Why is this result?

Is that about stack? i think the last *p++ is undefined.*p++ means *(p++) or *p;p++;?

void test123()
{

    char s[] = "123", * p;
    p = s;
    // 1 2 3
    cout << *p++ << endl;
    cout << *p++ << endl;
    cout << *p++ << endl;

}
void test321()
{
    char s[] = "123", * p;
    p = s;
    //321
    cout << *p++ << *p++ << *p++ << endl;

}
int main(void)
{
    cout << "123:" << endl;
    test123();
    cout << "123:" << endl;
    test321();
    cout << "hello world" << endl;
    return 0;
}

i think the result is undefined