software testing mtehododlogies path testing

Chapter - 2
THE TAXONOMY OF BUGS

The Consequences of BUGS
 The importance of Bugs depends on frequency,
correction cost, installation cost and consequences.
 Frequency: How often does that kind of bug occur?
Pay more attention to the more frequent bug types.
 Correction Cost: What does it cost to correct the bug
after it’s been found? That cost is the sum of two
factors.(1) the cost of discovery and (2) the cost of
correction. These costs go up dramatically the later
in the development cycle the bug is discovered.
Correction cost also depends on system size.

The Consequences of BUGS-
continued.,
 Installation Cost: Installation cost depends on
the number of installations: small for a single
user program but more for distributed systems.
Fixing one bug and distributing the fix could
exceed the entire system’s development cost.
 Consequences: what are the consequences of
the bug? This can be measured by the size of the
awards made by juries to the victims of the bug.
 A reasonable metric for bug importance is:
Importance($)=frequence*(correction_
Cost+installation_cost+consequential_cost)

How Bugs Affect Us-
Consequences
 Bug consequences range from mild to
catastrophic. These are measured in terms
of human rather than machine. Some
consequences on a scale of one to ten.
 Mild: The symptoms of the bug offend us
aesthetically; a misspelled output or a
misaligned printout.
 Moderate: outputs are misleading or
redundant. The bug impacts the system’s
performance.

How Bugs Affect Us-
Consequences-continued.,
 Annoying: The system’s behavior, because
of the bug is dehumanizing. Names are
truncated or arbitrarily modified.
 Disturbing: it refuses to handle legitimate
transactions. The ATM wont give you
money. My credit card is declared invalid.
 Serious: it loses track of transactions. Not
just the transaction it self but the fact that
the transaction occurred. Accountability is
lost.

How Bugs Affect Us-
 Very serious: The bug causes the system to do
the wrong transactions. Instead of losing your
paycheck, the system credits it to another
account or converts deposits into withdrawals.
 Extreme: the problems aren’t limited to a few
users or to a few transaction types. They are
frequent and arbitrary instead of sporadic or for
unusual cases.
 Intolerable: Long term un recoverable corruption
of the data base occurs and the corruption is not
easily discovered. Serious consideration is given
to shutting the system down.

How Bugs Affect Us-
 Catastrophic: the decision to shutdown is
taken out of our hands because the
system fails.
 Infectious: what can be worse than a failed
system? One that corrupts other systems
even though it does not fall in itself; that
erodes the social physical environment;
that melts nuclear reactors and starts war.

Flexible Severity Rather than
Absolutes
 Quality can be measured as a combination of factors, of
which the number of bugs and their severity is only one
component.
 Many organizations have designed and use satisfactory,
quantitative, quality metrics.
 Because bugs and their symptoms play a significant role in
such metrics, as testing progresses you see the quality rise
to a reasonable value which is deemed to be safe to ship the
product.
 The factors are:

Correction Cost

Context and Application Dependency

Creating Culture Dependency
 User Culture Dependency

The Software Development Phase

A Taxonomy for Bugs
 There is no universally correct way to categorize
bugs.
 The taxonomy is not rigid.
 A given bug can be put into one or another category
depending on its history and the programmer’s state
of mind.
 The major categories are:
 Requirements, features and functionality Bugs
 Structural Bugs
 Data Bugs
 Coding Bugs
 Interface, integration and system Bugs
 Test and Test Design Bugs

Requirements, Features and
Functionality Bugs
 Various categories in Requirements,
Features and Functionality:
 Requirements and Specifications
 Feature Bugs
 Feature Interaction Bugs

Requirements and
Specifications Bugs
 Requirement and Specification: Requirements and the
specifications developed from them can be incomplete,
ambiguous, or self-contradictory. They can be
misunderstood or impossible to understand.
 The specifications that don’t have flaws in them may
change while the design in progress. The features are
added, modified and deleted.
 Requirements, especially as expressed in a specification
are a major source of expensive bugs.

The range is from a few percentage to more than 50%,
depending upon the application and the environment.
 What hurts most about these bugs is that they’re the
earliest to invade the system and the last to leave.

Feature Bugs
 Feature Bugs: Specification problems usually
create corresponding feature problems. A
feature can be wrong, missing, or
superfluous. A missing feature or case is
easier to detect and correct.
 A wrong feature could have deep design
implications.
 Removing the features might complicate the
software, consume more resources, and
foster more bugs.

Feature Interactions Bugs
 Feature interactions: providing correct, clear,
implementable and testable feature specifications is
not enough.
 Features usually come in groups or related features.
 The features of each group and the interaction of
features within each group are usually well tested.
 The problem is unpredictable interactions between
feature groups or even between individual features.
 For example, your telephone is provided with call
holding and call forwarding. The interactions
between these two features may have bugs.
 Every application has its peculiar set of features and a
much bigger set of unspecified feature interaction
potentials and therefore feature interaction bugs.

Specification & feature bug
remedies
 Specification & feature bug Remedies: most feature
bugs are rooted in human to human communication
problems. One solution is the use of high level, formal
specification languages or systems.
 Such languages and systems provide short term support
but, in the long run, do not solve the problem.
 Short term support: specification languages facilitate
formalization of requirements and inconsistency and
ambiguity analysis.
 Long term Support: assume that we have a great
specification language and that it can be used to create
unambiguous, complete specifications with unambiguous,
complete tests and consistent test criteria.
 A specification written in that language could theoretically
be compiled into object code into HOL programming.
 The specification problem has been shifted to a higher
level but not eliminated.

Testing techniques for
functional Bugs.
 Most functional test techniques- that
is those techniques which are based
on a behavioral description of
software, such as transaction flow
testing, syntax testing, domain
testing, logic testing and state testing
are useful in testing functional bugs.

Structural Bugs
 Various categories in Structural Bugs:
 Control and Sequence Bugs
 Logic Bugs
 Processing Bugs
 Initialization Bugs
 Data flow Bugs and Anomalies

Control and Sequence Bugs
 Control and sequence bugs include paths left out,
unreachable code, improper nesting of loops, loop-back
or loop-termination criteria incorrect, missing process
steps, duplicated processing, unnecessary processing,
rampaging GOTO’s, ill-conceived switches, spaghetti
code, and worst of all pachinko code.
 One reason for control flow bugs is that this area is
amenable to theoretical treatment.
 Most of the control flow bugs are easily tested and
caught in unit testing.
 Another reason for control flow bugs is that use of old
code especially ALP & COBOL code are dominated by
control flow bugs.
 Control and sequence bugs at all levels are caught by
testing, especially structural testing more specifically
path testing combined with a bottom line functional test
based on a specification.

Logic Bugs
 Bugs in logic are related to:
 misunderstanding how case statements and logic
operators behave singly and combinations
 Include non existent cases
 Improper layout of cases
 Impossible cases that are not impossible
 a “don’t care” case that matters
 Improper negation of boolean expression
 Improper simplification and combination of cases
 Overlap of exclusive cases
 Confusing “exclusive OR” with “inclusive OR”
 The best defense against this kind of bugs is a
systematic analysis of cases.

Processing Bugs
 Processing Bugs include arithmetic bugs, algebraic,
mathematical function evaluation, algorithm selection
and general processing.
 The reasons for Processing Bugs:
 Incorrect conversion from one data representation to another.
 Ignoring overflow
 Ignoring the difference between +ve and -ve Zero
 Improper use of <,>,<=,>=
 Assumption to equality to zero in floating point
 Improper comparison between various formats
 Although these bugs are frequent 12% they tend to be caught
in good unit testing and also tend to have localized effects.

Initialization Bugs
 Initialization bugs are common, both improper
and superfluous initialization occur.
 Typical initialization bugs are:
 Forgetting initialize work space, registers
 Forgetting initialize data areas before first use or
assuming that they are initialized else where
 A bug in the first value of a loop control parameter
 Accepting the initial value without a validation
check
 Initializing to the wrong format, data
representation or type

Data flow Bugs and
Anomalies
 Most initialization bugs are special case of
data flow anomalies.
 A data flow anomaly occurs when there is a
path along which we expect to do something
unreasonable with data, such as using an
uninitialized variable, attempting to use a
variable before it exists, modifying and then
not storing or using the result, or initializing
twice without an intermediate use

Data Bugs
 Data bugs include all bugs that arise from
the specification of data objects, their
formats, the number of such objects, and
their initial values.
 The increase in the proportion of the source
statements devoted to data definition is a
direct consequence of two factors:
 The dramatic reduction in the cost of main
memory and disc storage.
 The high cost of creating and testing software.

Dynamic Versus Static
 Dynamic data are transitory. Whatever their purpose,
they have a relatively short lifetime, typically the
processing time of the transaction. A storage object
may be used to hold dynamic data of different types,
with different formats, attributes, and residues.
 The dynamic data bugs are due to leftover garbage in
a shared resource. This can be handled in one of three
ways:

Cleanup after use by the user

Common cleanup by the resource manager

No clean up
 Static data are fixed in form and content. They appear
in the source code or database directly or indirectly,
for example a number, a string of characters, or a bit
pattern.
 Compile time processing will solve the bugs caused by
static data.

Content, Structure and
Attributes
 Contents- the actual bit pattern, character string, or
number put into a data structure. Content is a pure bit
pattern and has no meaning unless it is interpreted by a
hardware or software processor. All data bugs result in
the corruption or misinterpretation of content.
 Structure- the size and shape and numbers that describe
the data object, that is the memory location used to
store the content.
 Attributes- the specification meaning that is the
semantics associated with the contents of a data object.
 The severity and subtlety of bugs increases as we go
from content to attributes because things get less formal
in the direction.

Coding Bugs
 Coding errors of all kinds can create any of
the other kinds of bugs.
 Syntax errors are generally not important in
the scheme of things if the source language
translator has adequate syntax checking.
 If a program has many syntax errors, then
we should expect many logic and coding
bugs
 The documentation bugs are also called as
coding bugs which may mislead the
maintenance programmers.

Interface, Integration, and
System Bugs
 Various categories of bugs in Interface,
Integration, and System Bugs are:
 External Interfaces
 Internal interfaces
 Hardware Architecture
 Operating system
 Software Architecture
 Control and Sequence Bugs
 Integration bugs
 System bugs

External Interfaces
 The external interfaces are the means used to
communicate with the world.
 These include devices, actuators, sensors, input
terminals, printers, and communication lines.
 The primary design criterion for an interface with
outside world should be robustness.
 All external interfaces, human or machine should
employ a protocol. The protocol may be wrong or
incorrectly implemented.
 Other external interface bugs are: invalid timing or
sequence assumptions related to external signals
 Misunderstanding external input or output formats
 Insufficient tolerance to bad input data.

Internal Interfaces
 Internal interfaces are in principle not different
from external interfaces but they are more
controlled.
 A best example for internal interfaces are
communicating routines.
 The external environment is fixed and the
system must adapt to it but the internal
environment, which consists of interfaces with
other components, can be negotiated.
 Internal interfaces have the same problem as
external interfaces.
 There is a trade-off between the internal
interfaces and the complexity of the interfaces.

Hardware Architecture
 Bugs related to hardware architecture originate mostly
from misunderstanding how the hardware works.
 Examples of hardware architecture bugs:

Paging mechanism ignored

Address generation error
 i/o device operation or instruction error

i/o device address error

Misunderstood device status code
 Device protocol error
 Expecting the device to respond too quickly

Waiting too long for a response

Ignoring channel throughput limits

Incorrect interrupt handling

Remedy for hardware
architecture and interface
problems
 The remedy for hardware
architecture and interface problems
is two fold
 Good programming and testing
 Centralization of hardware interface
software in programs written by
hardware interface specialists.

Operating system Bugs
 Program bugs related to the operating system
are a combination of hardware architecture
and interface bugs mostly caused by a
misunderstanding of what it is the operating
system does.
 Use operating system interface specialists, and
use explicit interface modules or macros for all
operating system calls.
 This approach may not eliminate the bugs but
at least will localize them and make testing
easier.

Software Architecture
 Software architecture bugs are the kind that called –
interactive.
 Routines can pass unit and integration testing without
revealing such bugs.
 Many of them depend on load, and their symptoms
emerge only when the system is stressed.
 Sample for such bugs:

Assumption that there will be no interrupts

Failure to block or un block interrupts

Assumption that Code is reentrant or not reentrant

Assumption that memory and registers were initialized or not
initialized
 Careful integration of modules and subjecting the final
system to a stress test are effective methods for these
bugs.

System level - Control and
sequence Bugs
 These bugs include:

Ignored timing

Assuming that events occur in a specified sequence

Starting a process before its prerequisites are met

Waiting for an impossible combination of
prerequisites

Not recognizing when prerequisites have been met

Specifying wrong priority, program state and level

Missing, wrong, redundant or superfluous process
steps.
 The remedy for these bugs is highly structured
sequence control.
 Specialize, internal, sequence control
mechanisms are helpful.

Resource management
problems
 Memory is subdivided into dynamically allocated
resources such as buffer blocks, queue blocks, task
control blocks, and overlay buffers.
 External mass storage units such as discs, are
subdivided into memory resource pools.
 Here are some resource management and usage
bugs:
 Required resource not obtained
 Wrong resource used
 Resource is already in use
 Resource not returned to the right pool
 Fractionated resources not properly recombined
 Resource dead lock

Resource management
Remedies
 A design remedy that prevents bugs
is always preferable to a test method
that discovers them
 The design remedy in resource
management is to keep the resource
structure simple: the fewest different
kinds of resources, the fewest pools,
and no private resource management

Resource management
Remedies-continued.,
 Complicated resource structures are often designed in a misguided
attempt to save memory by three sized transactions.
 The reasoning is faulty because:
 Memory is cheap and getting cheaper
 Complicated resource structures and multiple pools need
management software
 The complicated scheme takes additional processing time, and all
resources are held in use a little longer.
 The basis for sizing the resource is often wrong. A typical choice is to
make the buffer block’s length equal to the length required by an
average transaction-usually a poor choice.
 A correct analysis shows that the optimum resource size is usually
proportional to the square root of the transaction’s length.
 The second design remedy is that is to centralize the management of
all pools, either through centralized resource managers, common
resource management sub routines, resource management macros,
or a combination of these.

Integration Bugs
 Integration bugs are bugs having to do with
the integration of, and with the interfaces
between, working and tested components.
 These bugs results from inconsistencies or
incompatibilities between components.
 The communication methods include data
structures, call sequences, registers,
semaphores, communication links and
protocols results in integration bugs.
 The integration bugs do not constitute a big
bug category(9%) they are expensive category
because they are usually caught late in the
game and because they force changes in
several components and/or data structures.

System Bugs
 System bugs covering all kinds of bugs that
cannot be ascribed to a component or to their
simple interactions, but result from the totality
of interactions between many components
such as programs, data, hardware, and the
operating systems.
 There can be no meaningful system testing
until there has been thorough component and
integration testing.
 System bugs are infrequent(1.7%) but very
important because they are often found only
after the system has been fielded.

Test and Test Design Bugs
 Testing: testers have no immunity to bugs.
Tests require complicated scenarios and
databases.
 They require code or the equivalent to execute
and consequently they can have bugs.
 Test criteria: if the specification is correct, it is
correctly interpreted and implemented, and a
proper test has been designed; but the
criterion by which the software’s behavior is
judged is incorrect or impossible.

Test Design Bugs-Remedies
 The remedies for test bugs are:
 Test Debugging
 Test Quality Assurance
 Test Execution Automation
 Test Design Automation

Test Debugging
 The first remedy for test bugs is
testing and debugging the tests.
 The differences between the test
debugging and program debugging
are not fundamental.
 Test debugging is easier because
tests, when properly designed are
simpler than programs.
 Test debugging tend to have a
localized effects.

Test Quality Assurance
 Programmers have the right to ask
how quality in independent testing
and test design is monitored.
 The sequence like test testers, test
test testers? Should not converge.

Test Execution
Automation
 The history of software bug removal and
prevention is indistinguishable from the
history of programming automation aids.
 Assemblers, loaders, compilers are
developed to reduce the incidence of
programmer and operator errors.
 Test execution bugs are virtually
eliminated by various test execution
automation tools

Test Design Automation
 Just as much of software
development has been automated
much test design can be and has
been automated.
 For a given productivity rate,
automation reduces the bug count-
be it for software or be it for tests.

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