In this step, we will write the C code to calculate the factorial of a number. We'll create a function to handle the factorial calculation using recursion, which is a common approach for this type of mathematical problem.

#include <stdio.h>

// Function to calculate factorial recursively
unsigned long long factorial(int n) {
    if (n == 0)  // Base case
        return 1;
    else
        return n * factorial(n - 1);  // Recursive case
}

int main() {
    int number;
    printf("Enter a positive integer: ");
    scanf("%d", &number);

    if (number < 0)
        printf("Factorial of a negative number doesn't exist.\n");
    else
        printf("Factorial of %d = %llu", number, factorial(number));

    return 0;
}
            

This program prompts the user for a positive integer, then calculates and displays its factorial. The function factorial is a recursive function that multiplies the number by the factorial of the number minus one, continuing until it reaches zero, where it returns 1.

After writing the code, compile it using a C compiler to ensure there are no errors. If you are using GCC, the command would look like this:

gcc -o factorialProgram factorial.c

If you're using an IDE, you can typically compile the program by pressing a 'Build' button or a similar option. Refer to the Introduction Page for specific instructions on your IDE.

Once compiled, run the program to test it:

./factorialProgram

In your IDE, running the program usually involves clicking a 'Run' button. This will execute the program and allow you to input a number to see its factorial.