buff: "", n=0
after call: buff: "", n=0
res == buff: 1;
buff: "", n=3
after call: buff: "XXX", n=3
res == buff: 1;
buff: "Iorem ipsum dolor", n=3
after call: buff: "XXXem ipsum dolor", n=3
res == buff: 1;
buff: "Iorem ipsum dolor", n=16
after call: buff: "XXXXXXXXXXXXXXXXr", n=16
res == buff: 1;
buff: "Iorem ipsum dolor", n=17
after call: buff: "XXXXXXXXXXXXXXXXX", n=17
res == buff: 1;