Structure of a C Program

Basic Structure of a C Program

A C program is divided into different sections. There are six main sections in a basic C program.

The Six Sections Are:

1. Documentation Section

2. Include / Link / Preprocessor Section

3. Definition Section

4. Global Declarations Section (functions and variables)

5. Main Function

6. Subprogram Functions

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Overview

Now let us learn about each of these sections in detail.

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Figure: Basic Structure Of C Program

Sample Program

The C program below finds the area of a circle using a user-defined function and a global variable PI holding the value of π.

/* File Name: areaofcircle.c
   Author: Abhijith
   Date: 06/05/2021
   Description: A program to calculate area of circle.
   User enters the radius.
*/

#include <stdio.h>      // Link section
#define PI 3.14         // Definition section

float area(float r);    // Global declaration

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

int main()              // Main function
{
    float r;

    printf("Enter the radius:\n");
    scanf("%f", &r);

    printf("The area is: %f", area(r));
    return 0;
}

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

float area(float r)     // Subprogram function
{
    return PI * r * r;
}

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Sections of the Program

1. Documentation Section

The documentation section contains details about the program such as:

• File name

• Author name

• Date of creation

• Description of the program

It provides an overview for anyone reading the code.

/* File Name: areaofcircle.c
   Author: Abhijith
   Date: 06/05/2021
   Description: A program to calculate area of circle.
   User enters the radius.
*/

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2. Link Section

This section declares the header files used in the program.
It tells the compiler to include system libraries.

#include <stdio.h>

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3. Definition Section

In this section, constants are defined using the #define directive.

#define PI 3.14

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4. Global Declaration Section

This section declares:

• Global variables

• User-defined function prototypes

float area(float r);
int a = 7;

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5. Main Function Section

Every C program must have a main function.
It contains:

• Declaration part → where variables are declared

• Execution part → where statements are written

Both parts are enclosed within curly braces { }.

int main()
{
    float r;

    printf("Enter the radius:\n");
    scanf("%f", &r);

    printf("The area is: %f", area(r));
    return 0;
}

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6. Subprogram Section

This section contains user-defined functions used in the program.

float area(float r)
{
    return PI * r * r;
}