A function is a block of statements that can perform a particular task. As we all know, there is always at least one function in C, and that is main().

Example
In the example below, the function’s name is sum and the data type is int. This task of this function is to produce the sum of two numbers:

int sum(int a,int b)
{
return(a+b);
}

Below, the function is declared in main():

void main()
{
int sum(int,int); //function declaration
int x=5,y=6;
total = sum(x,y);
}

Formal parameters and actual parameters
When we call a function in main() or anywhere else in the program, and the function we created needs parameters, we would pass parameters to it while calling the function. In the example above, we passed variables x and y to obtain the sum of x and y.

According to the example above, the formal parameters are a and b, and the *actual *parameters are x and y.

Essentially, the variables being passed in the function call are actual parameters, and the variables being initialized and used in the function are formal* *parameters.

Advantages of a function
A function can be used any number of times after it is defined once.

Functions make programs manageable and easy to understand.

Function categories
There are 4 types of functions:

1. Functions with arguments and return values
This function has arguments and returns a value:
#include <stdio.h>

// Function declaration
int addNumbers(int a, int b);

int main() {
// Example usage
int num1 = 5, num2 = 7;
int sumResult = addNumbers(num1, num2);

// Printing the result
printf(“The sum of %d and %d is: %d\n”, num1, num2, sumResult);

return 0;
}

// Function definition
int addNumbers(int a, int b) {
// Adding two numbers and returning the result
int result = a + b;
return result;
}

2. Functions with arguments and without return values
This function has arguments, but it does not return a value:

#include <stdio.h>
int main()
{
void sum(float,float); //function with arguments and no return value
float x=10.56,y=7.22;
sum(x,y);
}
void sum(float a,float b) //function with arguments and no return value
{
float z = a+b;
printf(“x + y = %f”,z);
}
3. Functions without arguments and with return values
This function has no arguments, but it has a return value:
#include<stdio.h>
int main()
{
int sum();
int c = sum();
printf(“Sum = %d”,c);
}
int sum() //function with no arguments and return data type
{
int x=10,y=20,z=5;
printf(“x = %d ; y = %d ; z = %d \n”,x,y,z);
int sum = x+y+z;
return(sum);
}
4. Functions without arguments and without return values
This function has no arguments and no return value:
#include<stdio.h>
int main()
{
void sum();
sum();
}
void sum() //function with no arguments and return data type
{
int x=15,y=35,z=5;
printf(“x = %d ; y = %d ; z = %d \n”,x,y,z);
int sum = x+y+z;
printf(“Sum = %d”,sum);
}