Introduction to C Programming

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Copyright © 2011 to author(s). All rights reserved
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Amr Ali Abdel-Naby@2011
2

Course Objective
After completing this course, you will be able to:
Write programs in C language
Understand what are the constructs of the C language
Implement data structures and algorithms in C
3

Course Notes
Set your cell phone to vibrate.
I assume you know computer architecture.
Ask any time.
During labs, feel free to:
Check any material
Search internet
4

Course References
www.cprogramming.com
C by Example
www.opengroup.com
http://ocw.mit.edu/index.htm
5

Outline
Introduction to C
Variables and Data Types
Blocks and Compound Statements
Control Flow
Modular Programming
Variable Scope
I/O
Pointers and Memory Addresses
Arrays and Pointers Arithmetic
Search and Sorting Algorithms
User Defined Data Types
Data Structures
Callbacks
Hash Tables
Using External Libraries
Creating Libraries
Standard Library
6

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
7

What is C?
Invented by Dennis Ritchie – AT&T Bell Labs
1972
Widely used today
Extends to newer system architectures
Efficiency/performance
Low-level access
8

C Features
Few keywords
Structures, unions, compound data types..
Pointers, arrays…
Standard library
Compiles to native code
Macro preprocessor
9

C Evolution
1972 – C Invention
1978 – 1st specification published
1989 – C89 standard (ANSI C or standard C)
1990 – ANSI C adopted by ISO, AKA C90
1999 – C99 standard
Not completely implemented in many compilers
2007 – Work on new standard C1X announced
We will focus on ANSI/ISO C (C89/C90)
10

C Usage
Systems programming
Operating systems
Microcontrollers
Embedded processors
DSP processors
11

C vs. Others
Recent derivatives: C++, C#, Objective-C
Had an effect on: Java, Perl, Python
C lacks:
Exceptions
Range checking
Garbage collection
OOP
…
Lower level language
12

Editing C Code
*.c extension for C source files
*.h extension for C header files
They are editable by any text editor.
13

IDE – All in One Solution
Examples:
Eclipse CDT
MS VC++ express edition
KDevelop
Xcode
Compiler, editor, debugger…
Suitable for large programs
14

Structure of a C File
15

Comments
Ignored by the compiler
Can appear anywhere
16

The #include Macro
A header file has constants, functions, and other declarations.
#include reads the contents of the header file
#include <header-file> searches for the header file in the include paths
#include “header-file” searches for the header file in the current directory where the file it included it is in
17

Declaring variables
Must declare variables before use
General form:
type variable_name [=initial_value][,][…];
Uninitialized, variable assumes a default value
Can declare/initialize multiple variables at once
18

Arithmetic Expressions
19

Order of Operations
Orders of Operation
Use parentheses to override order of evaluation
20

Function Prototypes
Functions also must be declared before use
Declaration called function prototype
Prototypes for many common functions in header files for C Standard Library
General form:
return_type function_name(arg1, arg2, …)
21

The main() Function
C Program entry point
Can be one of:
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Function Definition
Must match prototype (if there is one)
Variable names don’t have to match
Curly braces define a block
Variables declared in a block exist only in that block
Variable declarations must be before any other statements
23

Our First Program
24

More About Strings
Strings stored as character array
Null-terminated
Last character in array is ‘0’
Not written explicitly in string literals
Special characters specified using (escape character):
- backslash, ’ - apostrophe, ” - quotation mark
b, t, r, n - backspace, tab, carriage return, linefeed
ooo, xhh - octal and hexadecimal ASCII character codes
x41 – ’A’,
060 – ’0’
25

Console IO
stdout, stdin: console output and input streams
puts(string): print string to stdout
putchar(char): print character to stdout
char = getchar(): return character from stdin
string = gets(string): read line from stdin into string
26

Preprocessor Macros
They begin with #.
They can take arguments.
Parentheses ensure order of operations.
Compiler performs inline replacement.
27

Conditional Preprocessor Macros
They can control which lines are compiled.
Evaluated before code itself is compiled, so conditions must be preprocessor defines or literals
Used in header files to ensure declarations happen only once
28

Extra Preprocessor Macros
Preprocessor directive
Trigger a custom compiler error/warning message
Remove the definition of a previously defined constant
29

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
30

Definitions
Datatype
Determines the set of values an object can have and what operations that can be performed on it
Operator
Specifies how an object can be manipulated
Expression
Combination of values, variables, operators, and functions
Variable
Named link/reference to a value stored in the system’s memory or an expression that can be evaluated
31

Variables Naming Rules
Can contain letters, digits and _
Should start with letters
Keywords (e.g., for,while etc.) cannot be used as variable names.
Names are case sensitive.
X is not as x.
32

Data Types
C has a small family of datatypes.
Numeric (int, float, double)
Character (char)
User defined (struct, union)
Numeric datatypes
33

Variables Sizes and Endianess
Sizes are machine/compiler dependent.
sizeof(char) < sizeof(short) <= sizeof(int) <= sizeof(long)
sizeof(char) < sizeof(short) <= sizeof(float) <= sizeof(double)
For datatypes spanning multiple bytes, the order of arrangement of the individual bytes is important.
Big endian vs. little endian
34

Big Endian vs. Little Endian
35

Constants
Literal/fixed values assigned to variables or used directly in expressions
Integers
3, 3UL, 0x12, 012
Floating point
3.141, 3.141F
Character
‘A’, ‘x41’, ‘0101’
String
“Hello world”, “Hello” “world”
Enumeration
enum bool{YES, NO}, enum color{R=1, G, B, Y=10}
36

Operators
Arithmetic
+, -, *, /, %
Relational
>, >=, <, <=, ==, !=
Logical
&&, ||, !
Increment and decrement
X++, Y—
--X, ++X
Bitwise
&, |, ^, ~, >>, <<
Assignment
+=, &=…
37

Conditional Expression
38

Type Conversions
C is a weakly typed language.
It allows implicit conversions as well as forced casting.
When variables are promoted to higher precision, data is preserved.
char  int
int  float
As a rule (with exceptions), the compiler promotes each term in an binary expression to the highest precision operand.
39

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
40

Curly braces {} combine statements into a compound statement/block
A block substitute a simple statement and compiled as a single unit.
Variables can be declared in a block.
A block can be empty or nested.
41

Examples
42

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
43

Booleans in C
No booleans in C
True is any non-zero value/result of a condition/expression.
False is any zero value/result of a condition/expression.
Expression must be numeric or a pointer.
44

The if Statement
45

The switch Statement
46

The while Loop
47

The for Loop
48

The do-while Loop
49

The break and continue Keywords
break exists the innermost loop or switch statement.
continue skips rest of innermost loop body, jumping to loop condition .
50

goto Keyword
Allows you to jump unconditionally to arbitrary part of your code (within the same function)
The location is identified using a label.
A label is a named location in the code. It has the same form as a variable followed by a ’:’ .
Do not use it.
51

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
52

Divide and Conquer
Let’s design a program to solve linear Diophantine equation.
ax + by = c where a, b, c, x, y are integers
get a, b, c from command line
compute g = gcd(a,b)
if (c is not a multiple of the gcd) no solutions exist;
Run Extended Euclidean algorithm on a, b
rescale x and y output by (c/g)
print solution
Extended Euclidean algorithm: finds integers x, y s.t. ax + by = gcd(a, b).
53

Returning Multiple Values
Extended Euclidean algorithm returns gcd, and two other state variables, x and y
Functions only return (up to) one value
Solution: Use global variables
Declare variables for other outputs outside the function.
Variables declared outside of a function block are global.
Persist throughout life of program
Can be accessed/modified in any function
54

In General
Break down problem into simpler sub-problems.
Consider iteration and recursion.
Writing pseudocode first can help.
55

Programming Modules in C
C programs do not need to be monolithic.
Module: interface and implementation
Interface: header files
Implementation: auxiliary source/object files
Same concept carries over to external libraries.
Lets take the extended Euclid as an example.
56

ecluid.c
57

ecluid.h
58

Using Euclid Module
59

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
60

Variable Scope
The region in which a variable is valid.
Many cases, corresponds to block with variable’s declaration
Variables declared outside of a function have global scope
Function definitions also have scope
61

Scope and Nested Declarations
62

Static Variables
static keyword has two meanings, depending on where the static variable is declared
Outside a function, static variables/functions only visible within that file
Not global (cannot be extern’ed)
Inside a function, static variables are local to that function
Initialized only during program initialization
Do not get reinitialized with each function call
63

Register Variables
During execution, data are processed in registers.
Explicitly store commonly used data in registers
Minimize load/store overhead
Can explicitly declare certain variables as registers using register keyword
Must be a simple type
Only local variables and function arguments eligible
excess/unallowed register declarations ignored,
Registers do not reside in addressed memory
Pointer of a register variable illegal
64

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
65

Preliminaries
IO facilities are provided by the standard library <stdio.h> and not by the language itself.
A text stream consists of a series of lines ending with ’n’.
The standard library takes care of conversion from ’rn’−’n’ .
A binary stream consists of a series of raw bytes.
The streams provided by standard library are buffered.
66

Standard IO
67

Formatted Output: printf
It takes in a variable number of arguments.
It returns the number of characters printed.
The format can contain literal strings as well as format specifiers (starts with %).
68

printf Format Specification - type
%[flags][width][. precision][length]<type>
69

printf Format Specification - width
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printf Format Specification - flag
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printf Format Specification - precision
72

printf Format Specification - modifier
73

Character Arrays
Strings are represented as an array of characters.
C does not restrict the length of the string. The end of the string is specified using 0.
“hello”  {‘h’, ‘e’, ‘l’, ‘l’, ‘o’, ‘0’}
74

Comparing Strings
strcmp in string.h compares two strings in dictionary order.
Lower case letters come after capital case.
‘a’ > ‘A’
The function returns a value <0 if s comes before t
The function return a value 0 if s is the same as t
The function return a value >0 if s comes after t
75

Formatted Input
scanf reads characters from standard input, interpreting them according to format specification
Similar to printf , scanf also takes variable number of arguments.
Arguments have to be address of variables.
The format specification is the same as that for printf .
When multiple items are to be read, each item is assumed to be separated by white space.
scanf ignores white spaces.
It returns the number of items read or EOF.
76

String IO
Formatted data can be written to or read from character arrays.
77

File IO
C allows us to read/write data from text/binary files.
We can:
Open a file
Close a file
Read a single character
Read a single line
Write a single character
Write a single line
Read formatted line
Write formatted line
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Project I: Game of Life
79
Required:
In lab-01.pdf, solve all problems.
Duration: 120 minutes

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
80

Physical vs. Virtual Memory
Physical memories are physical resources where data can be stored.
Caches
RAMs
Hard disks
Removable storage
Virtual memory is an OS abstracted addressable space accessible by your code.
81

Physical Memory Considerations
Different sizes and access speeds
Memory management is a major OS function.
You have to optimize your code to make the best usage of the physical memory.
OS moves data around physical memory during execution.
In embedded systems, it may be very limited.
82

Virtual Memory
How much physical memory do you have?
How much virtual memory do you have?
Depends on OS
Usable parts of virtual memory are stack and heap.
83

Addressing Variables
Every variables has an address in memory.
What does not have an address?
Register variables
Expressions unless result is a variable.
Constants, literals, and preprocessors
The & operator finds the address of a variable.
Address of a variable of type t has type t *.
84

Dereferencing Pointers
Using the * operator, I can access and modify addressed variable.
A pointer that dereferences nothing is called a NULL pointer.
85

Casting Pointers
Explicitly from any type to any type.
Implicitly to and from void *.
May cause segmentation faults and many difficult errors to debug.
86

Functions with Multiple Outputs
Recall extended Ecluid, it calculates g = gcd(a,b) and sets the global variables x and y.
Using pointers we can, extend the outputs of a function.
87

Accessing Caller’s Variables
88

Variable Passing Out of Scope
What is wrong with this code?
89

Pointer to Pointers
Address stored by a pointer is also data in memory.
A pointer to a pointer can address location of address in memory.
Uses in C:
Pointer arrays
String arrays
90

Pointer to Pointers Example
91
vs.

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
92

Arrays and Pointers
Arrays in C are implemented using a pointer to block of contiguous memory.
[] can be used for accessing array elements.
Array name is a pointer to its 1st element.
Not modifiable/reassignable like any pointer.
93

The sizeof() Operator
Can be used to find the byte size of:
A primitive type/variable
Primitive arrays
How to find array length?
94

Pointer Arithmetic
A pointer is not an integer but we can add/substract an integer from it.
pa + i points to arr[i];
Address value increments by i x size of data type.
If arr[0] has address 100, then pa + 3 has value of 112.
95

Strings as Arrays
Strings are null terminated character arrays.
There are some utilities defined in string.h.
strcpy
strncpy
strcmp
strncmp
strlen
strcat
strncat
strchr
strrchr
…
96

Pointer Arrays
Array of pointers
Each pointer can point to another array.
Example usage: An array int arr[100] that we want to sort without modifying
Solution:
Declare a pointer array int * sorted_arr[100] containing pointers to elements of arr.
Sort the pointers instead of the numbers themselves.
Good approach for sorting arrays whose elements are very large.
97

String Arrays
An array of strings, each stored as a pointer to an array of chars.
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Multidimensional Arrays
C also permits multidimensional arrays specified using [].
Higher dimensions are possible.
Multidimensional arrays are rectangular.
Pointer arrays can be arbitrary shaped.
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Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
100

Linear Search
101

Binary Search
102

Binary Search cont’d
Requires random access to array memory
Slow on sequential devices like hard disks
May be wasteful
Linear search may be useful in some cases.
Implemented in C standard library
bsearch in stdlib.h
103

Simple Sort
Iterate through an array until an out of order element found.
Insert out of order element into correct location.
Repeat until end of array reached.
104

Finding Out of Order Elements
105

Shifting Out of Order Elements
106

Quick Sort
Choose a pivot element.
Move all elements less than pivot to one side.
Greater ones are on other side.
Sort sides individually.
Implemented in C standard library
qsort in stdlib.h
107

Quick Sort cont’d
Not stable
Equal-valued elements can get switched.
Can sort in-place
Desirable for low-memory environments
Choice of pivot influences performance.
Easily parallelizable
Recursive
Can cause stack overflow on large array
108

Exercise 4
Required:
In assn-04.pdf, solve all problems except 4.3.
109

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
110

Structure
A collection of related variables grouped under a single name.
111

Structure cont’d
struct defines a new data type.
The name of the structure is optional.
The variables declared within a structure are called its members.
Variables can be declared like any other built in data-type.
Initialization is done by specifying values of every member.
Assignment operator copies every member of the structure.
Be careful with pointers.
112

Accessing Structure Members
The . operator is used to access structure members.
113

Structure Pointers
Structures are copied element wise.
For large structures it is more efficient to pass pointers.
Members can be accesses from structure pointers using the -> operator.
114

Array of Structures
115

Size of Structures
The size of a structure is greater than or equal to the sum of the sizes of its members.
Alignment and padding are issues.
Libraries
Precompiled files
SIMD instructions
Members can be explicitly aligned or padded using compiler extensions.
116

Union
May hold objects of different types/sizes in the same memory location.
Union size is equal to the size of its largest element.
The compiler does not test if the data is being read in the correct format.
117

Bit Fields
A bit-field is a set of adjacent bits within a single word.
It must be unsigned int.
The number after: specifies the width
118

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
119

Dynamic Memory Allocation
malloc() allocates an uninitialized block of memory.
calloc() allocates a zero initialized array of n elements.
free() frees allocated memory.
Common errors:
Accessing freed memory
Accessing uninitialized pointer
Memory leak
120

Linked List
A dynamic data structure that consists of a sequence of records where each element contains a link to the next and may be previous record in the sequence.
They can be:
Single
Double
Circular
121

Single Linked List
Every node has a payload and a link to the next node in the list.
The start (head) of the list is maintained in a separate variable.
End of the list is indicated by NULL (sentinel).
122

Single Linked List Utilities
123

Binary Tree
A dynamic data structure where each node has at most two children.
A binary search tree is a binary tree with ordering among its children.
Usually, all elements in the left subtree are less than the root element while elements in the right subtree are greater than the root element.
124

Binary Tree Utilities
Can be framed as recursive
Traversal (printing, searching):
Pre-order: root, left subtree, right subtree
In order: left subtree, root, right subtree
Post order: left subtree , right subtree, root
125

Exercise 5
Required:
In assn-05.pdf, solve all problems.
126

Stack
A special type of list where we read and write from same end of list.
LIFO list
Push operation writes to list end.
POP operation reads from list end.
Can be build as an array or as a linked list.
127

Stack as an Array
128

Stack as a Linked List
129

Queue
A special type of list where we read and write from different ends of list.
FIFO list
Enqueue operation writes to list end.
Dequeue operation reads from other list end.
Can be build as an array or as a linked list.
130

Queue as an Array
131

Queue as a Linked List
132

B Tree
Generalized binary search tree, used for databases and file systems
With variable number of children
Tree is balanced.
All leaves at same level
A node contains a list of keys to divide range of elements in children.
Initially, it contains a root node with no children.
133

Inserting Elements
Complicated due to maximum number of keys
At high level:
Traverse tree to leaf node
If leaf full, split into two
Move median key element into parent and split parent if already full
Split remaining keys into 2 leaves
Add elements to sorted list of keys
Can be done in one pass by splitting full parents during traversal in step 1
134

Insertion Example
135

Insertion Example cont’d
136

Searching a B Tree
Like searching a B Tree
Start at root
If node empty, element not in tree
Search list of keys for element (using linear or binary search)
If element in list, return element
Otherwise, element between keys, and repeat search on child node for that range
137

Deleting Elements
Complicated due to minimum number of children restriction
You need to ensure child nodes to be traversed have enough keys
If adjacent child node has at least t keys, move separating key from parent to child and closest key in adjacent child to parent
If no adjacent child nodes have extra keys, merge child node with adjacent child
When removing a key from a node with children, need to rearrange keys again
If child before or after removed key has enough keys, move closest key from child to parent
If neither child has enough keys, merge both children
If child not a leaf, have to repeat this process
138

Deletion Example
139

Deletion Example cont’d
140

Priority Queue
A queue where ordering data by priority, used for sorting, event simulation, and many other algorithms
Elements enqueued with priority, dequeued in order of highest priority
Common implementations: heap or binary search tree
141

Heaps
A tree with high-ordering property
Child priority ≤ Parent priority
Usually implemented as an array with top element at beginning
142

Extracting Data
Maximum priority element at top of heap
Can peek by looking at top element
We can remove top element, move last element to top, and swap top element down with its children until it satisfies heap-ordering property:
Start at top
Find largest of element and children; if element is largest, we are done
Otherwise, swap element with largest child and repeat with element in new position
143

Inserting Data/Increasing Priority
Element is inserted at the end.
Increase priority of element to real priority.
Start at element
If new priority less than parent’s, we are done.
Otherwise, swap element with parent and repeat.
144

Inserting Example
145

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
146

Void Pointers
C does not allow to declare and usage of void pointers.
Can be used as a return type or as a function parameter
C allows void pointers.
The can point to any data point.
They can’t be dereferenced.
They must be cast before used.
147

Function Pointers
In C, function itself is not a variable.
It is possible to declare pointer to functions.
Declaration examples:
Function pointers can be assigned, pass to and from functions, placed in arrays etc.
148

Callbacks
A piece of executable code passed to functions.
Callbacks are implemented by passing function pointers.
Example:
qsort() can sort an array of any data type.
qsort() calls a function whenever a comparison needs to be done.
Callback
The callback returns <0, 0, or >0 depending on iinputs.
149

qsort Callback
150

Linked List Callback
We can make a callback that iterates a function through the list.
151

Printing and Counting Nodes
152

Array of Function Pointers
153

Array of Function Pointers cont'd
154

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
155

Hash Table
Combines linked lists and arrays to provide an efficient structure for storing dynamic data.
They are commonly implemented as an array of linked lists.
156

Hash Table cont'd
Each data item is associated with a key that determines its location.
Hash functions are used to generate an evenly distributed hash value.
A hash collision is said to occur when two items have the same hash value.
Items with the same hash keys are chained
157

Hash Functions
A hash function maps its input into a finite range: hash value, hash code.
The hash value should ideally have uniform distribution.
Other uses of hash functions: cryptography, caches (computers/internet)…
Hash function types:
Division type
Multiplication type
…
158

Hash Table Example
159

Hash Table Example
160

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
161

Symbols and Libraries
External libraries provide a wealth of functionality .
C standard library
Programs access libraries’ functions and variables via identifiers known as symbols.
Header file declarations/prototypes mapped to symbols at compile time.
Symbols linked to definitions in external libraries during linking.
Our own program produces symbols too.
162

Functions and Variables as Symbols
What variables and functions are declared globally?
163

C Compilation Process
164

Static and Dynamic Linking
Functions, global variables must be allocated memory before used.
Can allocate at compile time (static) or at run time (shared)
Static linking links symbols in .o files and .lib files.
Dynamic linking uses symbols in .dll files.
165

Static vs. Dynamic Linking
Static
During compile time
Linked symbols are added to executable.
Larger executable
No libraries’ dependencies on run time
Library upgrade needs recompilation.
Dynamic
During run time
Linked symbols are loaded from shared library.
Smaller executable
Depends on shared libraries location on run time.
No recompilation needed
166

Symbol Resolution Issues
Symbols can be defined in multiple places.
Suppose we define our own puts() function .
But, puts() is defined in C standard library.
Which puts compiler uses?
Dynamic linking uses our puts as C standard library is known at run time only.
Statically linking against C standard library causes an error.
167

Outline
Introduction to C
Control Flow
Modular Programming
Variable Scope
I/O
Data Structures
Callbacks
Hash Tables
Creating Libraries
Standard Library
168

Standard Library
<stdio.h>
<ctype.h>
<stdlib.h>
<string.h>
<assert.h>
<stdarg.h>
<time.h>
…
169

<stdio.h>
fopen
freopen
fflush
rename
remove
tmpfile
tmpname
fread
fwrite
fseek
ftell
rewind
clearerr
feof
ferror
170

<ctype.h>
isalnum
isctrl
isdigit
islower
isprint
ispunct
isspace
isupper
171

<string.h>
memcpy
memmove
memcmp
memset
172

<stdlib.h>
atof
atoi
atol
rand
srand
abort
exit
atexit
system
bsearch
qsort
173

<assert.h>
assert
174

<time.h>
clock
time
difftime
mktime
asctime
localtime
ctime
strftime
175

Project II: BMP to JPEG Tool
176
Required:
Write a utility that converts a bmp image to a jpeg image.
Deadline:
A week from now
References:
http://en.wikipedia.org/wiki/BMP_file_format
http://en.wikipedia.org/wiki/JPEG
http://www.digicamsoft.com/itu/itu-t81-36.html
http://www.cs.cf.ac.uk/Dave/Multimedia/node234.html
http://www.w3.org/Graphics/JPEG/jfif3.pdf
Bmp to jpeg
utility

BMP File Format
File header
Image header
Color table
GAP1
Pixel array
GAP2
Color profile
177

BMP File Header
Signature:
2 bytes
“BM”
File size:
4 bytes
File size in bytes
Reserved 1 and 2:
Each 2 bytes
Used by the image creator application
File offset to pixel array:
4 bytes
Offset from file start to pixel array
178

BMP Image Header
Each field is either 4 bytes or 2 bytes.
Color space endpoints is 36 bytes.
Planes must be 1.
Bits per pixel can be 1, 4, 8, 16, 24, and 32.
We will only focus on uncompressed bmp.
Important color count is usually ignored.
Masks can be used to extract data from pixels.
179

BMP Color Table
Mandatory when bits per pixel are less than 8.
It defines colors used in the bitmap.
Can be used to index bitmap images.
A table entry is either 4 bytes or 3 bytes depending on the header version.
180

BMP Pixels
181

JPEG Compression
Color space transformation
Downsampling
Discrete cosine transform
Quantization
Zigzag scanning
DPCM on DC components
RLE on AC components
Entropy Coding
182

Color Space Transformation and Downsampling
Transforming RGB to YCbCr
YCbCr allows reduction in Cb and Cr components.
Downsampling
Known Downsampling are:
4:4:4
4:2:2
4:2:0
183

Discrete Cosine Transform
For 8×8 block, its values are shifted from a positive range to one centered around zero before computing the DCT.
DCT is performed after shifting as follows:
184

Quantization
Dividing DCT output with quantization matrix.
The output will is rounding of quantization.
185

Zigzag Scanning
Maps 8x8 matrix into a vector
186

DPCM and RLE
DPCM:
Store the difference between the current DC value and the previous one
RLE:
Encode the AC values as pairs of (skip, value).
Skip is the number of preceding zeroes.
Value is the next non-zero value.
(0,0) is used as end of block.
187

Entropy Coding
Categorize DC values into SSS
Send off SSS as Huffman symbol, followed by actual 3 bits.
For AC components (skip, value), encode the composite symbol (skip, SSS) using the Huffman coding.
Huffman Tables can be custom (sent in header) or default.
188

JPEG File Format
Let’s check http://www.w3.org/Graphics/JPEG/jfif3.pdf
189

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Introduction to C Programming

by amraldo on May 09, 2011

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Introduction to C Programming — Presentation Transcript