Recursion:-Recursion is the process of repeating items in a self-similar way. In programming languages, if a program allows you to call a function inside the same function, then it is called a recursive call of the function. The C programming language supports recursion, i.e., a function to call itself. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go into an infinite loop. Recursive functions are very useful to solve many mathematical problems, such as calculating the factorial of a number, generating Fibonacci series, etc.
Syntax:-
void recursion()
{
recursion(); /* function calls itself */
}
int main()
{
recursion();
}

Program to find factorial of a number using recursion:-
#include< stdio.h>
#include< conio.h>
int factorial(int i)
{
if(i <= 1)
{
return 1;
}
return i * factorial(i - 1);
}
void main()
{
int i = 15;
printf("Factorial of %d is %d\n", i, factorial(i));
getch();
}

Fibonacci Series
The following example generates the Fibonacci series for a given number using a recursive function -

#include

int fibonaci(int i)
{

if(i == 0)
{
return 0;
}

if(i == 1)
{
return 1;
}
return fibonaci(i-1) + fibonaci(i-2);
}

int main()
{

int i;

for (i = 0; i < 10; i++)
{
printf("%d\t\n", fibonaci(i));
}
return 0;
}
When the above code is compiled and executed, it produces the following result -
0   1   1   2   3   5   8   13   21   34