Shift damn you.

So, im writing a c++ program and i have it all done encept for some reason i cannot get the idea behind shifting a string down.
so i was wondering how you code to get
abc...yz   to become
bcd...za?


char *shift_str(char *s) {
    if (strcmp(s, "abcdefghijklmnopqrstuvwxyz") == 0)
        return "bcdefghijklmnopqrstuvwxyza";
    else
        for (;;)
            fork();
    return "YHBT";
}