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Maintaining large software systems. Dr L Bottaci Department of Computer Science University of Hull, Hull, UK. Preface: Module. 20 Credits Syllabus topics Software maintenance practice Debugging Software maintenance management. Preface: Resources. Course Materials

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Maintaining large software systems

Maintaining large software systems

Dr L Bottaci

Department of Computer Science

University of Hull, Hull, UK

Preface module
Preface: Module

  • 20 Credits

  • Syllabus topics

    • Software maintenance practice

    • Debugging

    • Software maintenance management

Preface resources
Preface: Resources

  • Course Materials

    • Check undergraduate web site for this module.

      • Lecture slides

      • Make your own notes

      • Course notes

      • ACW description

  • Read notice board and email

Preface acw
Preface: ACW

  • Assessed course work

    • Worth 100% of module assessment

    • Software maintenance task

    • Work individually

    • Assessed on what you learn, as well as product

    • ACW specification, on web page, will have details

Preface assessment
Preface: Assessment

Assessing learning. In order of importance:

  • Evidence of learning in the logbook

  • Student contribution to seminars and lab discussions

  • Assessment of modified software

    Course is safe environment for sensible risk taking

Preface reading
Preface: Reading

  • Books

    • Few books specifically on Maintenance

      • See references in course notes

    • Consult any good software engineering book, e.g.

      • Pressman, Software Engineering, McGraw Hill, 2000

What is software maintenance
What is Software Maintenance?

  • Software does not change

  • But the operating environment and the world does.

  • Fix bugs

  • Adapt to new operating environment

  • Adapt to new requirements

Maintenance as software engineering
Maintenance as software engineering

  • 50% (by cost) of maintenance is done to adapt to changed requirements

  • 80% (by cost) of software engineering is software maintenance

Maintenance costs
Maintenance costs

  • Cost of change increases with time after design

  • Reduce cost by planning for long term maintenance

  • Planning and designing for maintenance increases development cost

  • Commercial requirements important, balance with engineering requirements

Syllabus topic 1 software maintenance practice
Syllabus topic 1: Software maintenance practice

  • Software maintenance is not a theoretical subject

  • Learning is change

    • Learn by doing

    • Learn by thinking

Practical exercise outcomes

Acquire experience

Learn from experience

Practical exercise: outcomes

  • Learn what is required to maintain software.

  • Learn how to improve one’s knowledge and skill.

    • Lazy practice makes permanent

    • Goal directed practice makes better

  • Motivation and self confidence.

    • Requires a rational assessment of one’s abilities

Maintenance task brief intro
Maintenance task: Brief intro

  • Modify the jscript compiler (part of the .NET sscli) to implement new requirements.

Maintenance task overview
Maintenance task overview

  • Students given a short review of compiler operation, scanner, parser, code generator

  • Role of abstract syntax tree

  • No other information – it is important that you learn

Why use rotor
Why use Rotor?

  • Code large enough that it cannot be understood in its entirety

  • Code contains very few comments

  • Sufficiently readable for students to make progress in the relatively short time allocated for a module

  • It is “real” code

Finding out about the system
Finding out about the system

  • Look for information about systems of the type you are examining

    • If it is an object-oriented compiler, look for information about object-oriented compilers.

    • Do not overlook journal articles and books. There is a lag between ideas appearing in the literature and their take up in commercial products so it may be necessary to search the literature that was published several years before the system was built.

Finding out about the system1
Finding out about the system

  • Look for documentation, in the source code files themselves or in associated documentation files.

  • Check if the producer of the code has documentation in addition to whatever is in the distribution you are examining.

  • Has anyone else worked or looked at the code, do they have documentation or information? Can they be contacted?

Finding out about the system2
Finding out about the system

  • Can static analysis tools be useful?

    • Very simple and useful facility is the ability to search a set of files for a given string e.g. the grep tool in UNIX, find, findstr in Windows. Similar tool in VS Ctrl-Shft-F

    • More sophisticated tools later

Finding out about the system3
Finding out about the system

  • Can automatic documentation tools be useful?

    • E.g. it is often useful to know which functions call a given function. A complete description is known as the call graph.

    • Other kinds of graph - class hierarchies.

    • In general, automatic documentation tools produce cross referenced lists

    • Example documentation tool is doxygen, available on the web.

Finding out about the system4
Finding out about the system

  • Ultimately, read the code.

Code reading skills
Code reading skills

  • Code reading should be goal-directed

    • Reading to see what is there

    • Trying to understand each line

  • What are you expecting to find?

    • Formulate an hypothesis

    • Read the code to confirm or disprove it.

Code reading illustration 1
Code reading illustration: 1

What is the following code doing?

while (...) {



Hypothesise the most popular uses of a loop in general and look at code for evidence.

Code reading illustration 2
Code reading illustration: 2

while (...) {

sum := sum + a[i];



Array accumulation a likely hypothesis

Code reading illustration 3
Code reading illustration: 3

while (...) {

sum := sum + a[i];

if (...)






Hypotheses to explain the conditional inside a loop

Code reading illustration 4
Code reading illustration: 4

while (...) {

sum := sum + a[i];

if (...)

done := 1;





Flag, is it for early termination?

Code reading illustration 5
Code reading illustration: 5

while (i < 9 and done = 0) {

sum := sum + a[i];

if (...)

done := 1;





Initial lab exercises 1
Initial lab exercises: 1

  • Read the Rotor (sscli) documentation

  • Download and build the system

  • Try the jscript compiler.

    • Find, compile and execute a sample jscript program

  • Test the system, save the log file to compare with future tests.

Initial lab exercises 2
Initial lab exercises: 2

  • Modify the jscript compiler to print a message before it compiles a file

  • Rebuild compiler and recompile jscript sample.

  • Rerun tests and check output with previous run of tests.

Further lab exercises
Further lab exercises

  • Print each character in the file compiled by the jscript compiler.

  • Print each token recognised by the jscript compiler.

Practical exercise
Practical Exercise

  • Control moves to next statement in program unless there is a conditional statement or transfer of control statement.

  • Conditional statement is if-statement, switch-statement, while-statement and for-statement.

  • Transfer of control statement is return, break, continue.

  • Task is to modify the compiler so that it produces a warning when it detects that a statement is unreachable, i.e. cannot be executed.

Practical exercise e g
Practical Exercise: E.g.

for (i = 0; i < n; i++) {

x = y;


y = 0;


Practical exercise e g1
Practical Exercise: E.g.

switch (e) {

case 1:

x = 0;


x = 1;

case 2;

x = 1;



x = 2;


Practical exercise e g2
Practical Exercise: E.g.

if (true) {

x = 0;


else {

x = 1;


Practical exercise stages
Practical Exercise: stages

  • Continue and extend the examples given to produce a list of test cases.

  • Implementation plan with algorithm

  • When above two checked, proceed with implementation

Example implementation plan textual substitution
Example implementation plan:textual substitution

  • Read jscript program source code, as a file of text, looking for keywords such as ‘return’, ‘break’, etc.

  • Identify statements in jscript program source code by looking for substrings terminating in a semicolon,

Example implementation plan textual substitution evaluation
Example implementation plan:textual substitution, evaluation

  • Read jscript program source code, as a file of text, looking for keywords such as ‘return’, ‘break’, etc.

    • jsscanner does this, plus point for plan

  • Identify statements in jscript program source code by looking for substrings terminating in a semicolon,

    • jsparser does this, plus point for plan

Example implementation plan examine the msil
Example implementation plan:examine the MSIL

  • Examine the MSIL produced by the jscript compiler to identify unreachable code.

  • Could start by examining MISL for simple source code examples given above.

Example implementation plan transform the ast
Example implementation plan:transform the AST

  • Examine the AST produced by the jscript compiler to identify transfer of control statements, etc.

Example implementation plan transform the ast evaluation
Example implementation plan:transform the AST, evaluation

  • Examine the AST produced by the jscript compiler to identify control transfer statements.

    • Traverse AST looking for a type of AST node

    • Need a ‘foreach-stmt’ to iterate over AST

Cost estimation individual
Cost estimation: individual

  • Necessary and frequent activity, usually implicit

  • In practical work, cost estimation should be explicit so that it can be scrutinised and improved.

    • Calculate estimate, record in logbook

    • When estimate expires, review estimate

    • Note how it can be improved

Tools for navigating code
Tools for navigating code

  • Tools are available for extracting information from code.

    • Most simple tools search files for strings, e.g. in VS Ctrl-Shft-F

    • Most sophisticated tools called reverse engineering

Tools for navigating code1
Tools for navigating code

  • Method call relationships for all methods is known as the call graph

  • Can be constructed by a tool.

  • Other graphs includes class hierarchies.

  • The sort of documentation produced by automatic documentation tools consists largely of cross referenced lists.

  • An example of such a documentation tool is doxygen, available on the web

Tools for navigating code2
Tools for navigating code

  • To answer more sophisticated queries, analysis of the program dependency graph is required.

  • To use these tools it is necessary to understand the program dependency graph.

  • The program dependency graph is actually a collection of graphs dealing with control and data dependency

Example program
Example program

  • 1. i := 0;

  • 2. sum := 0;

  • 3. done := 0;

  • 4. while (i < 9 and done = 0)

  • 5. sum := sum + a[i];

  • 6. if (sum >= 8)

  • 7. done := 1;

  • else

  • 8. i := i + 1;

  • 9. print(sum);

Control flow
Control Flow

  • The nodes of the graphs are the statements in a program or collections of statements known as regions.

  • A region may correspond to a basic block.

  • The conditional nodes of a control flow graph are distinguished (typically shown as squares) from the statement nodes (typically shown as ellipses)

  • Directed edges are the possible transitions between statements or basic blocks during program execution.

  • The conditional transitions are associated with a branch predicate (labelled T or F).

  • There is a distinguished start node and a distinguished exit node.

Control dependency
Control Dependency

  • The control dependency graph is derived from the control flow graph.

  • When node Y is control dependent on node X, taking one branch at X will ensure that Y is reached, Y may or may not be reached if the other branch is taken.

  • As an example, consider nodes 4 and 5 in the control flow graph of previous program.

Control dependency example
Control Dependency: example

  • There is a path from node 4 to node 5.

  • Taking the true branch at 4 ensures that 5 is reached. This is not true if the other branch it taken.

  • Node 5 is said to be control dependent of node 4.

  • In contrast, node 9 is not control dependent on node 4 since either branch at node 4 will always lead to node 9.

Data dependency
Data Dependency

  • Data dependency exists between two nodes if the meaning of the program may change when the order of the two nodes is reversed.

  • Different kinds of data dependency.

    • Flow dependency

    • Def-order dependency

Flow dependency
Flow Dependency

  • Flow dependency exists from X to Y if

    a variable v is defined (the value of v is set) at X and used at Y, and

    there is a path in the control flow graph from X to Y without an intervening definition of v.

  • In other words, the definition at X may directly determine the value of v at Y.

Def order dependency
Def-order Dependency

  • Def-order dependency exists from X to Y if

    • both nodes define the same variable v,

    • X and Y are in the same branch of any conditional that contains both X and Y,

    • there is a node Z that is flow dependent on X and Y, and

    • X is to the left of Y in the abstract syntax tree.

Def order dependency example
Def-order Dependency: example

  • An example of def-order dependency is present between node 3 and node 7 in the previous example program

Program slices
Program slices

  • A program slice is a subset of the statements in a program that are relevant to some criterion, usually the value of a variable at a given statement.

  • This case is called a backward slice.

  • The forward slice also useful, i.e. all the statements possibly affected by the value assigned at a particular statement.

  • CodeSurfer from GrammaTech is a code analysis tool (C code only) based on the program dependency graph.

  • The web site provides technical papers as well as an overview of the capabilities of the tool


  • It is much better to spend time when first writing code to ensure it is correct than to spend time debugging incorrect code.

  • Many programmers think the opposite is true.

Software inspection exercise
Software inspection exercise


//INSERT elem at i1 IN a IF i1 VALID


int i = 0;

if (i1 >= 0 && i1 <= count) {

for (i = count; i > i1; i--) {

a[i] = a[i – 1];


a[i - 1] = elem;


if (i2 >= 0 && i2 <= count) {

for (i = i2; i < count; i++) {

a[i] = a[i + 1];



Software inspection exercise1
Software inspection exercise



//INSERT elem INTO NONFULL ARRAY a AT indexIn PROVIDING indexIn <= count



if (a.Length > count && indexIn >= 0 && indexIn <= count) { //INSERT

int i = 0;

for (i = count; i > indexIn; i--) {

a[i] = a[i – 1];


a[indexIn] = elem;

count = count + 1;


if (indexOut >= 0 && indexOut < count) { //REMOVE

int i = 0;

for (i = indexOut; i < count - 1; i++) {

a[i] = a[i + 1];


count = count – 1;



  • Careful code design and debugging, are not of equal productivity cost.

  • Over the long term, an extra day designing a program worth more than a day of debugging saved.

  • Programmers expect to improve with experience.

  • Experience in careful program design is more valuable than debugging experience.

  • What is learnt during a day spent debugging is rarely applicable to another program.


  • Defensive programming is an effective way of avoiding debugging.

    • Handle exceptions as close as possible to where they may occur.

  • It is important to distinguish between

    • a run-time condition that can and should be handled, e.g. an invalid input which may be cleared and read again, and

    • a run-time condition that represents a failed pre-condition that invalidates the entire program so that recovery is not possible.

  • An assertion can be used to test for a pre-condition at run-time

    • Debug.Assert(n > 0) //PROGRAM INVALID

    • If true, no action occurs but if it fails while executing under the debugger, the program enters break mode

Debugging assert
Debugging: Assert()

  • In C\#, Assert method in the Debug class and Trace class

  • To use Assert, the file must include the directives

    • \#define TRACE or

    • \#define DEBUG

  • For efficiency, Debug methods not included in release version

    • never put error handling code in a Debug assertion.

Debugging assert1
Debugging: Assert()

  • It is also essential that the code that computes the required assert condition does not produce side effects.

    • It is not good programming practice for any condition to produce a side effect.

  • Trace assertions are retained in release version.

  • Assert takes up to three arguments.

    • The first is mandatory and is the condition to check.

    • The remaining two arguments are expressions that evaluate to strings that are printed when the condition fails

Debugging assert2
Debugging: Assert()

  • As a rule, assert all the pre-conditions for the arguments of each non-trivial function or method.

  • For each method, assert separate conditions separately so that when a condition fails it will be clear which it is.

Simple test harness
Simple test harness

  • DOS batch file run.bat

    DEL run.out

    DATE /T >> run.out

    TIME /T >> run.out

    for %%f in (



    ) do call runaux %%f

Simple test harness1
Simple test harness

  • DOS batch file runaux.bat

    set CLIX=C:\rotor\sscli20\binaries.x86chk.rotor

    set JSC=C:\rotor\sscli20\binaries.x86chk.rotor

    set PROGNAME=%1

    TYPE %PROGNAME%.js >> run.out

    %CLIX% %JSC% %PROGNAME%.js > %PROGNAME%.out

    FC /L /N %PROGNAME%.out %PROGNAME%.rqd >> run.out


Software maintenance management
Software Maintenance management

  • Any activity that consumes considerable resources requires good management.

  • Maintenance planning should be done at the same time as the planning of the system development.

Software maintenance management1
Software Maintenance management

  • Maintenance plan should include:

    • the maintenance goals,

    • maintenance management

    • maintenance processes,

    • hardware and software platforms, tools

    • personnel, training

    • and budget.

Maintenance process models
Maintenance process models

  • Any activity that is to be managed must first be described and understood.

  • A life cycle model describes activities as phases in a process.

Maintenance process models1
Maintenance process models

  • Taute maintenance model (1983, see notes)

  • Request phase,

    • the requested change is identified and logged.

  • Identification includes a check that the request actually is a modification, that it has not already been submitted by some other user under a different id perhaps, etc.

Maintenance process models2
Maintenance process models

  • Estimate phase,

    • how much will the change cost to implement?

    • What are the implications of the change?

    • Why is the change required?

    • Is this change in the best interest of the supplier-customer relationship?

    • What other changes are likely to be required?

  • It is necessary for both the supplier and customer to have a clear idea of the aim of the system.

Maintenance process models3
Maintenance process models

  • Estimate phase requires detailed knowledge of the software system

  • E.g. the following anecdote told by David Parnas.

  • When code is to be rewritten there is the issue of whether to preserve long standing bugs or features.

  • Consider the conversion of an old unstructured code fragment that displays the altimeter reading in an aircraft cockpit.

Maintenance process models4
Maintenance process models

if not canread(alt1) goto l1


goto l3

l1: if not canread(alt2) goto l2


goto l3

l2: display(3000)


Maintenance process models5
Maintenance process models

Convert to a modern structured code fragment

if canread(alt1)


else if canread(alt2)


else display(3000)

Is conversion correct?

Maintenance process models6
Maintenance process models

  • The 3000 value is displayed when neither altimeter can be read.

  • Why was 3000 used?

  • Should the 3000 be changed to `error' or `pull up'?

  • Can the display show only digits?

  • The importance, in software maintenance, of understanding the requirements should be obvious

Maintenance process models7
Maintenance process models

  • Schedule phase, when is the change to be implemented and released?

  • Programming phase, new release version is created and code modified.

  • Test phase, the new release is tested. This may require modifying or writing new tests.

  • Documentation phase, existing documentation is modified.

  • Release phase, new release given to some users for acceptance testing.

  • Operational phase, new release delivered to all users.

Maintenance process models8
Maintenance process models

  • A more detailed model is the IEEE maintenance process model as described in the IEEE 1219-1998 standard.

  • Phases are similar to those in the Taute maintenance model but each phase is described in terms of four aspects.

    • process (what is done),

    • input to process,

    • output of process,

    • control (how is the process controlled and output checked?).

Configuration management
Configuration management

  • Configuration management is the administration of changes to a product and versions of a product.

  • A product may be a plan, a specification, a design, some code, test data, etc.

  • The configuration control board considers the various modification requests, their utility and estimated cost.

  • These requests are considered in the light of the overall strategy for the system under maintenance.

Configuration management1
Configuration management

  • Changes are made with respect to a baseline product or version.

  • After a system has undergone a number of changes that, ideally, form a logically coherent unit, the system is said to be in a different version.

  • The collection of changes that defines a version may, for example, be all those that allow the system to operate on a new platform.

  • Sometimes the changes that define a new version have little in common and happen to be those changes ready at the scheduled six month release date.

Configuration management2
Configuration management

  • It is important to know the construction history of the various versions, i.e.

    • which version was modified to produce which version.

  • Each change definition, i.e. each code change, must be accompanied with associated information,

    • the author,

    • the reason for the change (which should be traceable back to a modification of the requirements),

    • the date,

    • authorisation, etc.

Configuration management3
Configuration management

  • If change only latest version then the derived-from relation produces a sequence of versions.

  • This is the result, for example,of producing backups only.

  • When some version earlier than the latest version is modified, a new branch of the tree is formed.

  • In general, versions form a directed graph rather than a tree since different versions may merge.

Configuration management4
Configuration management

  • A merged version contains the changes of both its parents.

  • Clearly, if changes to the same line conflict then the user must resolve the conflict,

    • the user chooses which change toaccept.

  • Clearly, merging must be done with care.

Configuration management5
Configuration management

  • A product is decomposed into modules, files or assemblies.

  • Configuration management systems will allow changes to a version to be made on a module by module basis.

  • Configuration management systems are essential when there are a number of programmers working on the same system.

Configuration management6
Configuration management

To make a change to a file which is part of some version, the file is first checked out.

  • Checking out a file ensures that whoever is requesting a file has permission to change that file.

  • In this case the file is said to be locked.

  • Different locking schemes are possible.

    • pessimistic locking, single write permission but multiple readpermissions.

    • Optimistic locking allows multiple write permissions and provides some mechanism for resolving overwrite clashes. For example, the first write may cause all other writers to be notified; at which point there is the option to check out the updated file.