第十三號艦隊
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Reverse debugging for gdb

Posted on Edited on In ,

在gdb 7.0之後加入的Reverse debugging，終於有時間可以玩玩看了。
同樣的，來個程式範例。

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#include <stdio.h>

void foo() {
     printf("inside foo()");
     int x = 6;
     x += 2;
}

int main() {
     int x = 0;
     x = x + 2;
     foo();
     printf("x = %d\n", x);
     x = 4;
     return 0;
}

編譯它

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gdb -g -o test test.c

使用gdb來玩玩看，最簡單的用法

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$ gdb test
(gdb) start
Temporary breakpoint 1, main () at test.c:10
10	     int x = 0;
(gdb) record
(gdb) next
11	     x = x + 2;
(gdb) disp x
1: x = 0
(gdb) next
12	     foo();
1: x = 2
(gdb) reverse-next
11	     x = x + 2;
1: x = 0

在使用之前，要先用record記錄操作順序，才有機會還原狀態。不用的時候可以使用record stop停止。
接著常用的step, next, stepi跟nexti都有逆向版。可以參考GDB and Reverse Debugging。

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(gdb) b 15
Breakpoint 2 at 0x400578: file test.c, line 15.
(gdb) continue
Continuing.
inside foo()x = 2

Breakpoint 2, main () at test.c:15
15	     return 0;
(gdb) b foo
Breakpoint 3 at 0x400524: file test.c, line 4.
(gdb) reverse-continue
Continuing.

Breakpoint 3, foo () at test.c:4
4	     printf("inside foo()");

想要追蹤之前執行過的中斷點該怎麼辦？ 使用reverse-continue。
也可以用watchpointer來追蹤到底如何被修改的

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(gdb) set can-use-hw-watchpoints 0
(gdb) watch x
Watchpoint 3: x
(gdb) continue
Continuing.
Watchpoint 3: x

Old value = 0
New value = 2
(gdb) reverse-continue
Continuing.
Watchpoint 3: x

Old value = 2
New value = 0
main () at test.c:11
11	     x = x + 2;

更多的gdb使用教學，可以參考