gistfile1.c

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

char *prepend_lol(const char *nope)
{
        char *yep = malloc(strlen(nope) + 5);
        sprintf(yep, "lol, %s", nope);
        return yep;
}

char *is_even(int n)
{
        if (n < 0)
                return is_even(n + 2);
        if (n > 1)
                return is_even(n - 2);

        if (n == 0)
                return prepend_lol("yes, that's even");
        else
                return prepend_lol("no, that's not even");
}

int usage(char *argv0)
{
        printf("use it like this: %s <some number>\n", argv0);
        return 1;
}

int main(int argc, char *argv[])
{
        int n;

        if (argc < 2)
                return usage(argv[0]);

        n = atoi(argv[1]);
        printf("you want to know if %d is even?\n", n);
        puts(is_even(n));

        return 0;
}