# Part I
输入如下命令：
$ gcc -o phase1 -g -std=c99 phase1.c
$ gdb ./phase1
按照 handout 依次输入命令并观察结果。
# 打断点
(gdb) break main
Breakpoint 1 at 0x13b2: file phase1.c, line 54.
(gdb) break unscramble
Breakpoint 2 at 0x1308: file phase1.c, line 39.
(gdb) break reverse
Breakpoint 3 at 0x1274: file phase1.c, line 30.
(gdb) break toggleCase
Breakpoint 4 at 0x11e7: file phase1.c, line 18.
(gdb) info break
Num Type Disp Enb Address What
1 breakpoint keep y 0x00000000000013b2 in main at phase1.c:54
2 breakpoint keep y 0x0000000000001308 in unscramble at phase1.c:39
3 breakpoint keep y 0x0000000000001274 in reverse at phase1.c:30
4 breakpoint keep y 0x00000000000011e7 in toggleCase at phase1.c:18
# 运行程序 & 打印值
(gdb) run
Starting program: /home/gwen/Documents/report/code/15213/Ch3 Machine Level Programming/lab2_handout/phase1
Breakpoint 1, main () at phase1.c:54
54 int main () {
(gdb) n
55 word_t *secret_msg = (word_t *) calloc(1, sizeof(word_t));
(gdb) n
56 secret_msg->str = (char *) calloc(length, sizeof(char));
(gdb) print *(word_t*)secret_msg
$1 = {str = 0x0, len = 0}
(gdb) next
57 strncpy(secret_msg->str, "!312 Av XPHy QBBt", length);;
(gdb) print *(word_t*)secret_msg
$2 = {str = 0x5555555592c0 "", len = 0}
# 单步调试 & watchpoint
Breakpoint 2, unscramble (msg=0x555555555480 <__libc_csu_init>) at phase1.c:39
39 void unscramble (word_t *msg) {
(gdb) step
41 char chng = 13;
(gdb) next
42 for (int i = 0; i < msg->len; i++) {
(gdb) next
43 ltr = msg->str[i];
(gdb) print(ltr)
$3 = 85 'U'
(gdb) print isAlpha(ltr)
$4 = true
(gdb) watch ltr
Hardware watchpoint 5: ltr
(gdb) c
Continuing.
Hardware watchpoint 5: ltr
Old value = 85 'U'
New value = 33 '!'
unscramble (msg=0x5555555592a0) at phase1.c:44
44 if (isAlpha(ltr)) {
(gdb) step
isAlpha (ltr=85 'U') at phase1.c:13
13 bool isAlpha (char ltr) {
(gdb) backtrace
#0 isAlpha (ltr=85 'U') at phase1.c:13
#1 0x0000555555555345 in unscramble (msg=0x5555555592a0) at phase1.c:44
#2 0x0000555555555429 in main () at phase1.c:60
# 3 Questions
- What does
unscramble(word_t *msg)do? And how?
将secret_msg指向的结构体中的!312 Av XPHy QBBt替换成了!312 Ni KCUl DOOg。
unscramble的作用是：判断msg_str当前的字符，如果M < ltr <= Z，就将其字符值减去 13。否则加上 13。 - What does
reverse(word_t *msg)do? And how?
将secret_msg指向的结构体中的!312 Ni KCUl DOOg替换成了gOOD lUCK iN 213!。该函数反转字符串。 - What does
toggleCase(word_t *msg)do? And how?
将secret_msg指向的结构体中的gOOD lUCK iN 213!替换成了Good Luck In 213!。该函数反转字符串大小写。
# Part II
输入 gcc -std=c99 -o phase2 -g phase2.c 编译生成 phase2 ，对其利用 gdb 进行排错发现 S->top->len 为 0，导致浮点数相除时发生错误。我们进行如下修改：
- 在初始化栈的时候让栈顶的长度为 0
push新结点的时候让结点的len为栈顶结点的长度加 1，然后让栈顶指向结点pop时结点弹出，栈顶指向的新结点的长度值为弹出结点长度减 1
