Functions in C Programming

In a C programming language a function is a group of statements that together perform a special task. In every C program has at least one function, which is main().

In a C library has a numerous predefined or built-in functions. For example, strcat() to concatenate two strings, main() from where program execution starts. 

In a C programming function has a 3 part:

• Function declaration
• Function definition
• Function call

Function declaration : function declaration tells the compiler about a function’s name, return type, and parameters.

A syntax of function declaration –

return_type   function_name( parameter list );

Example of  the function declaration is as follows −

int max(int num1, int num2);

here int is a return type of function which tells function will return integer value and max is a name of the function.

“ int num1, int num2” is a parameter list which tells max function will receive two integer arguments.

Arguments are optional which depends on program requirements.

Function definition : function definition is an actual body of the function.

Syntax of function definition:

return_type function_name( parameter list ) 
{
   body of the function
}

function definition consist a return type, function name, parameter list and body of the function.

Function call

whenever  program calls a function, the flow of control is transferred to the called function definition.

A called function performs a defined task and when flow of control reached to function-ending closing brace, flow of control returns to back to the main program.

Function can be categories as:

• Function with no argument, no return value
• Function with  argument but  no return value
• Function with  argument and  return value
• Function with no argument, but  return value

Function  with  no  argument  & no return value

Example: Write a program to find the area of circle using function with no argument, no return value.

The formula for the area of the circle is :

Area_circle = Π * r * r

Program

#include 

void areaOfcircle(); //Function declaration
int main() {
    areaOfcircle(); // Function Call
    return 0;
}
void areaOfcircle() // function definition to calculate area of circle	
{
    float area, radius;
    printf("Enter the radius of circle : ");
    scanf("%f", & radius); // take radius as input
    area = 3.14 * radius * radius;
    printf("\n Area of circle= : %.2f", area);
}

Output

Enter the radius of circle : 
 Area of circle= : 78.50

Function  with  argument  but  no return value

Example: Write a program to find the area of circle using function with argument but no return value.

#include 

void areaOfcircle(float r); //Function declaration with argument

int main() {
    float radius;
    printf("Enter the radius of circle : ");
    scanf("%f", & radius); // take radius as input
    areaOfcircle(radius); // Function Call
    return 0;
}
void areaOfcircle(float r) // function definition to calculate area of circle	              
{
    float area;
    area = 3.14 * r * r;
    printf("\n Area of circle : %.2f", area);
}

Output

Enter the radius of circle : 
 Area of circle= : 78.50

Function  with  argument  &  return value

Example: Write a program to find the area of circle using function with argument but return value.

#include 								                  float areaOfcircle(float r);		//Function declaration with argument

int main() {
    float radius, a;
    printf("Enter the radius of circle : ");
    scanf("%f", & radius); // take radius as input
    a = areaOfcircle(radius); // Function Call 	
    printf("\n Area of circle : %.2f", a);
    return 0;
}
float areaOfcircle(float r) // function definition to calculate area of circle	                  
{
    float area;
    area = 3.14 * r * r;
    return area;
}

Output

Enter the radius of circle : 
 Area of circle= : 78.50

Function with no argument but  return value

Example: Write a program to find the area of circle using function with no argument but return value.

#include 							                        float areaOfcircle();		//Function declaration

int main() {
    float a;
    a = areaOfcircle(); // Function Call 
    printf("\n Area of circle : %.2f", a);
    return 0;
}
float areaOfcircle() { // function definition to calculate area of circle
    float area, radius;
    printf("Enter the radius of circle : ");
    scanf("%f", & radius); // take radius as input			
    area = 3.14 * radius * radius;
    return area; // return value
}

Output

Enter the radius of circle : 
 Area of circle= : 78.50