char **all_rotations(const char *str)
{
    char *buf, **rots;
    int len = strlen(str), i;

    buf = calloc(2*len+1, sizeof(char));
    strcpy(buf, str);
    strcpy(buf+strlen(str), str);

    rots = malloc(len * sizeof(char*));
    for (i = 0; i < len; ++i) {
        rots[i] = calloc(len+1, sizeof(char));
        strncpy(rots[i], buf+i, len);
    }

    free(buf);
    return rots;
}