C Tokens

C-Tokens

•  Identifier
•  Variable
•  Constants
•  Keywords
•  Data types

Identifier

•  Simply references to memory locations,which can hold values (data).
• Formed by combining
• Letters (both upper and lowercase)
• Digits (0–9)
• Underscore ( _ )

Identifier Rules

• The first character of an identifier must be a letter,an underscore ( _ ) also counts as a letter
• The blank or white space character(ascii value) is not permitted in an identifier.
• Can be any length. Internal identifier (do not have the external linkage) such as preprocessor macro names at least the first 31 characters are significant, also implementation dependent.
• Reserved words/keywords and characters such as main and # also cannot be used.

Variable

• Identifier that value may change during the program execution.
• Initializing a variable means, give a value to the variable, that is the variable’s initial value and can be changed later on.

Syntax - variable

1.  Type
2.  Name
3.  Value

Syntax :
            datatype name = <value>;
                   Ex: int x=4;

Keywords

 C - keyword List

• Keywords are the words whose meaning has already been explained to the ‘C’ compiler.
• The keywords are also called as reserved words.
• There are 32 keywords available in ‘C’.

Constants

• integer-constant
• character-constant
• floating-constant
• enumeration-constant

Integer-constant

• consisting of a sequence of digits
• number preceded by 0 : octal number
• number preceded by 0x: hexadecimal
• integer constant may be suffixed by letter

     – The suffix U (or u) forces the constant to be unsigned. It is                unsigned long if the value of the number itself is greater than            decimal 2,147,483,647, regardless of which base is used.
     – The suffix L (or l) attached to any constant forces the constant          to be represented as a long.
      – UL or ul : unsigned long


character-constant

• sequence of one or more characters enclosed in single quotes    Ex: ‘a’
• Character constants do not contain the newlines character in order to represent them following escape sequences may be used
• There is an extended set of characters that cannot be represented in the char type. 
• A constant in this extended set is written with a preceding L

                     Ex: L‘a’

Float & Enumeration Constant

Floating constant

• floating constant consists of a decimal part and a fraction part.
• A floating-point constant that is not suffixed is of type double.
• F or f for a float and L and I for long double.

Enumeration constant

• Identifiers declared as enumerators are constants of type int.
• Enumeration constants are identifiers defined in enum type declarations

        Ex: enum group { green, blue, red, white}


Why data type?

Variable

• VALUE (100, 1000 etc…)
• TYPE (char, int etc…)

                    ------- data type

int

• int will normally be the natural size for aparticular machine
• To develop portable programs, two suffixes are used : “short” and “long”
• Short is often 16 bits, long 32 bits

Categories in data types

Type Specifiers:
                     char       double         enum
                      int         struct           typedef
                      float      union           void

Special type specifiers:

• signed short
• unsigned long

 Type qualifiers

• Const
• Volatile

Special Type Specifiers

Signed & unsigned

• shows effect on sign of the variable.
• can be used with int and char.
• cannot be used with float and double.

Long & short

• shows effect on size of the variable.
• can be used with int and char.
• cannot be used with float and double.
• long can be used with double.

How to write a C Program & what are Requirements ?

Creating the program

• A programmer uses a text editor to create or modify files                 containing C code
• The contents must obey C syntax
• The filename must end by convention “.c”
   Ex: sample.c
• Code is also known as source code
• A file containing source code is called a source file

Compiling the program

• There are many C compilers around.
• Sun compiler - cc
• GNU C - gcc
• Borland compiler, Turbo C compiler, Quick C compiler
• root@home> gcc –Wall sample.c
• Note: There may be a minor variations of
  library functions between different compilers

Running the sample program in Linux based Operating Systems

• If there are no errors in sample.c, this command produces an executable file in the same directory by name a.out
• root@home> ./a.out
• This executes your program, printing any results to the screen

Structure of C program

program header comment

preprocessor directives (if any)

int main ( )
{
statement(s)
return 0 ;
}

Description of hello.c 

•  A C program starts execution with a function called main()‫‏‬
• When a C program is started by the OS, a special startup routine is called before main(). 
• This startup routine takes values from the OS (command line arguments and the environment) and sets things up so that main() is called first.
• The startup routine also invoked exit() on return from main(). exit() releases all to reusable resources and then return to the kernel

Comments

• A comment is descriptive text used to help a reader of the program understand its content.
• All comments must begin with characters /*and end with the characters */.
• These are called comment delimiters.
• Nested comments are not allowed.

Preprocessor Directives

• Lines that begin with a # are called preprocessor directives   
• Example: the #include <stdio.h> directive causes the preprocessor to include a copy of the standard input/output header file stdio.h at this point in the code.
• This header file was included because it contains information about the printf ( ) function that is used in this program.

int main ()

• Every program must have a function called main. This is where program execution begins.
• The reserved word “int” indicates that main() returns an integer value.
• The parentheses following the reserved word “main” indicate that it is a function

The Function Body

• A left brace (curly bracket) -- { -- begins the body of every function.
• A corresponding right brace -- } -- ends the function body

printf (“Hello, World!\n”) ;

• This line is a C statement
•  It is a call to the function printf ( ) with a single argument (parameter), namely the string “Hello, World!\n”
• Notice that this line ends with a semicolon
• Note: All statements in C end with a semicolon

return 0

•  Because function main() returns an integer value, there must be a statement that indicates what this value is.
• The statement “return 0 ;” indicates that main() returns a value of zero to the operating system.
• A value of 0 indicates that the program successfully terminated execution.
• Don't worry about this concept now. Just remember to use the statement.

Why C ?

•  'C' is popular because
• It is reliable, simple and easy to use
• It encourages modularity and good program organization
• It is portable
• It is a middle level
• It provides good data-structuring facilities

Loop holes in 'C'

• close to the hardware & malpractice prone to undesired behavior of system.
• ‘C’ offers only straightforward, single-thread control flow: tests, loops, grouping and subprograms, but not multi programming, parallel operations & synchronization.
• C provides no operations to deal directly with composite objects such as strings or arrays.

INTRODUCTION TO C PROGRAMMING LANGUAGE

C ? 

• C is a programming language.
• Developed at AT &T’s Bell Laboratories of USA in 1972.
•  It was designed and written by Dennis Ritchie.

Versions of ‘C’

• K & R C
•  ANSI C
•  ISO C
•  ISO 9899
•  C99

First Edition of ‘C’

•  In 1978, the first edition of Kernighan and Ritchie's The C  Programming Language was published.
•  It introduced the following features to the existing versions of  C
•  structure data type
•  long int data type
•  unsigned int data type
• The += operator

C Programming Tutorial-Dennis Ritchie

ANSI C

In 1989, C was first officially standardized by ANSI standards committees also included following new features

1.  functions returning struct or union
2.  void * data type
3.  const qualifier to make an object Read Only
4.  assignment for struct data types

ISO 9899

•  The ANSI C standard, with a few minor modifications, was adopted as ISO standard number ISO 9899. 
• The first ISO edition of this document was published in 1990(ISO 9899:1990)
• ISO 9899:1999, which was published in 1999 is commonly referred to as "C99". 
• It was adopted as an ANSI standard in March 2000.

New features in C99

• Inline functions
• Freeing of restrictions on the location of variable declarations 
• The addition of several new data types, including long long int
• An explicit Boolean data type
• Official support for comments beginning with “// ” as in C++
• Several new library functions and Header files