Types of user defined function in C language

A function may or may not accept any argument. It may or may not return any value. Based on these facts, There are four different types of user defined function in C language.

1. Function without arguments and without return value
2. Function without arguments and with return value
3. Function with arguments and without return value
4. Function with arguments and with return value

Example

Function without argument and return value

#include<stdio.h>  
void printName();  
void main ()  
{  
    printf("Hello ");  
    printName();  
}  
void printName()  
{  
    printf("EyWiah.com");  
}  

#include<stdio.h>  
void sum();  
void main()  
{  
    printf("\nGoing to calculate the sum of two numbers:");  
    sum();  
}  
void sum()  
{  
    int a,b;   
    printf("\nEnter two numbers");  
    scanf("%d %d",&a,&b);   
    printf("The sum is %d",a+b);  
}  

Function without argument and with return value

#include<stdio.h>  
int sum();  
void main()  
{  
    int result;   
    printf("\nGoing to calculate the sum of two numbers:");  
    result = sum();  
    printf("%d",result);  
}  
int sum()  
{  
    int a,b;   
    printf("\nEnter two numbers");  
    scanf("%d %d",&a,&b);  
    return a+b;   
}  

C Program to calculate the area of the square is given below.

#include<stdio.h>  
int sum();  
void main()  
{  
    printf("Going to calculate the area of the square\n");  
    float area = square();  
    printf("The area of the square: %f\n",area);  
}  
int square()  
{  
    float side;  
    printf("Enter the length of the side in meters: ");  
    scanf("%f",&side);  
    return side * side;  
}  

Function with argument and without return value

#include<stdio.h>  
void sum(int, int);  
void main()  
{  
    int a,b,result;   
    printf("\nGoing to calculate the sum of two numbers:");  
    printf("\nEnter two numbers:");  
    scanf("%d %d",&a,&b);  
    sum(a,b);  
}  
void sum(int a, int b)  
{  
    printf("\nThe sum is %d",a+b);      
}  

Function with argument and with return value

#include<stdio.h>  
int sum(int, int);  
void main()  
{  
    int a,b,result;   
    printf("\nGoing to calculate the sum of two numbers:");  
    printf("\nEnter two numbers:");  
    scanf("%d %d",&a,&b);  
    result = sum(a,b);  
    printf("\nThe sum is : %d",result);  
}  
int sum(int a, int b)  
{  
    return a+b;  
}  

Program to check whether a number is even or odd is given below.

#include<stdio.h>  
int even_odd(int);  
void main()  
{  
 int n,flag=0;  
 printf("\nGoing to check whether a number is even or odd");  
 printf("\nEnter the number: ");  
 scanf("%d",&n);  
 flag = even_odd(n);  
 if(flag == 0)  
 {  
    printf("\nThe number is odd");  
 }  
 else   
 {  
    printf("\nThe number is even");  
 }  
}  
int even_odd(int n)  
{  
    if(n%2 == 0)  
    {  
        return 1;  
    }  
    else   
    {  
        return 0;  
    }  
}