L2: Program Development

Question 1

(CRE)² Criteria

(Program Fitness Checklist)

Answer 1

A Good Program should be:

• Correct
  
  • Does what specification said
  • What client wants
  • What user needs
• Clear
  
  • Understandable Implementation
• Robust
  
  • Resilient wrt the real world, human abilities, future requirements
• Ready
  
  • Delivered within schedule, budget constraints
• Efficient
  
  • Within limits of time, space, cost
• Ethical
  
  • Meets standards set by law, professional ethics, and safety needs

Question 2

Convergent Programming Steps

Answer 2

1. Understand the Problem
   
   • State the Problem
   • Specify the Interface
   • List representative and stressful cases
2. Code into C
   
   • Diagram your approach
   • Decide on variables and invariants
   • Outline overall logic
   • Convert logic into code
3. Verify the Solution
   
   • “Desk Check” -mental test cases
   • Perform test cases on computer
   • Use Fitness Test (CRE)²
   • Peer Review
4. Return to earlier steps as needed

Question 3

Convergent Programming:

Basic Idea

Answer 3

• Clearly define the logic:
  
  • in english,
  • mathematical notation
  • diagrams
• THEN, express that logic in C

Code INTO C, not IN C

Question 4

Convergent Programming Steps:

Step 1) Understand the Problem

-Substeps

Answer 4

• State the problem
• Specify the Interface
• List Representative Cases
• List Stressful Cases

Question 5

Convergent Programming Steps:

Step 2) Code Into C:

-Substeps

Answer 5

Code into C

• Diagram your approach
• Decide on variables and invariants
• Outline overall logic
• Convert logic into code

Question 6

Convergent Programming Steps:

Step 3) Verify the Solution

-Substeps

Answer 6

Verify the Solution

• “Desk Check” -mental test cases
• Computer Test: Perform test cases on computer
• Use Fitness Test (CRE)2
• Peer Review

Question 7

2 “Families” of Common Errors

Answer 7

• Exogenous (Outward error)
  
  • Hardware Issues
    
    • Computer Crash/error
  • Software Issues
  • Data Issues:
    
    • incorrect data
    • too much data
    • too fast/slow
• Endogenous (Problems with development process/team)
  
  • Analysis
    
    • Wrong Problem
    • Wrong Goals
    • Wrong User
  • Specification
    
    • incorrect
    • incomplete
    • inconsistent
  • Logic
    
    • forgetfulness
    • stupidity
    • laziness

Question 8

make:

-n switch

Answer 8

%make -n

Do not actually perform the commands in the recipes,

just echo them

Question 9

Programming as a Craft:

Maintain Respect for ________

Answer 9

• The User
  
  • Will they feel good about using your product?
• The Tools
  
  • Recognize their limits, and your own
• Yourself
  
  • Use best known practices
  • Take pride in your work

Question 10

Make Files:

• file name
• format

Answer 10

• Always have the name “makefile”
• Format:

target: prerequisites

recipe

dependencies

Example:

foobar: main.o util.o

cc -c main.c

Question 11

Key Files

during

Compiling and Linking

Answer 11

• Source Files
  
  • c language
  • text format
  • end in .c and .h
• Object Files
  
  • machine instructions, binary format
  • external names
  • data initialization
  • end in .o
• Libraries
  
  • Collections of object files
  • binary format
  • end in .a
• Executable Files
  
  • instructions
  • data
  • entry point (“main”)
  • other details

Question 12

Steps of Creating an Executable Program

Answer 12

1. Edit
   
   • Create source files(.h, .c) with editor
2. Compile
   
   • Translate each source into an object file
3. Link
   
   • Combine the object files into an executable file
4. Execute
   
   • Use the new program

Question 13

Compiler:

Useful switches for the

cc program

Answer 13

* -c

* compile into an object(.o) file * -D
* Define a variable, equivalent to #define
* example: cc -DFOO=this * -E
* Send preprocessor output to stdout and stop * -S
* Save .s files instead of deleting them

More advanced switches:

• -g
  
  • create expanded symbol table for debugging
• -p
  
  • link with the profiling facility “prof”
• -O
  
  • optimize the code

Question 14

Compiler Steps

Answer 14

• Preprocessing
  
  • Replace #define, #include, etc
• Compilation
  
  • Produce assembly language from c text
  • generates .s files
• Assembly
  
  • Produce object files from assembly text
  • generates .o files

Question 15

Using External Variables

Answer 15

• Use the “extern” keyword to declare an external variable
• Allows use of a variable from another object that is linked
• Example:
  
  • extern int flag
• Ensure exactly one definition for each external name
• Always initialize external variables
• Ensure definition & import agree on type(the linker doesn’t check this)

Question 16

What happens during Linking?

(Overview)

Answer 16

• A program is usually specified in pieces, each in separate source files
• Also, system routines may be referenced, which are in the System Library files (“.a” extensions)
• The linker combines your objects and those needed system routines
• Creates an executable program, no extensions, executable attribute

Question 17

What happens during Compiling?

(overview)

Answer 17

• The compiler(cc) translates the source files(.c and .h) into a form suitable for efficient machine interpretation
• The files created are Object Files( “.o” extensions)
• Substeps:
  
  • Preprocessing
  • Assembly

Question 18

Using Malloc to Allocate storage

(Overview)

Answer 18

• “malloc” routine gets storage from the heap
• must include stdlib
• the heap is a storage pool that you manage
• Anything allocated persists until explicitly freed using free()
• Initial bits in allocated storage are unpredictable(when using malloc())

Question 19

Allocating a Structure Instance

with malloc

Answer 19

• Use sizeof() to determine the number of bits to be allocated
• Malloc returns a block of memory
• Cast the memory as the required type

name = (myType*) malloc(sizeof(myType))

Question 20

Declaring a Structure

Answer 20

typedef struct _myStruct{

char *myName;

int myNum;

struct _myStruct *other;

} MyStruct;

• typedef : creates a new type name
• struct : lays out a structure
• fields of the struct are defined
• within the definition, can reference the struct with _myStruct

Question 21

Pointer/Structures vs Arrays

Answer 21

• Arrays are great when
  
  • Data items can be conveniently numbered without many gaps
  • Max number is known
  • Reorganizations will not be frequent
• Pointers/Structures
  
  • Needed when things are more complicated
  • No obvious # of elements
  • Insertions are common

Question 22

Fundamental Principle of Pointer Programming

Answer 22

Diagram your f******

Pointer Logic

Question 23

Pointer Programming

Steps

Answer 23

1. Think
   
   • of a specific example of the situation and the action at hand
2. Diagram
   
   • how things are organized before the action(“before”)
3. Modify
   
   • to show effects of the action (“after”)
4. Generalize
   
   • to capture the general case
5. Code
   
   • the general case into c

Question 24

make operations:

Recipes

Answer 24

• Can have any number of commands
• Make process stops when a command fails
• Always precede the commands with a tab

Question 25

make operations: targets

Answer 25

* Can be specified on make command line * %mk foo.o * A rule may have multiple targets * A target with no prerequisites is useful to automate a task sometimes

Question 26

Benefits of make and makefiles

Answer 26

* Records dependence between executables, objects and sources * Check last modified times on all files * Performs compiles and links ONLY when changes occur

Question 27

Common Make Errors

Answer 27

* Circular Dependency * making a file dependent on itself * Insufficient Prerequisites * Ensure you have the .h files IN the prerequisites * Missing Recipe * Triggers a built-in recipe for .c -\> .o generation, which is probably not what you want * No tab before recipe * Make will think the command is a target * $ -substitutions * make and shell each have $var variables for substitution

Question 28

Program Memory: Two Methods to get an area from the Heap

Answer 28

* malloc(size) * memory allocation * allocates a single area of the heap of the specified size, then is casted to a pointer * calloc(n, size) * allocate multiple n chunks of memory, for use by arrays

Question 29

Memory Layout for a program

Answer 29

* Memory is a linear array of bytes * Stack changes with call and return * Heap changes with malloc/calloc and free Sections of Memory, in order: * Code * Data * Heap * Available(used for temporary storage) * Stack * Shell Data

Question 30

When to use the Heap

Answer 30

* When data should persist after a function returns * Size of data is not known until execution time

Question 31

Program Memory: Variables used to track Program State

Answer 31

* Key Addresses * Current _Instruction Counter (IC)_ * Stack Pointer, SP * Frame Pointer, FP * Heap Bottom, Stack Bottom * Register Values * Scheduling Priority * Current Working Directory * Process id's : * This process, pid * Parent Process, ppid * User and Group IDs

Question 32

3 Common Pointer Errors

Answer 32

* Pointer is null * Unitialized fields * pointer is not null, but points to wrong thing or garbage data * Forgetting the asterisk: * person p - wrong * person\* p - correct