C Programming Tutorial

C Programming Tutorial

C PROGRAMMING TUTORIAL

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T able of Contents
C Language Overview .............................................................. 1
Facts about C ............................................................................................... 1 Why to use C ? ............................................................................................. 2 C Programs .................................................................................................. 2

C Environment Setup ............................................................... 3
Text Editor ................................................................................................... 3 The C Compiler ............................................................................................ 3 Installation on Unix/Linux ............................................................................. 4 Installation on Mac OS .................................................................................. 4 Installation on Windows ............................................................................... 4

C Program Structure ................................................................ 5
C Hello World Example ................................................................................. 5 Compile & Execute C Program ....................................................................... 6

C Basic Syntax ......................................................................... 7
Tokens in C .................................................................................................. 7 Semicolons ; ................................................................................................ 7 Comments ................................................................................................... 8 Identifiers .................................................................................................... 8 Keywords .................................................................................................... 8 Whitespace in C ........................................................................................... 9

C Data Types ......................................................................... 10
Integer Types ............................................................................................. 10 Floating-Point Types ................................................................................... 11 The void Type ............................................................................................ 12

C Variables ............................................................................ 13
Variable Declaration in C ............................................................................. 13 Variable Initialization in C ............................................................................ 14 Lvalues and Rvalues in C ............................................................................. 15

C Constants and Literals ........................................................ 16
Integer literals............................................................................................ 16 Floating-point literals.................................................................................. 17 Character constants.................................................................................... 17 iii String literals.............................................................................................. 18 Defining Constants ..................................................................................... 18
The #define Preprocessor ...................................................................... 18 The const Keyword ................................................................................. 19

C Storage Classes ................................................................. 21
The auto Storage Class ................................................................................ 21 The register Storage Class ........................................................................... 21 The static Storage Class............................................................................... 22 The extern Storage Class ............................................................................. 23

C Operators ........................................................................... 24
Arithmetic Operators .................................................................................. 24 Relational Operators................................................................................... 25 Logical Operators ....................................................................................... 27 Bitwise Operators....................................................................................... 28 Assignment Operators ................................................................................ 30 Misc Operators ↦ sizeof & ternary .............................................................. 32 Operators Precedence in C .......................................................................... 32

Decision Making in C.............................................................. 34 if statement ............................................................................................... 35
Syntax ..................................................................................................... 35 Flow Diagram .......................................................................................... 35 Example .................................................................................................. 35

if...else statement ...................................................................................... 36
Syntax ..................................................................................................... 36 Flow Diagram .......................................................................................... 37 Example .................................................................................................. 37

The if...else if...else Statement ..................................................................... 38
Syntax ..................................................................................................... 38 Example .................................................................................................. 38

Nested if statements .................................................................................. 39
Syntax ..................................................................................................... 39 Example .................................................................................................. 39

switch statement ....................................................................................... 40
Syntax ..................................................................................................... 40 Flow Diagram .......................................................................................... 41 Example .................................................................................................. 41

Nested switch statements ........................................................................... 42
Syntax ..................................................................................................... 42 Example .................................................................................................. 42 iii The ? : Operator ......................................................................................... 43

C Loops.................................................................................. 44 while loop in C ........................................................................................... 45
Syntax ..................................................................................................... 45 Flow Diagram .......................................................................................... 45 Example .................................................................................................. 46

for loop in C ............................................................................................... 46
Syntax ..................................................................................................... 46 Flow Diagram .......................................................................................... 47 Example .................................................................................................. 47

do...while loop in C ..................................................................................... 48
Syntax ..................................................................................................... 48 Flow Diagram .......................................................................................... 49 Example .................................................................................................. 49

nested loops in C ........................................................................................ 50
Syntax ..................................................................................................... 50 Example .................................................................................................. 51

break statement in C .................................................................................. 52
Syntax ..................................................................................................... 52 Flow Diagram .......................................................................................... 52 Example .................................................................................................. 53

continue statement in C .............................................................................. 53
Syntax ..................................................................................................... 53 Flow Diagram .......................................................................................... 54 Example .................................................................................................. 54

goto statement in C .................................................................................... 55
Syntax ..................................................................................................... 55 Flow Diagram .......................................................................................... 55 Example .................................................................................................. 56

The Infinite Loop ........................................................................................ 56

C Functions ............................................................................ 58
Defining a Function .................................................................................... 58
Example .................................................................................................. 59

Function Declarations ................................................................................. 59 Calling a Function ....................................................................................... 60 Function Arguments ................................................................................... 61
Function call by value ............................................................................. 61 Function call by reference ....................................................................... 62

C Scope Rules ....................................................................... 64 iii Local Variables ........................................................................................... 64 Global Variables ......................................................................................... 65 Formal Parameters ..................................................................................... 66 Initializing Local and Global Variables ........................................................... 66

C Arrays ................................................................................. 68
Declaring Arrays ......................................................................................... 68 Initializing Arrays ........................................................................................ 69 Accessing Array Elements ............................................................................ 69 Multi-dimensional Arrays ............................................................................ 70 Two-Dimensional Arrays ............................................................................. 70 Initializing Two-Dimensional Arrays.............................................................. 71 Accessing Two-Dimensional Array Elements ................................................. 71 Passing Arrays as Function Arguments.......................................................... 72
Way-1 ...................................................................................................... 72 Way-2 ...................................................................................................... 73

Way-3....................................................................................................... 73
Example .................................................................................................. 73

Return array from function.......................................................................... 74 Pointer to an Array ..................................................................................... 76

C Pointers .............................................................................. 78
What Are Pointers? .................................................................................... 79 How to use Pointers? .................................................................................. 79 NULL Pointers in C ...................................................................................... 80 Pointer arithmetic ...................................................................................... 80 Incrementing a Pointer ............................................................................... 81 Decrementing a Pointer .............................................................................. 82 Pointer Comparisons .................................................................................. 82 Array of pointers ........................................................................................ 83 Pointer to Pointer....................................................................................... 85 Passing pointers to functions ....................................................................... 86 Return pointer from functions ..................................................................... 87

C Strings ................................................................................ 90 C Structures ........................................................................... 93
Defining a Structure.................................................................................... 93 Accessing Structure Members ..................................................................... 94 Structures as Function Arguments ............................................................... 95 Pointers to Structures ................................................................................. 96

C Unions ................................................................................ 99
Defining a Union ........................................................................................ 99 iii Accessing Union Members ........................................................................ 100

Bit Fields .............................................................................. 102
Bit Field Declaration ................................................................................. 103

Typedef ................................................................................ 105 typedef vs #define .................................................................................... 106

Input & Output ...................................................................... 107
The Standard Files .................................................................................... 107 The getchar() & putchar() functions ........................................................... 107 The gets() & puts() functions ..................................................................... 108 The scanf() and printf() functions ............................................................... 109

File I/O ................................................................................. 110
Opening Files ........................................................................................... 110 Closing a File ............................................................................................ 111 Writing a File ........................................................................................... 111 Reading a File........................................................................................... 112 Binary I/O Functions ................................................................................. 113

Preprocessors ...................................................................... 114
Preprocessors Examples............................................................................ 114 Predefined Macros ................................................................................... 115 Preprocessor Operators ............................................................................ 116
Macro Continuation (\) .......................................................................... 116 Stringize (#) ........................................................................................... 116 Token Pasting (##)................................................................................ 117 The defined() Operator ......................................................................... 117

Parameterized Macros .............................................................................. 118

Header Files ......................................................................... 119
Include Syntax.......................................................................................... 119 Include Operation .................................................................................... 120 Once-Only Headers .................................................................................. 120 Computed Includes................................................................................... 121

Type Casting ........................................................................ 122
Integer Promotion .................................................................................... 123 Usual Arithmetic Conversion ..................................................................... 123

Error Handling ...................................................................... 125
The errno, perror() and strerror() ............................................................... 125 Divide by zero errors ................................................................................ 126 Program Exit Status .................................................................................. 127

Recursion ............................................................................. 128
Number Factorial ..................................................................................... 128 iii Fibonacci Series ....................................................................................... 129

Variable Arguments .............................................................. 130 Memory Management .......................................................... 132
Allocating Memory Dynamically ................................................................. 132 Resizing and Releasing Memory ................................................................. 133

Command Line Arguments ................................................... 135

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CHAPTER

1

C Language Overview
This chapter describes the basic detail about C programming language, how it emerged, what are strengths of C and why we should use C.

T
    

he C programming language is a general purpose high level language that was

originally developed by Dennis M. Ritchie to develop the Unix operating system at Bell Labs. C was originally first implemented on the DEC PDP-11 computer in 1972. In 1978, Brian Kernighan and Dennis Ritchie produced the first publicly available description of C, now known as the K&R standard. The UNIX operating system, the C compiler, and essentially all UNIX applications programs have been written in C. The C has now become a widely used professional language for various reasons.

Easy to learn Structured language It produces efficient programs. It can handle low-level activities. It can be compiled on a variety of computer platforms.

Facts about C
  
(ANSI). C was invented to write an operating system called UNIX. C is a successor of B language which was introduced around 1970 The language was formalized in 1988 by the American National Standard Institute.



The UNIX OS was totally written in C By 1973.

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  

Today C is the most widely used and popular System Programming Language. Most of the state of the art software’s have been implemented using C. Today's most popular Linux OS and RBDMS MySQL have been written in C.

Why to use C ?
C was initially used for system development work, in particular the programs that make-up the operating system. C was adopted as a system development language because it produces code that runs nearly as fast as code written in assembly language. Some examples of the use of C might be:

         

Operating Systems Language Compilers Assemblers Text Editors Print Spoolers Network Drivers Modern Programs Data Bases Language Interpreters Utilities

C Programs
A C program can vary from 3 lines to millions of lines and it should be written into one or more text files with extension ".c" for example hello.c. You can use "vi", "vim" or any other text editor to write your C program into a file. This tutorial assumes that you know how to edit a text file and how to write source code using any programming language.

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CHAPTER

2

C Environment Setup
This section describes how to setup your system environment before you start doing your programming using C language.
Before you start doing programming using C programming language, you need following two software's available on your computer, (a) Text Editor and (b) The C Compiler.

Text Editor
This will be used to type your program. Examples of few editors include Windows Notepad, OS Edit command, Brief, Epsilon, EMACS, and vim or vi Name and version of text editor can vary on different operating systems. For example Notepad will be used on Windows and vim or vi can be used on windows as well as Linux, or Unix. The files you create with your editor are called source files and contain program source code. The source files for C programs are typically named with the extension .c. Before starting your programming, make sure you have one text editor in place and you have enough experience to write a computer program, save it in a file, compile it and finally execute it.

The C Compiler
The source code written in source file is the human readable source for your program. It needs to be "compiled", to turn into machine language so that your CPU can actually execute the program as per instructions given. This C programming language compiler will be used to compile your source code into final executable program. I assume you have basic knowledge about a programming language compiler. Most frequently used and free available compiler is GNU C/C++ compiler, otherwise you can have compilers either from HP or Solaris if you have respective Operating Systems. Following section guides you on how to install GNU C/C++ compiler on various OS. I'm mentioning C/C++ together because GNU gcc compiler works for both C and C++ programming languages.

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Installation on Unix/Linux
If you are using Linux or Unix then check whether GCC is installed on your system by entering the following command from the command line:

$ gcc -v
If you have GNU compiler installed on your machine then it should print a message something as follows:

Using built-in specs. Target: i386-redhat-linux Configured with: ../configure --prefix=/usr ....... Thread model: posix gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)
If GCC is not installed, then you will have to install it yourself using the detailed instructions available athttp://gcc.gnu.org/install/ This tutorial has been written based on Linux and all the given examples have been compiled on Cent OS flavor of Linux system.

Installation on Mac OS
If you use Mac OS X, the easiest way to obtain GCC is to download the Xcode development environment from Apple's web site and follow the simple installation instructions. Once you have Xcode setup, you will be able to use GNU compiler for C/C++. Xcode is currently available at developer.apple.com/technologies/tools/.

Installation on Windows
To install GCC at Windows you need to install MinGW. To install MinGW, go to the MinGW homepage,www.mingw.org, and follow the link to the MinGW download page. Download the latest version of the MinGW installation program, which should be named MinGW.exe. While installing MinWG, at a minimum, you must install gcc-core, gcc-g++, binutils, and the MinGW runtime, but you may wish to install more. Add the bin subdirectory of your MinGW installation to your PATH environment variable so that you can specify these tools on the command line by their simple names. When the installation is complete, you will be able to run gcc, g++, ar, ranlib, dlltool, and several other GNU tools from the Windows command line.

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CHAPTER

3

C Program Structure
Let’s look into Hello World example using C Programming Language.

B
    

efore we study basic building blocks of the C programming language, let us look a

bare minimum C program structure so that we can take it as a reference in upcoming chapters.

C Hello World Example
A C program basically consists of the following parts:

Preprocessor Commands Functions Variables Statements & Expressions Comments

Let us look at a simple code that would print the words "Hello World":

#include int main() { /* my first program in C */ printf("Hello, World! \n"); return 0; }
Let us look various parts of the above program:

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1. 2. 3. 4. 5.

The first line of the program #include is a preprocessor command which tells a C compiler to include stdio.h file before going to actual compilation. The next line int main() is the main function where program execution begins. The next line /*...*/ will be ignored by the compiler and it has been put to add additional comments in the program. So such lines are called comments in the program. The next line printf(...) is another function available in C which causes the message "Hello, World!" to be displayed on the screen. The next line return 0; terminates main()function and returns the value 0.

Compile & Execute C Program
Let’s look at how to save the source code in a file, and how to compile and run it. Following are the simple steps:

1. 2. 3. 4. 5. 6. 7.

Open a text editor and add the above mentioned code. Save the file as hello.c Open a command prompt and go to the directory where you saved the file. Type gcc hello.c and press enter to compile your code. If there are no errors in your code the command prompt will take you to the next line and would generate a.out executable file. Now type a.out to execute your program. You will be able to see "Hello World" printed on the screen

$ gcc hello.c $ ./a.out Hello, World!
Make sure that gcc compiler is in your path and that you are running it in the directory containing source file hello.c.

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CHAPTER

4

C Basic Syntax
This chapter will give detail about all the basic syntax about C programming language including tokens, keywords, identifiers etc.

Y

ou have seen a basic structure of C program, so it will be easy to understand other

basic building blocks of the C programming language.

Tokens in C
A C program consists of various tokens and a token is either a keyword, an identifier, a constant, a string literal, or a symbol. For example, the following C statement consists of five tokens:

printf("Hello, World! \n");
The individual tokens are:

printf ( "Hello, World! \n" ) ;

Semicolons ;
In C program, the semicolon is a statement terminator. That is, each individual statement must be ended with a semicolon. It indicates the end of one logical entity. For example, following are two different statements:

printf("Hello, World! \n"); return 0;

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Comments
Comments are like helping text in your C program and they are ignored by the compiler. They start with /* and terminates with the characters */ as shown below:

/* my first program in C */
You can not have comments with in comments and they do not occur within a string or character literals.

Identifiers
A C identifier is a name used to identify a variable, function, or any other user-defined item. An identifier starts with a letter A to Z or a to z or an underscore _ followed by zero or more letters, underscores, and digits (0 to 9). C does not allow punctuation characters such as @, $, and % within identifiers. C is a case sensitive programming language. Thus Manpower and manpower are two different identifiers in C. Here are some examples of acceptable identifiers:

mohd myname50

zara _temp

abc j

move_name a23b9

a_123 retVal

Keywords
The following list shows the reserved words in C. These reserved words may not be used as constant or variable or any other identifier names.

auto break case char const continue default do double

else enum extern float for goto if int

long register return short signed sizeof static struct

switch typedef union unsigned void volatile while _packed

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Whitespace in C
A line containing only whitespace, possibly with a comment, is known as a blank line, and a C compiler totally ignores it. Whitespace is the term used in C to describe blanks, tabs, newline characters and comments. Whitespace separates one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins. Therefore, in the following statement:

int age;
There must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them. On the other hand, in the following statement

fruit = apples + oranges;

// get the total fruit

No whitespace characters are necessary between fruit and =, or between = and apples, although you are free to include some if you wish for readability purpose.

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CHAPTER

5

C Data Types

I
1 2

n the C programming language, data types refers to an extensive system used for

declaring variables or functions of different types. The type of a variable determines how much space it occupies in storage and how the bit pattern stored is interpreted. The types in C can be classified as follows:

S.N. Types and Description Basic Types: They are arithmetic types and consists of the two types: (a) integer types and (b) floatingpoint types. Enumerated types: They are again arithmetic types and they are used to define variables that can only be assigned certain discrete integer values throughout the program. The type void: The type specifier void indicates that no value is available. Derived types: They include (a) Pointer types, (b) Array types, (c) Structure types, (d) Union types and (e) Function types.

3

4

The array types and structure types are referred to collectively as the aggregate types. The type of a function specifies the type of the function's return value. We will see basic types in the following section where as other types will be covered in the upcoming chapters.

Integer Types
Following table gives you detail about standard integer types with its storage sizes and value ranges:

Type char unsigned char

Storage size 1 byte 1 byte

Value range -128 to 127 or 0 to 255 0 to 255

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signed char int unsigned int short unsigned short long unsigned long

1 byte 2 or 4 bytes 2 or 4 bytes 2 bytes 2 bytes 4 bytes 4 bytes

-128 to 127 -32,768 to 32,767 or -2,147,483,648 to 2,147,483,647 0 to 65,535 or 0 to 4,294,967,295 -32,768 to 32,767 0 to 65,535 -2,147,483,648 to 2,147,483,647 0 to 4,294,967,295

To get the exact size of a type or a variable on a particular platform, you can use the sizeof operator. The expressions sizeof(type) yields the storage size of the object or type in bytes. Following is an example to get the size of int type on any machine:

#include #include int main() { printf("Storage size for int : %d \n", sizeof(int)); return 0; }
When you compile and execute the above program it produces following result on Linux:

Storage size for int : 4

Floating-Point Types
Following table gives you detail about standard float-point types with storage sizes and value ranges and their precision:

Type float double long double

Storage size 4 byte 8 byte 10 byte

Value range 1.2E-38 to 3.4E+38 2.3E-308 to 1.7E+308 3.4E-4932 to 1.1E+4932

Precision 6 decimal places 15 decimal places 19 decimal places

The header file float.h defines macros that allow you to use these values and other details about the binary representation of real numbers in your programs. Following example will print storage space taken by a float type and its range values:

#include #include int main() {

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printf("Storage size for float : %d \n", sizeof(float)); printf("Minimum float positive value: %E\n", FLT_MIN ); printf("Maximum float positive value: %E\n", FLT_MAX ); printf("Precision value: %d\n", FLT_DIG ); return 0; }
When you compile and execute the above program it produces following result on Linux:

Storage size for float : 4 Minimum float positive value: 1.175494E-38 Maximum float positive value: 3.402823E+38 Precision value: 6

The void Type
The void type specifies that no value is available. It is used in three kinds of situations:

S.N. Types and Description Function returns as void There are various functions in C who do not return value or you can say they return void. A function with no return value has the return type as void. For example void exit (int status); Function arguments as void There are various functions in C who do not accept any parameter. A function with no parameter can accept as a void. For example int rand(void); Pointers to void A pointer of type void * represents the address of an object, but not its type. For example a memory allocation function void *malloc( size_t size ); returns a pointer to void which can be casted to any data type.

1

2

3

The void type may not be understood to you at this point, so let us proceed and we will cover these concepts in upcoming chapters.

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CHAPTER

6

C Variables

A
Type char int float double void

variable is nothing but a name given to a storage area that our programs can

manipulate. Each variable in C has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable. The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because C is case-sensitive. Based on the basic types explained in previous chapter, there will be following basic variable types:

Description Typically a single octet(one byte). This is an integer type. The most natural size of integer for the machine. A single-precision floating point value. A double-precision floating point value. Represents the absence of type.

C programming language also allows to define various other type of variables which we will cover in subsequent chapters like Enumeration, Pointer, Array, Structure, Union etc. For this chapter, let us study only basic variable types.

Variable Declaration in C
All variables must be declared before we use them in C program, although certain declarations can be made implicitly by content. A declaration specifies a type, and contains a list of one or more variables of that type as follows:

type variable_list;
Here, type must be a valid C data type including char, int, float, double, or any user defined data type etc., and variable_list may consist of one or more identifier names separated by commas. Some valid variable declarations are shown here:

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int char float double

i, j, k; c, ch; f, salary; d;

A variable declaration does not allocate any memory space for the variable but a variable definition allocate required memory space for that variable. A variable declaration with an initial value as shown below will become variable definition and required memory is allocated for the variable.

int

i = 100;

An extern declaration is not a definition and does not allocate storage. In effect, it claims that a definition of the variable exists some where else in the program. A variable can be declared multiple times in a program, but it must be defined only once. Following is the declaration of a variable with extern keyword:

extern int

i;

Variable Initialization in C
Variables are initialized (assigned an value) with an equal sign followed by a constant expression. The general form of initialization is:

variable_name = value;
Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows:

type variable_name = value;
Some examples are:

int d = 3, f = 5; byte z = 22; double pi = 3.14159; char x = 'x';

/* /* /* /*

initializing d and f. */ initializes z. */ declares an approximation of pi. */ the variable x has the value 'x'. */

It is a good programming practice to initialize variables properly otherwise, sometime program would produce unexpected result. Try following example which makes use of various types of variables:

#include int main () { /* variable declaration: */ int a, b; int c; float f;

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/* actual initialization */ a = 10; b = 20; c = a + b; printf("value of c : %d \n", c); f = 70.0/3.0; printf("value of f : %f \n", f); return 0; }
When the above code is compiled and executed, it produces following result:

value of c : 30 value of f : 23.333334

Lvalues and Rvalues in C
There are two kinds of expressions in C:

1.

lvalue: An expression that is an lvalue may appear as either the left-hand or right-hand side of an assignment. rvalue: An expression that is an rvalue may appear on the right- but not left-hand side of an assignment.

2.

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and cannot appear on the left-hand side. Following is a valid statement:

int g = 20;
But following is not a valid statement and would generate compile-time error:

10 = 20;

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CHAPTER

7

C Constants and Literals

T

he constants refer to fixed values that the program may not alter during its

execution. These fixed values are also called literals.

Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are also enumeration constants as well. The constants are treated just like regular variables except that their values cannot be modified after their definition.

Integer literals
An integer literal can be a decimal, octal, or hexadecimal constant. A prefix specifies the base or radix: 0x or 0X for hexadecimal, 0 for octal, and nothing for decimal. An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively. The suffix can be uppercase or lowercase and can be in any order. Here are some examples of integer literals:

212 215u 0xFeeL 078 032UU

/* /* /* /* /*

Legal */ Legal */ Legal */ Illegal: 8 is not an octal digit */ Illegal: cannot repeat a suffix */

Following are other examples of various types of Integer literals:

85 0213 0x4b 30 30u 30l 30ul

/* /* /* /* /* /* /*

decimal */ octal */ hexadecimal */ int */ unsigned int */ long */ unsigned long */

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Floating-point literals
A floating-point literal has an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating point literals either in decimal form or exponential form. While representing using decimal form, you must include the decimal point, the exponent, or both and while representing using exponential form, you must include the integer part, the fractional part, or both. The signed exponent is introduced by e or E. Here are some examples of floating-point literals:

3.14159 314159E-5L 510E 210f .e55

/* /* /* /* /*

Legal */ Legal */ Illegal: incomplete exponent */ Illegal: no decimal or exponent */ Illegal: missing integer or fraction */

Character constants
Character literals are enclosed in single quotes e.g., 'x' and can be stored in a simple variable of char type. A character literal can be a plain character (e.g., 'x'), an escape sequence (e.g., '\t'), or a universal character (e.g., '\u02C0'). There are certain characters in C when they are proceeded by a back slash they will have special meaning and they are used to represent like newline (\n) or tab (\t). Here you have a list of some of such escape sequence codes:

Escape sequence \\ \' \" \? \a \b \f \n \r \t \v \ooo

Meaning \ character ' character " character ? character Alert or bell Backspace Form feed Newline Carriage return Horizontal tab Vertical tab Octal number of one to three digits

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\xhh . . .

Hexadecimal number of one or more digits

Following is the example to show few escape sequence characters:

#include

int main()
{ printf("Hello\tWorld\n\n");

return 0;
}
When the above code is compiled and executed, it produces following result:

Hello

World

String literals
String literals or constants are enclosed in double quotes "". A string contains characters that are similar to character literals: plain characters, escape sequences, and universal characters. You can break a long lines into multiple lines using string literals and separating them using whitespaces. Here are some examples of string literals. All the three forms are identical strings.

"hello, dear" "hello, \ dear" "hello, " "d" "ear"

Defining Constants
There are two simple ways in C to define constants:

1. 2.

Using #define preprocessor. Using const keyword.

The #define Preprocessor
Following is the form to use #define preprocessor to define a constant:

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#define identifier value
Following example explains it in detail:

#include #define LENGTH 10 #define WIDTH 5 #define NEWLINE '\n'

int main()
{

int area; area = LENGTH * WIDTH; printf("value of area : %d", area); printf("%c", NEWLINE);

return 0;
}
When the above code is compiled and executed, it produces following result:

value of area : 50

The const Keyword
You can use const prefix to declare constants with a specific type as follows:

const type variable = value;
Following example explains it in detail:

#include

int main()
{

const int LENGTH = 10; const int WIDTH = 5; const char NEWLINE = '\n'; int area; area = LENGTH * WIDTH; printf("value of area : %d", area); printf("%c", NEWLINE);

return 0;
} When the above code is compiled and executed, it produces following result:

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value of area : 50 Note that it is a good programming practice to define constants in CAPITALS.

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CHAPTER

8

C Storage Classes

A
   

storage class defines the scope (visibility) and life time of variables and/or functions

within a C Program. These specifiers precede the type that they modify. There are following storage classes which can be used in a C Program auto register static extern

The auto Storage Class
The auto storage class is the default storage class for all local variables.

{ int mount; auto int month; }
The example above defines two variables with the same storage class, auto can only be used within functions, i.e. local variables.

The register Storage Class
The register storage class is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and can't have the unary '&' operator applied to it (as it does not have a memory location).

{ register int } miles;

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The register should only be used for variables that require quick access such as counters. It should also be noted that defining 'register' goes not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register depending on hardware and implementation restrictions.

The static Storage Class
The static storage class instructs the compiler to keep a local variable in existence during the lifetime of the program instead of creating and destroying it each time it comes into and goes out of scope. Therefore, making local variables static allows them to maintain their values between function calls. The static modifier may also be applied to global variables. When this is done, it causes that variable's scope to be restricted to the file in which it is declared. In C programming, when static is used on a class data member, it causes only one copy of that member to be shared by all objects of its class.

#include /* function declaration */ void func(void); static int count = 5; /* global variable */ main() { while(count--) { func(); } return 0; } /* function definition */ void func( void ) { static int i = 5; /* local static variable */ i++; printf("i is %d and count is %d\n", i, count); }
You may not understand this example at this time because I have used function and global variables which I have not explained so far. So for now let us proceed even if you do not understand it completely. When the above code is compiled and executed, it produces following result:

i is 6 and count is 4 i is 7 and count is 3 i is 8 and count is 2 i is 9 and count is 1 i is 10 and count is 0

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The extern Storage Class
The extern storage class is used to give a reference of a global variable that is visible to ALL the program files. When you use 'extern' the variable cannot be initialized as all it does is point the variable name at a storage location that has been previously defined. When you have multiple files and you define a global variable or function which will be used in other files also, then extern will be used in another file to give reference of defined variable or function. Just for understanding extern is used to declare a global variable or function in another files. The extern modifier is most commonly used when there are two or more files sharing the same global variables or functions as explained below.

First File: main.c #include int count ; extern void write_extern(); main() { write_extern(); } Second File: write.c #include extern int count; void write_extern(void) { count = 5; printf("count is %d\n", count); }
Here extern keyword is being used to declare count in the second file where as it has its definition in the first file. Now compile these two files as follows:

$gcc main.c write.c
This will produce a.out executable program, when this program is executed, it produces following result:

5

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CHAPTER

9

C Operators

A
     

n operator is a symbol that tells the compiler to perform specific mathematical or logical

manipulations. C language is rich in built-in operators and provides following type of operators: Arithmetic Operators Relational Operators Logical Operators Bitwise Operators Assignment Operators Misc Operators

This tutorial will explain the arithmetic, relational, and logical, bitwise, assignment and other operators one by one.

Arithmetic Operators
Following table shows all the arithmetic operators supported by C language. Assume variable A holds 10 and variable B holds 20 then:

Operator Description + * / % ++ Adds two operands Subtracts second operand from the first Multiply both operands Divide numerator by de-numerator

Example A + B will give 30 A - B will give -10 A * B will give 200 B / A will give 2

Modulus Operator and remainder of after an integer division B % A will give 0 Increment operator increases integer value by one A++ will give 11

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--

Decrement operator decreases integer value by one

A-- will give 9

Try following example to understand all the arithmetic operators available in C programming language:

#include main() { int a = 21; int b = 10; int c ; c = a + b; printf("Line c = a - b; printf("Line c = a * b; printf("Line c = a / b; printf("Line c = a % b; printf("Line c = a++; printf("Line c = a--; printf("Line }
When you compile and execute the above program it produces following result:

1 - Value of c is %d\n", c ); 2 - Value of c is %d\n", c ); 3 - Value of c is %d\n", c ); 4 - Value of c is %d\n", c ); 5 - Value of c is %d\n", c ); 6 - Value of c is %d\n", c ); 7 - Value of c is %d\n", c );

Line 1 - Value of c is 31 Line 2 - Value of c is 11 Line 3 - Value of c is 210 Line 4 - Value of c is 2 Line 5 - Value of c is 1 Line 6 - Value of c is 21 Line 7 - Value of c is 22

Relational Operators
Following table shows all the relational operators supported by C language. Assume variable A holds 10 and variable B holds 20 then:

Operator Description == Checks if the value of two operands is equal or not, if yes then condition becomes true.

Example (A == B) is not true.

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!=

Checks if the value of two operands is equal or not, if values are not equal then condition becomes true. Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true.

(A != B) is true.

>

(A > B) is not true.

<

(A < B) is true.

>=

(A >= B) is not true.

> 2 will give 15 which is 0000 1111

Try following example to understand all the bitwise operators available in C programming language:

#include main() { unsigned int a = 60; unsigned int b = 13; int c = 0; /* 60 = 0011 1100 */ /* 13 = 0000 1101 */

c = a & b; /* 12 = 0000 1100 */ printf("Line 1 - Value of c is %d\n", c ); c = a | b; /* 61 = 0011 1101 */ printf("Line 2 - Value of c is %d\n", c ); c = a ^ b; /* 49 = 0011 0001 */ printf("Line 3 - Value of c is %d\n", c ); c = ~a; /*-61 = 1100 0011 */ printf("Line 4 - Value of c is %d\n", c ); c = a > 2; /* 15 = 0000 1111 */ printf("Line 6 - Value of c is %d\n", c ); }

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When you compile and execute the above program it produces following result:

Line 1 - Value of c is 12 Line 2 - Value of c is 61 Line 3 - Value of c is 49 Line 4 - Value of c is -61 Line 5 - Value of c is 240 Line 6 - Value of c is 15

Assignment Operators
There are following assignment operators supported by C language:

Operator Description = Simple assignment operator, Assigns values from right side operands to left side operand

Example C = A + B will assign value of A + B into C

+=

Add AND assignment operator, It adds right operand to the left operand and assign the result C += A is equivalent to C = C + A to left operand Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand Left shift AND assignment operator Right shift AND assignment operator Bitwise AND assignment operator bitwise exclusive OR and assignment operator bitwise inclusive OR and assignment operator C -= A is equivalent to C = C - A

-=

*=

C *= A is equivalent to C = C * A

/=

C /= A is equivalent to C = C / A

%= = &= ^= |=

C %= A is equivalent to C = C % A C > 2 C &= 2 is same as C = C & 2 C ^= 2 is same as C = C ^ 2 C |= 2 is same as C = C | 2

Try following example to understand all the assignment operators available in C programming language:

#include main()

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{ int a = 21; int c ; c = a; printf("Line 1 - = Operator Example, Value of c = %d\n", c );

c += a; printf("Line 2 - += Operator Example, Value of c = %d\n", c ); c -= a; printf("Line 3 - -= Operator Example, Value of c = %d\n", c ); c *= a; printf("Line 4 - *= Operator Example, Value of c = %d\n", c ); c /= a; printf("Line 5 - /= Operator Example, Value of c = %d\n", c ); c = 200; c %= a; printf("Line 6 - %= Operator Example, Value of c = %d\n", c ); c = 2; printf("Line 8 - >>= Operator Example, Value of c = %d\n", c ); c &= 2; printf("Line 9 - &= Operator Example, Value of c = %d\n", c ); c ^= 2; printf("Line 10 - ^= Operator Example, Value of c = %d\n", c ); c |= 2; printf("Line 11 - |= Operator Example, Value of c = %d\n", c ); }
When you compile and execute the above program it produces following result:

Line 1 - =

Operator Example, Value of c = 21

Line 2 - += Operator Example, Value of c = 42 Line 3 - -= Operator Example, Value of c = 21 Line 4 - *= Operator Example, Value of c = 441 Line 5 - /= Operator Example, Value of c = 21 Line 6 - %= Operator Example, Value of c = 11 Line 7 - = Operator Example, Value of c = 11 Line 9 - &= Operator Example, Value of c = 2 Line 10 - ^= Operator Example, Value of c = 0

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Line 11 - |= Operator Example, Value of c = 2

Misc Operators ↦sizeof & ternary
There are few other important operators including sizeof and ? : supported by C Language.

Operator Description sizeof() & * ?: Returns the size of an variable. Returns the address of an variable. Pointer to a variable. Conditional Expression

Example sizeof(a), where a is integer, will return 4. &a; will give actual address of the variable. *a; will pointer to a variable. If Condition is true ? Then value X : Otherwise value Y

Operators Precedence in C
Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator: For example x = 7 + 3 * 2; Here x is assigned 13, not 20 because operator * has higher precedence than + so it first get multiplied with 3*2 and then adds into 7. Here operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.

Category Postfix Unary Multiplicative Additive Shift Relational Equality Bitwise AND Bitwise XOR Bitwise OR Logical AND Logical OR

Operator () [] -> . ++ - + - ! ~ ++ - - (type)* & sizeof */% +> < >= == != & ^ | && ||

Associativity Left to right Right to left Left to right Left to right Left to right Left to right Left to right Left to right Left to right Left to right Left to right Left to right

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Conditional Assignment Comma

?: = += -= *= /= %=>>= 0; i--) { printf("Address of var[%d] = %x\n", i, ptr ); printf("Value of var[%d] = %d\n", i, *ptr ); /* move to the previous location */ ptr--; } return 0; }
When the above code is compiled and executed, it produces result something as follows:

Address of var[3] = bfedbcd8 Value of var[3] = 200 Address of var[2] = bfedbcd4 Value of var[2] = 100 Address of var[1] = bfedbcd0 Value of var[1] = 10

Pointer Comparisons
Pointers may be compared by using relational operators, such as ==, . If p1 and p2 point to variables that are related to each other, such as elements of the same array, then p1 and p2 can be meaningfully compared. The following program modifies the previous example one by incrementing the variable pointer so long as the address to which it points is either less than or equal to the address of the last element of the array, which is &var[MAX - 1]:

#include

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const int MAX = 3; int main () { int var[] = {10, 100, 200}; int i, *ptr; /* let us have address of the first element in pointer */ ptr = var; i = 0; while ( ptr operator as follows:

struct_pointer->title;
Let us re-write above example using structure pointer, hope this will be easy for you to understand the concept:

#include

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#include struct Books { char title[50]; char author[50]; char subject[100]; int book_id; };

/* function declaration */ void printBook( struct Books *book ); int main( ) { struct Books Book1; /* Declare Book1 of type Book */ struct Books Book2; /* Declare Book2 of type Book */

/* book 1 specification */ strcpy( Book1.title, "C Programming"); strcpy( Book1.author, "Nuha Ali"); strcpy( Book1.subject, "C Programming Tutorial");
Book1.book_id = 6495407;

/* book 2 specification */ strcpy( Book2.title, "Telecom Billing"); strcpy( Book2.author, "Zara Ali"); strcpy( Book2.subject, "Telecom Billing Tutorial");
Book2.book_id = 6495700;

/* print Book1 info by passing address of Book1 */ printBook( &Book1 );

/* print Book2 info by passing address of Book2 */ printBook( &Book2 ); return 0; } void printBook( struct Books *book ) { printf( "Book title : %s\n", book->title); printf( "Book author : %s\n", book->author); printf( "Book subject : %s\n", book->subject); printf( "Book book_id : %d\n", book->book_id); }
When the above code is compiled and executed, it produces following result:

Book title : C Programming Book author : Nuha Ali Book subject : C Programming Tutorial Book book_id : 6495407 Book title : Telecom Billing Book author : Zara Ali

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Book subject : Telecom Billing Tutorial Book book_id : 6495700

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CHAPTER

18
C Unions

A

union is a special data type available in C that enables you to store different data

types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multi-purpose.

Defining a Union
To define a union, you must use the union statement in very similar was as you did while defining structure. The union statement defines a new data type, with more than one member for your program. The format of the union statement is as follows:

union [union tag] { member definition; member definition; ... member definition; } [one or more union variables];
The union tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the union's definition, before the final semicolon, you can specify one or more union variables but it is optional. Here is the way you would define a union type named Data which has the three members i, f, and str:

union Data { int i; float f; char str[20]; } data;
Now a variable of Data type can store an integer, a floating-point number, or a string of characters. This means that a single variable i.e. same memory location can be used to store multiple types of data. You can use any built-in or user defined data types inside a union based on your requirement.

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The memory occupied by a union will be large enough to hold the largest member of the union. For example, in above example Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by character string. Following is the example which will display total memory size occupied by the above union:

#include #include union Data { int i; float f; char str[20]; }; int main( ) { union Data data; printf( "Memory size occupied by data : %d\n", sizeof(data)); return 0; }
When the above code is compiled and executed, it produces following result:

Memory size occupied by data : 20

Accessing Union Members
To access any member of a union, we use the member access operator (.). The member access operator is coded as a period between the union variable name and the union member that we wish to access. You would use union keyword to define variables of union type. Following is the example to explain usage of union:

#include #include union Data { int i; float f; char str[20]; }; int main( ) { union Data data; data.i = 10; data.f = 220.5; strcpy( data.str, "C Programming"); printf( "data.i : %d\n", data.i); printf( "data.f : %f\n", data.f);

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printf( "data.str : %s\n", data.str); return 0; }
When the above code is compiled and executed, it produces following result:

data.i : 1917853763 data.f : 4122360580327794860452759994368.000000 data.str : C Programming
Here we can see that values of i and f members of union got corrupted because final value assigned to the variable has occupied the memory location and this is the reason that the value if str member is getting printed very well. Now let's look into the same example once again where we will use one variable at a time which is the main purpose of having union:

#include #include union Data { int i; float f; char str[20]; }; int main( ) { union Data data; data.i = 10; printf( "data.i : %d\n", data.i); data.f = 220.5; printf( "data.f : %f\n", data.f); strcpy( data.str, "C Programming"); printf( "data.str : %s\n", data.str); return 0; }
When the above code is compiled and executed, it produces following result:

data.i : 10 data.f : 220.500000 data.str : C Programming
Here all the members are getting printed very well because one member is being used at a time.

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CHAPTER

19
Bit Fields

S

uppose your C program contains a number of TRUE/FALSE variables grouped in a

structure called status, as follows:

struct { unsigned int widthValidated; unsigned int heightValidated; } status;
This structure requires 8 bytes of memory space but in actual we are going to store either 0 or 1 in each of the variables. The C programming language offers a better way to utilize the memory space in such situation. If you are using such variables inside a structure then you can define the width of a variable which tells the C compiler that you are going to use only those number of bytes. For example above structure can be re-written as follows:

struct { unsigned int widthValidated : 1; unsigned int heightValidated : 1; } status;
Now the above structure will require 4 bytes of memory space for status variable but only 2 bits will be used to store the values. If you will use upto 32 variables each one with a width of 1 bit , then also status structure will use 4 bytes, but as soon as you will have 33 variables then it will allocate next slot of the memory and it will start using 64 bytes. Let us check the following example to understand the concept:

#include #include /* define simple structure */ struct { unsigned int widthValidated; unsigned int heightValidated; } status1; /* define a structure with bit fields */

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struct { unsigned int widthValidated : 1; unsigned int heightValidated : 1; } status2; int main( ) { printf( "Memory size occupied by status1 : %d\n", sizeof(status1)); printf( "Memory size occupied by status2 : %d\n", sizeof(status2)); return 0; }
When the above code is compiled and executed, it produces following result:

Memory size occupied by status1 : 8 Memory size occupied by status2 : 4

Bit Field Declaration
The declaration of a bit-field has the form inside a structure:

struct { type [member_name] : width ; };
Below the description of variable elements of a bit field:

Elements type member_name width

Description An integer type that determines how the bit-field's value is interpreted. The type may be int, signed int, unsigned int. The name of the bit-field. The number of bits in the bit-field. The width must be less than or equal to the bit width of the specified type.

The variables defined with a predefined width are called bit fields. A bit field can hold more than a single bit for example if you need a variable to store a value from 0 to 7 only then you can define a bit field with a width of 3 bits as follows:

struct { unsigned int age : 3; } Age;
The above structure definition instructs C compiler that age variable is going to use only 3 bits to store the value, if you will try to use more than 3 bits then it will not allow you to do so. Let us try the following example:

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#include #include struct { unsigned int age : 3; } Age; int main( ) { Age.age = 4; printf( "Sizeof( Age ) : %d\n", sizeof(Age) ); printf( "Age.age : %d\n", Age.age );

Age.age = 7; printf( "Age.age : %d\n", Age.age );

Age.age = 8; printf( "Age.age : %d\n", Age.age ); return 0; }
When the above code is compiled it will compile with warning and when executed, it produces following result:

Sizeof( Age ) : 4 Age.age : 4 Age.age : 7 Age.age : 0

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CHAPTER

20
Typedef

T
BYTE

he C programming language provides a keyword called typedef which you can use to

give a type a new name. Following is an example to define a term BYTE for one-byte numbers:

typedef unsigned char BYTE;
After this type definitions, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example:.

b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows:

typedef unsigned char byte;
You can use typedef to give a name to user defined data type as well. For example you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows:

#include #include typedef struct Books { char title[50]; char author[50]; char subject[100]; int book_id; } Book; int main( ) { Book book; strcpy( book.title, "C Programming"); strcpy( book.author, "Nuha Ali"); strcpy( book.subject, "C Programming Tutorial");

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book.book_id = 6495407; printf( printf( printf( printf( "Book "Book "Book "Book title : %s\n", book.title); author : %s\n", book.author); subject : %s\n", book.subject); book_id : %d\n", book.book_id);

return 0; }
When the above code is compiled and executed, it produces following result:

Book Book Book Book

title : C Programming author : Nuha Ali subject : C Programming Tutorial book_id : 6495407

typedef vs #define
The #define is a C-directive which is also used to define the aliases for various data types similar totypedef but with three differences:

 

The typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well, like you can define 1 as ONE etc. The typedef interpretation is performed by the compiler where as #define statements are processed by the pre-processor.

Following is a simplest usage of #define:

#include #define TRUE 1 #define FALSE 0 int main( ) { printf( "Value of TRUE : %d\n", TRUE); printf( "Value of FALSE : %d\n", FALSE); return 0; }
When the above code is compiled and executed, it produces following result:

Value of TRUE : 1 Value of FALSE : 0

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CHAPTER

21
Input & Output

W

hen we are saying Input that means to feed some data into program. This can

be given in the form of file or from command line. C programming language provides a set of built-in functions to read given input and feed it to the program as per requirement. When we are saying Output that means to display some data on screen, printer or in any file. C programming language provides a set of built-in functions to output the data on the computer screen as well as you can save that data in text or binary files.

The Standard Files
C programming language treats all the devices as files. So devices such as the display are addressed in the same way as files and following three file are automatically opened when a program executes to provide access to the keyboard and screen.

Standard File Standard input Standard output Standard error

File Pointer stdin stdout stderr

Device Keyboard Screen Your screen

The file points are the means to access the file for reading and writing purpose. This section will explain you how to read values from the screen and how to print the result on the screen.

The getchar() & putchar() functions
The int getchar(void) function reads the next available character from the screen and returns it as an integer. This function reads only single character at a time. You can use this method in the loop in case you want to read more than one characters from the screen. The int putchar(int c) function puts the passed character on the screen and returns the same character. This function puts only single character at a time. You can use this method

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in the loop in case you want to display more than one characters on the screen. Check the following example:

#include int main( ) { int c; printf( "Enter a value :"); c = getchar( ); printf( "\nYou entered: "); putchar( c ); return 0; }
When the above code is compiled and executed, it waits for you to input some text when you enter a text and press enter then program proceeds and reads only a single character and displays it as follows:

$./a.out Enter a value : this is test You entered: t

The gets() & puts() functions
The char *gets(char *s) function reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF. The int puts(const char *s) function writes the string s and a trailing newline to stdout.

#include int main( ) { char str[100]; printf( "Enter a value :"); str = gets( str ); printf( "\nYou entered: "); puts( str ); return 0; }
When the above code is compiled and executed, it waits for you to input some text when you enter a text and press enter then program proceeds and reads the complete line till end and displays it as follows:

$./a.out Enter a value : this is test

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You entered: This is test

The scanf() and printf() functions
The int scanf(const char *format, ...) function reads input from the standard input stream stdin and scans that input according to format provided. The int printf(const char *format, ...) function writes output to the standard output stream stdout and produces output according to a format provided. The format can be a simple constant string, but you can specify %s, %d, %c, %f etc to print or read strings, integer, character or float respectively. There are many other formatting options available which can be used based on requirements. For a complete detail you can refer to a man page for these function. For now let us proceed with a simple example which makes things clear:

#include int main( ) { char str[100]; int i; printf( "Enter a value :"); scanf("%s %d", str, &i); printf( "\nYou entered: %s, %d ", str, i); return 0; }
When the above code is compiled and executed, it waits for you to input some text when you enter a text and press enter then program proceeds and reads the input and displays it as follows:

$./a.out Enter a value : seven 7 You entered: seven 7
Here it should be noted that scanf() expect input in the same format as you provided %s and %d, which means you have to provide valid input like "string integer", if you provide "string string" or "integer integer" then it will be assumed as wrong input. Second, while reading a string scanf() stops reading as soon as it encounters a space so "this is test" are three strings for scanf().

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CHAPTER

22
File I/O

L

ast chapter explained about standard input and output devices handled by C

programming language. This chapter we will see how C programmers can create, open, close text or binary files for their data storage. A file represents a sequence of bytes, does not matter if it is a text file or binary file. C programming language provides access on high level functions as well as low level (OS level) calls to handle file on your storage devices. This chapter will take you through important calls for the file management.

Opening Files
You can use the fopen( ) function to create a new file or to open an existing file, this call will initialize an object of the type FILE, which contains all the information necessary to control the stream. Following is the prototype of this function call:

FILE *fopen( const char * filename, const char * mode );
Here filename is string literal which you will use to name your file and access mode can have one of the following values:

Mode Description r w a r+ w+ a+ Opens an existing text file for reading purpose. Opens a text file for writing, if it does not exist then a new file is created. Here your program will start writing content from the beginning of the file. Opens a text file for writing in appending mode, if it does not exist then a new file is created. Here your program will start appending content in the existing file content. Opens a text file for reading and writing both. Opens a text file for reading and writing both. It first truncate the file to zero length if it exists otherwise create the file if it does not exist. Opens a text file for reading and writing both. It creates the file if it does not exist. The reading will start from the beginning but writing can only be appended.

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If you are going to handle binary files then you will use below mentioned access modes instead of the above mentioned:

"rb", "wb", "ab", "ab+", "a+b", "wb+", "w+b", "ab+", "a+b"

Closing a File
To close a file, use the fclose( ) function. The prototype of this function is:

int fclose( FILE *fp );
The fclose( ) function returns zero on success, or EOF if there is an error in closing the file. This function actually, flushes any data still pending in the buffer to the file, closes the file, and releases any memory used for the file. The EOF is a constant defined in the header file stdio.h. There are various functions provide by C standard library to read and write a file character by character or in the form of a fixed length string. Let us see few of the in the next section.

Writing a File
Following is the simplest function to write individual characters to a stream:

int fputc( int c, FILE *fp );
The function fputc() writes the character value of the argument c to the output stream referenced by fp. It returns the written character written on success otherwise EOF if there is an error. You can use the following functions to write a null-terminated string to a stream:

int fputs( const char *s, FILE *fp );
The function fputs() writes the string s to the output stream referenced by fp. It returns a non-negative value on success, otherwise EOF is returned in case of any error. You can use int fprintf(FILE *fp,const char *format, ...) function as well to write a string into a file. Try the following example:

#include main() { FILE *fp; fp = fopen("/tmp/test.txt", "w+"); fprintf(fp, "This is testing for fprintf...\n"); fputs("This is testing for fputs...\n", fp); fclose(fp); }

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When the above code is compiled and executed, it creates a new file test.txt in /tmp directory and writes two lines using two different functions. Let us read this file in next section.

Reading a File
Following is the simplest function to read a single character from a file:

int fgetc( FILE * fp );
The fgetc() function reads a character from the input file referenced by fp. The return value is the character read, or in case of any error it returns EOF. The following functions allow you to read a string from a stream:

char *fgets( char *buf, int n, FILE *fp );
The functions fgets() reads up to n - 1 characters from the input stream referenced by fp. It copies the read string into the buffer buf, appending a null character to terminate the string. If this function encounters a newline character '\n' or the end of the file EOF before they have read the maximum number of characters, then it returns only the characters read up to that point including new line character. You can also use int fscanf(FILE *fp, const char *format, ...) function to read strings from a file but it stops reading after the first space character encounters.

#include main() { FILE *fp; char buff[100]; fp = fopen("/tmp/test.txt", "r"); fscanf(fp, "%s", buff); printf("1 : %s\n", buff ); fgets(buff, 255, (FILE*)fp); printf("2: %s\n", buff ); fgets(buff, 255, (FILE*)fp); printf("3: %s\n", buff ); fclose(fp); }
When the above code is compiled and executed, it reads the file created in previous section and produces following result:

1 : This 2: is testing for fprintf...

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3: This is testing for fputs...
Let's see a little more detail about what happened here. First fscanf() method read just This because after that it encountered a space, second call is for fgets() which read the remaining line till it encountered end of line. Finally last call fgets() read second line completely.

Binary I/O Functions
There are following two functions which can be used for binary input and output:

size_t fread(void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file); size_t fwrite(const void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file);
Both of these functions should be used to read or write blocks of memories - usually arrays or structures.

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CHAPTER

23
Preprocessors

T
#define

he C Preprocessor is not part of the compiler, but is a separate step in the

compilation process. In simplistic terms, a C Preprocessor is just a text substitution tool and they instruct compiler to do required pre-processing before actual compilation. We'll refer to the C Preprocessor as the CPP. All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in first column. Following section lists down all important preprocessor directives:

Directive

Description Substitutes a preprocessor macro Inserts a particular header from another file Undefines a preprocessor macro Returns true if this macro is defined Returns true if this macro is not defined Tests if a compile time condition is true The alternative for #if #else an #if in one statement Ends preprocessor conditional Prints error message on stderr Issues special commands to the compiler, using a standardized method

#include #undef #ifdef #ifndef #if #else #elif #endif #error #pragma

Preprocessors Examples
Analyze following examples to understand various directives.

#define MAX_ARRAY_LENGTH 20

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This directive tells the CPP to replace instances of MAX_ARRAY_LENGTH with 20. Use #define for constants to increase readability.

#include #include "myheader.h"
These directives tell the CPP to get stdio.h from System Libraries and add the text to the current source file. The next line tells CPP to get myheader.h from the local directory and add the content to the current source file.

#undef FILE_SIZE #define FILE_SIZE 42
This tells the CPP to undefine existing FILE_SIZE and define it as 42.

#ifndef MESSAGE #define MESSAGE "You wish!" #endif
This tells the CPP to define MESSAGE only if MESSAGE isn't already defined.

#ifdef DEBUG /* Your debugging statements here */ #endif
This tells the CPP to do the process the statements enclosed if DEBUG is defined. This is useful if you pass the -DDEBUG flag to gcc compiler at the time of compilation. This will define DEBUG, so you can turn debugging on and off on the fly during compilation.

Predefined Macros
ANSI C defines a number of macros. Although each one is available for your use in programming, the predefined macros should not be directly modified.

Macro __DATE__ __TIME__ __FILE__ __LINE__ __STDC__

Description The current date as a character literal in "MMM DD YYYY" format The current time as a character literal in "HH:MM:SS" format This contains the current filename as a string literal. This contains the current line number as a decimal constant. Defined as 1 when the compiler complies with the ANSI standard.

Let's try the following example:

#include main()

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{ printf("File printf("Date printf("Time printf("Line printf("ANSI }
When the above code in a file test.c is compiled and executed, it produces the following result:

:%s\n", :%s\n", :%s\n", :%d\n", :%d\n",

__FILE__ __DATE__ __TIME__ __LINE__ __STDC__

); ); ); ); );

File :test.c Date :Jun 2 2012 Time :03:36:24 Line :8 ANSI :1

Preprocessor Operators
The C preprocessor offers following operators to help you in creating macros:

Macro Continuation (\)
A macro usually must be contained on a single line. The macro continuation operator is used to continue a macro that is too long for a single line. For example:

#define message_for(a, b) \ printf(#a " and " #b ": We love you!\n")

Stringize (#)
The stringize or number-sign operator ('#'), when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro that has a specified argument or parameter list. For example:

#include #define message_for(a, b) \ printf(#a " and " #b ": We love you!\n")

int main(void)
{ message_for(Carole, Debra); return 0; }
When the above code is compiled and executed, it produces the following result:

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Carole and Debra: We love you!

Token Pasting (##)
The token-pasting operator (##) within a macro definition combines two arguments. It permits two separate tokens in the macro definition to be joined into a single token. For example:

#include #define tokenpaster(n) printf ("token" #n " = %d", token##n)

int main(void)
{

int token34 = 40; tokenpaster(34); return 0; }
When the above code is compiled and executed, it produces the following result:

token34 = 40
How it happened, because this example results in the following actual output from the preprocessor:

printf ("token34 = %d", token34);
This example shows the concatenation of token##n into token34 and here we have used both stringize and token-pasting.

The defined() Operator
The preprocessor defined operator is used in constant expressions to determine if an identifier is defined using #define. If the specified identifier is defined, the value is true (non-zero). If the symbol is not defined, the value is false (zero). The defined operator is specified as follows:

#include #if !defined (MESSAGE) #define MESSAGE "You wish!" #endif

int main(void)
{ printf("Here is the message: %s\n", MESSAGE); return 0; }

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When the above code is compiled and executed, it produces the following result:

Here is the message: You wish!

Parameterized Macros
One of the powerful functions of the CPP is the ability to simulate functions using parameterized macros. For example, we might have some code to square a number as follows:

int square(int x) { return x * x;
}
We can rewrite above code using a macro as follows:

#define square(x) ((x) * (x))
Macros with arguments must be defined using the #define directive before they can be used. The argument list is enclosed in parentheses and must immediately follow the macro name. Spaces are not allowed between and macro name and open parenthesis. For example:

#include #define MAX(x,y) ((x) > (y) ? (x) : (y))

int main(void)
{ printf("Max between 20 and 10 is %d\n", MAX(10, 20)); return 0; }
When the above code is compiled and executed, it produces the following result:

Max between 20 and 10 is 20

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CHAPTER

24
Header Files

A

header file is a file with extension .h which contains C function declarations and

macro definitions and to be shared between several source files. There are two types of header files: the files that the programmer writes and the files that come with your compiler. You request the use of a header file in your program by including it, with the C preprocessing directive #include like you have seen inclusion of stdio.h header file which comes along with your compiler. Including a header file is equal to copying the content of the header file but we do not do it because it will be very much error-prone and it is not a good idea to copy the content of header file in the source files, specially if we have multiple source file comprising our program. A simple practice in C or C++ programs is that we keep all the constants, macros, system wide global variables, and function prototypes in header files and include that header file wherever it is required.

Include Syntax
Both user and system header files are included using the preprocessing directive #include. It has following two forms:

#include
This form is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the -I option while compiling your source code.

#include "file"
This form is used for header files of your own program. It searches for a file named file in the directory containing the current file. You can prepend directories to this list with the -I option while compiling your source code.

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Include Operation
The #include directive works by directing the C preprocessor to scan the specified file as input before continuing with the rest of the current source file. The output from the preprocessor contains the output already generated, followed by the output resulting from the included file, followed by the output that comes from the text after the #include directive. For example, if you have a header file header.h as follows:

char *test (void); and a main program called program.c that uses the header file, like this:

int x; #include "header.h" int main (void) { puts (test ()); } the compiler will see the same token stream as it would if program.c read

int x; char *test (void); int main (void) { puts (test ()); }

Once-Only Headers
If a header file happens to be included twice, the compiler will process its contents twice and will result an error. The standard way to prevent this is to enclose the entire real contents of the file in a conditional, like this:

#ifndef HEADER_FILE #define HEADER_FILE the entire header file file #endif
This construct is commonly known as a wrapper #ifndef. When the header is included again, the conditional will be false, because HEADER_FILE is defined. The preprocessor will skip over the entire contents of the file, and the compiler will not see it twice.

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Computed Includes
Sometimes it is necessary to select one of several different header files to be included into your program. They might specify configuration parameters to be used on different sorts of operating systems, for instance. You could do this with a series of conditionals as follows:

#if SYSTEM_1 # include "system_1.h" #elif SYSTEM_2 # include "system_2.h" #elif SYSTEM_3 ... #endif
But as it grows, it becomes tedious, instead the preprocessor offers the ability to use a macro for the header name. This is called a computed include. Instead of writing a header name as the direct argument of #include, you simply put a macro name there instead:

#define SYSTEM_H "system_1.h" ... #include SYSTEM_H
SYSTEM_H will be expanded, and the preprocessor will look for system_1.h as if the #include had been written that way originally. SYSTEM_H could be defined by your Makefile with a -D option.

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CHAPTER

25
Type Casting

T

ypecasting is a way to convert a variable from one data type to another data type.

For example if you want to store a long value into a simple integer then you can type cast long to int. You can convert values from one type to another explicitly using the cast operator as follows:

(type_name) expression
Consider the following example where the cast operator causes the division of one integer variable by another to be performed as a floating-point operation:

#include main() { int sum = 17, count = 5; double mean; mean = (double) sum / count; printf("Value of mean : %f\n", mean ); }
When the above code is compiled and executed, it produces the following result:

Value of mean : 3.400000
It should be noted here that the cast operator has precedence over division, so the value of sum is first converted to type double and finally it gets divided by count yielding a double value. Type conversions can be implicit which is performed by the compiler automatically, or it can be specified explicitly through the use of the cast operator. It is considered good programming practice to use the cast operator whenever type conversions are necessary.

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Integer Promotion
The Integer promotion is the process by which values of integer type "smaller" than int or unsigned int are converted either to int or unsigned int. Consider an example of adding a character in an int:

#include main() { int i = 17; char c = 'c'; /* ascii value is 99 */ int sum; sum = i + c; printf("Value of sum : %d\n", sum ); }
When the above code is compiled and executed, it produces the following result:

Value of sum : 116
Here value of sum is coming as 116 because compiler is doing integer promotion and converting the value of 'c' to ascii before performing actual addition operation.

Usual Arithmetic Conversion
The usual arithmetic conversions are implicitly performed to cast their values in a common type. Compiler first performs integer promotion, if operands still have different types then they are converted to the type that appears highest in the following hierarchy:

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The usual arithmetic conversions are not performed for the assignment operators, nor for the logical operators && and ||. Let us take following example to understand the concept:

#include main() { int i = 17; char c = 'c'; /* ascii value is 99 */ float sum; sum = i + c; printf("Value of sum : %f\n", sum ); }
When the above code is compiled and executed, it produces the following result:

Value of sum : 116.000000
Here it is simple to understand that first c gets converted to integer but because final value is double, so usual arithmetic conversion applies and compiler convert i and c into float and add them yielding a float result.

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CHAPTER

26
Error Handling

A

s such C programming does not provide direct support for error handling but

being a system programming language, it provides you access at lower level in the form of return values. Most of the C or even Unix function calls return -1 or NULL in case of any error and sets an error code errno is set which is global variable and indicates an error occurred during any function call. You can find various error codes defined in header file. So a C programmer can check the returned values and can take appropriate action depending on the return value. As a good practice, developer should set errno to 0 at the time of initialization of the program. A value of 0 indicates that there is no error in the program.

The errno, perror() and strerror()
The C programming language provides perror() and strerror() functions which can be used to display the text message associated with errno.

 

The perror() function displays the string you pass to it, followed by a colon, a space, and then the textual representation of the current errno value. The strerror() function, which returns a pointer to the textual representation of the current errno value.

Let's try to simulate an error condition and try to open a file which does not exist. Here I'm using both the functions to show the usage, but you can use one or more ways of printing your errors. Second important point to note is that you should use stderr file stream to output all the errors.

#include #include #include extern int errno ; int main () { FILE * pf; int errnum;

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pf = fopen ("unexist.txt", "rb"); if (pf == NULL) { errnum = errno; fprintf(stderr, "Value of errno: %d\n", errno); perror("Error printed by perror"); fprintf(stderr, "Error opening file: %s\n", strerror( errnum )); } else { fclose (pf); } return 0; }
When the above code is compiled and executed, it produces the following result:

Value of errno: 2 Error printed by perror: No such file or directory Error opening file: No such file or directory

Divide by zero errors
It is a common problem that at the time of dividing any number, programmers do not check if a divisor is zero and finally it creates a runtime error. The code below fixes this by checking if the divisor is zero before dividing:

#include #include main() { int dividend = 20; int divisor = 0; int quotient; if( divisor == 0){ fprintf(stderr, "Division by zero! Exiting...\n"); exit(-1); } quotient = dividend / divisor; fprintf(stderr, "Value of quotient : %d\n", quotient ); exit(0); }
When the above code is compiled and executed, it produces the following result:

Division by zero! Exiting...

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Program Exit Status
It is a common practice to exit with a value of EXIT_SUCCESS in case of programming is coming out after a successful operation. Here EXIT_SUCCESS is a macro and it is defined as 0. If you have an error condition in your program and you are coming out then you should exit with a status EXIT_FAILURE which is defined as -1. So let's write above program as follows:

#include #include main() { int dividend = 20; int divisor = 5; int quotient; if( divisor == 0){ fprintf(stderr, "Division by zero! Exiting...\n"); exit(EXIT_FAILURE); } quotient = dividend / divisor; fprintf(stderr, "Value of quotient : %d\n", quotient ); exit(EXIT_SUCCESS); }
When the above code is compiled and executed, it produces the following result:

Value of quotient : 4

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CHAPTER

27
Recursion

R

ecursion is the process of repeating items in a self-similar way. Same applies in

programming languages as well where if a programming allows you to call a function inside the same function that is called recursive call of the function as follows.

void recursion() { recursion(); /* function calls itself */ } int main() { recursion(); }
The C programming language supports recursion ie. a function to call itself. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go in infinite loop. Recursive function are very useful to solve many mathematical problems like to calculate factorial of a number, generating fibonacci series etc.

Number Factorial
Following is an example which calculate factorial for a given number using a recursive function:

#include int factorial(unsigned int i) { if(i 2 ) { printf("Too many arguments supplied.\n"); } else { printf("One argument expected.\n"); } }
When the above code is compiled and executed with a single argument, it produces the following result.

$./a.out testing The argument supplied is testing
When the above code is compiled and executed with a two arguments, it produces the following result.

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$./a.out testing1 testing2 Too many arguments supplied.
When the above code is compiled and executed without passing any argument, it produces the following result.

$./a.out One argument expected
It should be noted that argv[0] holds the name of the program itself and argv[1] is a pointer to the first command line argument supplied, and *argv[n] is the last argument. If no arguments are supplied, argc will be one, otherwise and if you pass one argument then argc is set at 2. You pass all the command line arguments separated by a space, but if argument itself has a space then you can pass such arguments by putting them inside double quotes "" or single quotes ''. Let us re-write above example once again where we will print program name and we also pass a command line argument by putting inside double quotes:

#include int main( int argc, char *argv[] ) { printf("Program name %s\n", argv[0]); if( argc == 2 ) { printf("The argument supplied is %s\n", argv[1]); } else if( argc > 2 ) { printf("Too many arguments supplied.\n"); } else { printf("One argument expected.\n"); } }
When the above code is compiled and executed with a single argument separated by space but inside double quotes, it produces the following result.

$./a.out "testing1 testing2"

Progranm name ./a.out The argument supplied is testing1 testing2

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