Dr. Jonathan validation verification.ppt

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 19 Slide 1
Verification and Validation
Dr.K.N Jonathan,PhD
Senior Lecturer,UNILAK

Verification and Validation
 Assuring that a software
system meets a user's needs

Objectives
 To introduce software verification and validation
and to discuss the distinction between them
 To describe the program inspection process and
its role in V & V
 To explain static analysis as a verification
technique
 To describe the Cleanroom software
development process

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 its specification
 Validation:
"Are we building the right product"
 The software should do what the user really
requires
Verification vs validation

 Is a whole life-cycle process - V & V must be
applied at each stage in the software process.
 Has two principal objectives
• The discovery of defects in a system
• The assessment of whether or not the system is usable in
an operational situation.
The V & V process

 Software inspections Concerned with analysis
of
the static system representation to discover
problems (static verification)
• May be supplement by tool-based document and code analysis
 Software testing Concerned with exercising and
observing 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
Formal
specification
High-level
design
Requirements
specification
Detailed
design
Program
Prototype
Dynamic
validation
Static
verification

 Can reveal the presence of errors NOT their
absence
 A successful test is a test which discovers one
or more errors
 The only validation technique for non-functional
requirements
 Should be used in conjunction with static
verification to provide full V&V coverage
Program testing

 Defect testing
• Tests designed to discover system defects.
• A successful defect test is one which reveals the presence
of defects in a system.
• Covered in Chapter 20
 Statistical testing
• tests designed to reflect the frequence of user inputs. Used
for reliability estimation.
• Covered in Chapter 21
Types of testing

V& V goals
 Verification and validation should establish
confidence that the software is fit for purpose
 This does NOT mean completely free of defects
 Rather, it must be good enough for its intended
use and the type of use will determine the
degree of confidence that is needed

V & V confidence
 Depends on system’s purpose, user
expectations and marketing environment
• Software function
» The level of confidence depends on how critical the software 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

 Defect testing and debugging are distinct
processes
 Verification and validation is concerned with
establishing the existence of defects in a
program
 Debugging is concerned with locating and
repairing these errors
 Debugging involves formulating a hypothesis
about program behaviour then testing these
hypotheses to find the system error
Testing and debugging

The debugging process
Locate
error
Design
error repair
Repair
error
Re-test
program
Test
results Specification Test
cases

 Careful planning is required to get the most out
of testing and inspection processes
 Planning should start early in the development
process
 The plan should identify the balance between
static verification and testing
 Test planning is about defining standards for the
testing process rather than describing product
tests
V & V planning

The V-model of development
Requirements
specification
System
specification
System
design
Detailed
design
Module and
unit code
and tess
Sub-system
integration
test plan
System
integration
test plan
Acceptance
test plan
Service
Acceptance
test
System
integration test
Sub-system
integration test

The structure of a software test plan
 The testing process
 Requirements traceability
 Tested items
 Testing schedule
 Test recording procedures
 Hardware and software requirements
 Constraints

Software inspections
 Involve people examining the source
representation with the aim of discovering
anomalies and defects
 Do not require execution of a system so may be
used before implementation
 May be applied to any representation of the
system (requirements, design, test data, etc.)
 Very effective technique for discovering errors

Inspection success
 Many diffreent defects may be discovered in a
single inspection. In testing, one defect ,may
mask another so several executions are required
 The reuse domain and programming knowledge
so reviewers are likely to have seen the types of
error that commonly arise

Inspections and testing
 Inspections and testing are complementary and
not opposing verification techniques
 Both should be used during the V & V process
 Inspections can check conformance with a
specification but not conformance with the
customer’s real requirements
 Inspections cannot check non-functional
characteristics such as performance, usability,
etc.

Program inspections
 Formalised approach to document reviews
 Intended explicitly for defect DETECTION (not
correction)
 Defects may be logical errors, anomalies in the
code that might indicate an erroneous condition
(e.g. an uninitialised variable) or non-compliance
with standards

Inspection pre-conditions
 A precise specification must be available
 Team members must be familiar with the
organisation standards
 Syntactically correct code must be available
 An error checklist should be prepared
 Management must accept that inspection will
increase costs early in the software process
 Management must not use inspections for
staff
appraisal

The inspection process
Inspection
meeting
Individual
preparation
Overview
Planning
Rework
Follow-up

Inspection procedure
 System overview presented to inspection team
 Code and associated documents are
distributed to inspection team in advance
 Inspection takes place and discovered errors
are noted
 Modifications are made to repair discovered
errors
 Re-inspection may or may not be required

Inspection teams
 Made up of at least 4 members
 Author of the code being inspected
 Inspector who finds errors, omissions and
inconsistencies
 Reader who reads the code to the team
 Moderator who chairs the meeting and notes
discovered errors
 Other roles are Scribe and Chief moderator

Inspection checklists
 Checklist of common errors should be used to
drive the inspection
 Error checklist is programming language
dependent
 The 'weaker' the type checking, the larger the
checklist
 Examples: Initialisation, Constant naming, loop
termination, array bounds, etc.

Inspection checks
Fault class Inspectioncheck
Data faults Are all programvariables initialised before their values
are used?
Have all constants been named?
Should the lower bound of arrays be 0, 1, or something
else?
Should the upper bound of arrays beequalto the size of
the array or Size -1?
If character strings are used, is a delimiter explicitly
assigned?
Controlfaults For each conditionalstatement, is the condition correct?
Is each loop certain to terminate?
Are compound statements correctly bracketed?
In case statements, are allpossible cases accounted for?
Input/output faults Are all input variables used?
Are all output variables assigneda value before they are
output?
Interface faults Do allfunction and procedure calls have the correct
number of parameters?
Do formaland actual parameter types match?
Are the parameters in the right order?
If components access shared memory, do they have the
same model of the shared memory structure?
Storage management
faults
If a linked structure is modified, have alllinks been
correctly reassigned?
If dynamic storage is used, has space been allocated
correctly?
Is space explicitly de-allocated after it is no longer
required?
Exception
management faults
Have all possible error conditions been taken into
account?

Inspection rate
 500 statements/hour during overview
 125 source statement/hour during individual
preparation
 90-125 statements/hour can be inspected
 Inspection is therefore an expensive process
 Inspecting 500 lines costs about 40 man/hours
effort = £2800

Automated static analysis
 Static analysers are software tools for source
text processing
 They parse the program text and try to discover
potentially erroneous conditions and bring these
to the attention of the V & V team
 Very effective as an aid to inspections. A
supplement to but not a replacement for
inspections

Static analysis checks
Fault class Static analysis check
Data faults Variables used before initialisation
Variables declared but never used
Variables assigned twice but never used
between assignments
Possible array bound violations
Undeclared variables
Controlfaults Unreachable code
Unconditionalbranches into loops
Input/output faults Variables output twice with no intervening
assignment
Interface faults Parameter type mismatches
Parameter number mismatches
Non-usage of the results of functions
Uncalled functions and procedures
Storage management
faults
Unassigned pointers
Pointer arithmetic

Stages of static analysis
 Control flow analysis. Checks for loops with
multiple exit or entry points, finds unreachable
code, etc.
 Data use analysis. Detects uninitialised
variables, variables written twice without an
intervening assignment, variables which are
declared but never used, etc.
 Interface analysis. Checks the consistency of
routine and procedure declarations and their
use

Stages of static analysis
 Information flow analysis. Identifies the
dependencies of output variables. Does not
detect anomalies itself but highlights
information for code inspection or review
 Path analysis. Identifies paths through the
program and sets out the statements
executed in that path. Again, potentially
useful in the review process
 Both these stages generate vast amounts of
information. 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.c
140% lint lint_ex.c
lint_ex.c(10): warning: c may be used before set
lint_ex.c(10): warning: i may be used before set
printarray: 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 such as C
is used which has weak typing and hence many
errors are undetected by the compiler
 Less cost-effective for languages like Java that
have strong type checking and can therefore
detect many errors during compilation

 The name is derived from the 'Cleanroom'
process in semiconductor fabrication. The
philosophy is defect avoidance rather than
defect removal
 Software development process based on:
• Incremental development
• Formal specification.
• Static verification using correctness arguments
• Statistical testing to determine program reliability.
Cleanroom software development

The Cleanroom process
Construct
structured
program
Define
software
increments
Formally
verify
code
Integrate
increment
Formally
specify
system
Develop
operational
profile
Design
statistical
tests
Test
integrated
system
Error rework

Cleanroom process characteristics
 Formal specification using a state transition
model
 Incremental development
 Structured programming - limited control and
abstraction constructs are used
 Static verification using rigorous inspections
 Statistical testing of the system (covered in Ch.
21).

Incremental development
Formal
specification
Develop s/w
increment
Establish
rerquirements
Deliver
software
Frozen
specification
Requirements change request

Formal specification and inspections
 The state based model is a system specification
and the inspection process checks the program
against this model
 Programming approach is defined so that the
correspondence between the model and the
system is clear
 Mathematical arguments (not proofs) are used to
increase confidence in the inspection process

 Specification team. Responsible for developing
and maintaining the system specification
 Development team. Responsible for
developing and verifying the software. The
software is NOT executed or even compiled
during this process
 Certification team. Responsible for developing
a set of statistical tests to exercise the software
after development. Reliability growth models
used to determine when reliability is acceptable
Cleanroom process teams

 Results in IBM have been very impressive with
few discovered faults in delivered systems
 Independent assessment shows that the
process is no more expensive than other
approaches
 Fewer errors than in a 'traditional' development
process
 Not clear how this approach can be transferred
to an environment with less skilled or less
highly motivated engineers
Cleanroom process evaluation

Key points
 Verification and validation are not the same
thing. Verification shows conformance with
specification; validation shows that the program
meets the customer’s needs
 Test plans should be drawn up to guide the
testing process.
 Static verification techniques involve examination
and analysis of the program for error detection

Key points
 Program inspections are very effective in
discovering errors
 Program code in inspections is checked by a
small team to locate software faults
 Static analysis tools can discover program
anomalies which may be an indication of faults in
the code
 The Cleanroom development process depends
on incremental development, static verification
and statistical testing

Dr. Jonathan validation verification.ppt

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