Super simple recursive factorial function in C.

simple_factorial.c

#include <stdio.h>
#include <stdlib.h>

long factorial(long n) {
    if (n == 0)
        return 1;
    else
        return (n * factorial(n - 1));
}

int main(int argc, char **argv) {
    char *ptr;
    long number;

    // check perquisites
    if (argc != 2) {
        printf("Exactly one character needed. I am going to die now :-(");
        exit(1);
    }

    // convert user input to number
    number = strtol(argv[1], &ptr, 10);

    // there may was some malformed input
    if (ptr[0] != 0) {
        printf("Your input could not be parsed completely. Unparsed part is |%s|", ptr);
        exit(1);
    }
    if (number < 0) {
        printf("Number must be bigger or equal than 0.");
        exit(1);
    }

    // this wont quite work out with our standard data types
    if (number >= 21) {
        printf("You should enter a number that is equal or less than 21, otherwise it cannot be stored in a long data type");
        exit(1);
    }

    printf("The factorial of %ld is %ld\n", number, factorial(number));

    exit(0);
}