C - Basic structure of a C program

In this tutorial we will learn about the basic structure of a C program.

Following is the basic structure of a C program.

Lets explore the sections with an example.

Write a program to print area of a circle.

In the following example we will find the area of a circle for a given radius 10cm.

Formula

The formula to compute the area of a circle is πr2 where π is PI = 3.1416 (approx.) and r is the radius of the circle.

Lets write the C code to compute the area of the circle.

/**
 * file: circle.c
 * author: yusuf shakeel
 * date: 2010-11-25
 * description: program to find the area of a circle
 *              using the radius r
 */

#include <stdio.h>

#define PI 3.1416

float area(float r);

int main(void)
{
  float r = 10;
  printf("Area: %.2f", area(r));
  return 0;
}

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

The above code will give the following output.

Area: 314.16

Different sections of the above code

Documentation

This section contains a multi line comment describing the code.

/**
 * file: circle.c
 * author: yusuf shakeel
 * date: 2010-11-25
 * description: program to find the area of a circle
 *              using the radius r
 */

In C, we can create single line comment using two forward slash // and we can create multi line comment using /* */.

Comments are ignored by the compiler and is used to write notes and document code.

Link

This section includes header file.

#include <stdio.h>

We are including the stdio.h input/output header file from the C library.

Definition

This section contains constant.

#define PI 3.1416

In the above code we have created a constant PI and assigned 3.1416 to it.

The #define is a preprocessor compiler directive which is used to create constants. We generally use uppercase letters to create constants.

The #define is not a statement and must not end with a ; semicolon.

Global declaration

This section contains function declaration.

float area(float r);

We have declared an area function which takes a floating number (i.e., number with decimal parts) as argument and returns floating number.

main( ) function

This section contains the main() function.

int main(void)
{
  float r = 10;
  printf("Area: %.2f", area(r));
  return 0;
}

This is the main() function of the code. Inside this function we have created a floating variable r and assigned 10 to it.

Then we have called the printf() function. The first argument contains "Area: %.2f" which means we will print floating number having only 2 decimal place. In the second argument we are calling the area() function and passing the value of r to it.

Subprograms

This section contains a subprogram, an area() function that is called from the main() function.

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

This is the definition of the area() function. It receives the value of radius in variable r and then returns the area of the circle using the following formula PI * r * r.