For simplicity we divide our code into small pieces / modules known as functions. To use functions in C programming we need to define function prototype, function definition and finally we call the function to perform that functionality. Here we will define a simple sum function, which will accept 2 numbers as an input and will return sum of these numbers.

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#include <stdio.h>

int sum(int x, int y); // this is prototype of our sum function.

int main(void){
// for simplicity we divide our code into small pieces / modules known as functions
// to use functions in C programming we need to define function prototype, function definition
// and finally we call the function to perform certain functionality
// here we will defin a simple sum function, which will accept 2 numbers as input and will return their sum

int number1, number2;
int a;

printf("Please Enter First Numbern");
scanf("%d", &number1);

printf("Please Enter Second Numbern");
scanf("%d", &number2);

// here we called our sum function with 2 input numbers entered by user
//Now this function will return the sum of these numbers which is assigned to variable a
a = sum( number1 , number2);

printf("Sum = %d", a);

return 0;
}

// following is the definition of our sum function
int sum(int x, int y){
int s;
s = x + y;
return s;
}

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Output: