Structure of C language

The structure of a C program is a protocol (rules) to the programmer, while writing a C program. The general basic structure of C program is shown in the figure below. The whole program is controlled within main ( ) along with left brace denoted by “{” and right braces denoted by “}”. If you need to declare local variables and executable program structures are enclosed within “{” and “}” is called the body of the main function. The main ( ) function can be preceded by documentation, preprocessor statements and global declarations.


The documentation section consist of a set of comment lines giving the name of the program, the another name and other details, which the programmer would like to use later.

Preprocessor Statements

The preprocessor statement begin with # symbol and are also called the preprocessor directive. These statements instruct the compiler to include C preprocessors such as header files and symbolic constants before compiling the C program. Some of the preprocessor statements are listed below.

Global Declarations

The variables are declared before the main ( ) function as well as user defined functions are called global variables. These global variables can be accessed by all the user defined functions including main ( ) function.

The main ( ) function

Each and Every C program should contain only one main ( ). The C program execution starts with main ( ) function. No C program is executed without the main function. The main ( ) function should be written in small (lowercase) letters and it should not be terminated by semicolon. Main ( ) executes user defined program statements, library functions and user defined functions and all these statements should be enclosed within left and right braces.


Every C program should have a pair of curly braces ({, }). The left braces indicates the beginning of the main ( ) function and the right braces indicates the end of the main ( ) function. These braces can also be used to indicate the user-defined functions beginning and ending. These two braces can also be used in compound statements.

Local Declarations

The variable declaration is a part of C program and all the variables are used in main ( ) function should be declared in the local declaration section is called local variables. Not only variables, we can also declare arrays, functions, pointers etc. These variables can also be initialized with basic data types. For examples.

Main ( )
int sum = 0;
int x;
float y;

Here, the variable sum is declared as integer variable and it is initialized to zero. Other variables declared as int and float and these variables inside any function are called local variables.

Program statements

These statements are building blocks of a program. They represent instructions to the computer to perform a specific task (operations). An instruction may contain an input-output statements, arithmetic statements, control statements, simple assignment statements and any other statements and it also includes comments that are enclosed within /* and */ . The comment statements are not compiled and executed and each executable statement should be terminated with semicolon.

User defined functions

These are subprograms, generally, a subprogram is a function and these functions are written by the user are called user ; defined functions. These functions are performed by user specific tasks and this also contains set of program statements. They may be written before or after a main () function and called within main () function. This is an optional to the programmer.

Now, let us write a small program to display some message shown below.
Sample Program:

# include
printf (“welcome to the world of C/n”);

I hope you have understood about the basic structure of C language.:)