A recursive function requires a base condition that terminates the recursive calls. Without a reachable base condition, recursion can continue until the program exhausts its available stack space.

#include <stdio.h>

int factorial(int n)
{
    if (n == 0)
    {
        return 1;
    }

    return n * factorial(n - 1);
}

int main(void)
{
    int number = 5;

    printf("Factorial of %d = %d\n",
           number, factorial(number));

    return 0;
}

Example Output #

Factorial of 5 = 120

Explanation #

The condition:

if (n == 0)
{
    return 1;
}

is the base condition. When n reaches 0, the function returns 1 without making another recursive call.

The recursive step is:

return n * factorial(n - 1);

Each call decreases n by one, moving the computation toward the base condition.

Recursive Execution #

For factorial(5), the calls proceed as follows:

As the calls return, the pending multiplications are evaluated:\[ 5 \times 4 \times 3 \times 2 \times 1 = 120 \]

Base Condition and Recursive Step #

The function assumes a non-negative integer input. For negative values, the condition n == 0 is never reached through repeated decrements, so the function does not terminate normally. Input validation is necessary if values outside the intended range are possible.