● Inspection takes place and discovered errorsare noted.

● Modifications are made to repair discoverederrors.

● Re-inspection may or may not be required.

Inspection roles

Author or owner The programmer or designer responsible forproducing the program or document. Responsiblefor fixing defects discovered during the inspectionprocess.

Inspector Finds errors, omissions and inconsistencies inprograms and documents. May also identifybroader issues that are outside the scope of theinspection team.

Reader Presents the code or document at an inspectionmeeting.

Scribe Records the results of the inspection meeting.

Chairman or moderator Manages the process and facilitates the inspection.Reports process results to the Chief moderator.

Chief moderator Responsible for inspection process improvements,checklist updating, standards development etc.

Inspection checklists

● Checklist of common errors should be used todrive the inspection.

● Error checklists are programming languagedependent and reflect the characteristic errors thatare likely to arise in the language.

● In general, the 'weaker' the type checking, the largerthe checklist.

● Examples: Initialisation, Constant naming, looptermination, array bounds, etc.

Inspection checks 1

Data faults Are all program variables initialised before their values areused?Have all constants been named?Should the upper bound of arrays be equal to the size of thearray or Size -1?If character strings are used, is a de limiter explicitlyassigned?Is there any possibility of buffer overflow?

Control faults For each conditional statement, is the condition correct?Is each loop certain to terminate?Are compound statements correctly bracketed?In case statements, are all possible cases accounted for?If a break is required after each case in case statements, hasit been included?

Input/output faults Are all input variables used?Are all output variables assigned a value before they areoutput?Can unexpected inputs cause corruption?

Inspection checks 2

Interface faults Do all function and method calls have the correct numberof parameters?Do formal and actual parameter types match?Are the parameters in the right order?If components access shared memory, do they have thesame model of the shared memory structure?

Storagemanagement faults

If a linked structure is modified, have all links beencorrectly reassigned?If dynamic storage is used, has space been allocatedcorrectly?Is space explicitly de-allocated after it is no longerrequired?

Exceptionmanagement faults

Have all possible error conditions been taken into account?

Inspection rate

● 500 statements/hour during overview.

● 125 source statement/hour during individualpreparation.

● 90-125 statements/hour can be inspected.

● Inspection is therefore an expensiveprocess.

● Inspecting 500 lines costs about 40man/hours effort - about £2800 at UK rates.

Automated static analysis

● Static analysers are software tools for sourcetext processing.

● They parse the program text and try todiscover potentially erroneous conditions andbring these to the attention of the V & Vteam.

● They are very effective as an aid toinspections - they are a supplement to butnot a replacement for inspections.

Static analysis checks

Fault class Static analysis check

Data faults Variables used before initialisationVariables declared but never usedVariables assigned twice but never used betweenassignmentsPossible array bound violationsUndeclared variables

Control faults Unreachable codeUnconditional branches into loops

Input/output faults Variables output twice with no interveningassignment

Interface faults Parameter type mismatchesParameter number mismatchesNon-usage of the results of functionsUncalled functions and procedures

Storage managementfaults

Unassigned pointersPointer arithmetic

Stages of static analysis

● Control flow analysis. Checks for loops withmultiple exit or entry points, finds unreachablecode, etc.

● Data use analysis. Detects uninitialisedvariables, variables written twice without anintervening assignment, variables which aredeclared but never used, etc.

● Interface analysis. Checks the consistency ofroutine and procedure declarations and theiruse

Stages of static analysis

● Information flow analysis. Identifies thedependencies of output variables. Does notdetect anomalies itself but highlightsinformation for code inspection or review

● Path analysis. Identifies paths through theprogram and sets out the statementsexecuted in that path. Again, potentiallyuseful in the review process

● Both these stages generate vast amounts ofinformation. They must be used with care.

LINT static analysis

138% more lint_ex.c#include <stdio.h>printarray (Anarray) int Anarray;{ printf(“%d”,Anarray); }

main (){ int Anarray[5]; int i; char c; printarray (Anarray, i, c); printarray (Anarray) ;}

139% cc lint_ex.c140% lint lint_ex.c

lint_ex.c(10): warning: c may be used before setlint_ex.c(10): warning: i may be used before setprintarray: variable # of args. lint_ex.c(4) :: lint_ex.c(10)printarray, arg. 1 used inconsistently lint_ex.c(4) :: lint_ex.c(10)printarray, arg. 1 used inconsistently lint_ex.c(4) :: lint_ex.c(11)printf returns value which is always ignored

Use of static analysis

● Particularly valuable when a language suchas C is used which has weak typing andhence many errors are undetected by thecompiler,

● Less cost-effective for languages like Javathat have strong type checking and cantherefore detect many errors duringcompilation.

Verification and formal methods

● Formal methods can be used when amathematical specification of the system isproduced.

● They are the ultimate static verificationtechnique.

● They involve detailed mathematical analysisof the specification and may develop formalarguments that a program conforms to itsmathematical specification.

Arguments for formal methods

● Producing a mathematical specificationrequires a detailed analysis of therequirements and this is likely to uncovererrors.

● They can detect implementation errorsbefore testing when the program is analysedalongside the specification.

Arguments against formal methods

● Require specialised notations that cannot beunderstood by domain experts.

● Very expensive to develop a specificationand even more expensive to show that aprogram meets that specification.

● It may be possible to reach the same level ofconfidence in a program more cheaply usingother V & V techniques.

● The name is derived from the 'Cleanroom'process in semiconductor fabrication. Thephilosophy is defect avoidance rather thandefect removal.

● This software development process is based on:• Incremental development;

• Formal specification;

• Static verification using correctness arguments;

• Statistical testing to determine program reliability.

Cleanroom software development

The Cleanroom process

Cleanroom process characteristics

● Formal specification using a state transitionmodel.

● Incremental development where thecustomer prioritises increments.

● Structured programming - limited control andabstraction constructs are used in theprogram.

● Static verification using rigorous inspections.● Statistical testing of the system (covered in

Ch. 24).

Formal specification and inspections

● The state based model is a systemspecification and the inspection processchecks the program against this mode.l

● The programming approach is defined sothat the correspondence between the modeland the system is clear.

● Mathematical arguments (not proofs) areused to increase confidence in the inspectionprocess.

● Specification team. Responsible for developingand maintaining the system specification.

● Development team. Responsible fordeveloping and verifying the software. Thesoftware is NOT executed or even compiledduring this process.

● Certification team. Responsible for developinga set of statistical tests to exercise the softwareafter development. Reliability growth modelsused to determine when reliability is acceptable.

Cleanroom process teams

● The results of using the Cleanroom process havebeen very impressive with few discovered faults indelivered systems.

● Independent assessment shows that theprocess is no more expensive than otherapproaches.

● There were fewer errors than in a 'traditional'development process.

● However, the process is not widely used. It is notclear how this approach can be transferredto an environment with less skilled or lessmotivated software engineers.

Cleanroom process evaluation

Key points

● Verification and validation are not the samething. Verification shows conformance withspecification; validation shows that theprogram meets the customer’s needs.

● Test plans should be drawn up to guide thetesting process.

● Static verification techniques involveexamination and analysis of the program forerror detection.

Key points

● Program inspections are very effective indiscovering errors.

● Program code in inspections is systematicallychecked by a small team to locate software faults.

● Static analysis tools can discover programanomalies which may be an indication of faults in thecode.

● The Cleanroom development process depends onincremental development, static verification andstatistical testing.

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