設置觀察點

例子

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
int a = 0;

void *thread1_func(void *p_arg)
{
        while (1)
        {
                a++;
                sleep(10);
        }
}

int main(int argc, char* argv[])
{
        pthread_t t1;

        pthread_create(&t1, NULL, thread1_func, "Thread 1");
		
        sleep(1000);
        return 0;
}

技巧

gdb可以使用“watch”命令設置觀察點，也就是當一個變量值發生變化時，程序會停下來。以上面程序為例:

(gdb) start
Temporary breakpoint 1 at 0x4005a8: file a.c, line 19.
Starting program: /data2/home/nanxiao/a
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Temporary breakpoint 1, main () at a.c:19
19              pthread_create(&t1, NULL, thread1_func, "Thread 1");
(gdb) watch a
Hardware watchpoint 2: a
(gdb) r
Starting program: /data2/home/nanxiao/a
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[New Thread 0x7ffff782c700 (LWP 8813)]
[Switching to Thread 0x7ffff782c700 (LWP 8813)]
Hardware watchpoint 2: a

Old value = 0
New value = 1
thread1_func (p_arg=0x4006d8) at a.c:11
11                      sleep(10);
(gdb) c
Continuing.
Hardware watchpoint 2: a

Old value = 1
New value = 2
thread1_func (p_arg=0x4006d8) at a.c:11
11                      sleep(10);

可以看到，使用“watch a”命令以後，當a的值變化：由0變成1，由1變成2，程序都會停下來。
此外也可以使用“watch *(data type*)address”這樣的命令，仍以上面程序為例:

(gdb) p &a
$1 = (int *) 0x6009c8 <a>
(gdb) watch *(int*)0x6009c8
Hardware watchpoint 2: *(int*)0x6009c8
(gdb) r
Starting program: /data2/home/nanxiao/a
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[New Thread 0x7ffff782c700 (LWP 15431)]
[Switching to Thread 0x7ffff782c700 (LWP 15431)]
Hardware watchpoint 2: *(int*)0x6009c8

Old value = 0
New value = 1
thread1_func (p_arg=0x4006d8) at a.c:11
11                      sleep(10);
(gdb) c
Continuing.
Hardware watchpoint 2: *(int*)0x6009c8

Old value = 1
New value = 2
thread1_func (p_arg=0x4006d8) at a.c:11
11                      sleep(10);

先得到a的地址：0x6009c8，接著用“watch *(int*)0x6009c8”設置觀察點，可以看到同“watch a”命令效果一樣。
觀察點可以通過軟件或硬件的方式實現，取決於具體的系統。但是軟件實現的觀察點會導致程序運行很慢，使用時需注意。參見gdb手冊.

如果系統支持硬件觀測的話，當設置觀測點是會打印如下信息： Hardware watchpoint num: expr

如果不想用硬件觀測點的話可如下設置： set can-use-hw-watchpoints

查看斷點

列出當前所設置了的所有觀察點：
info watchpoints

watch 所設置的斷點也可以用控制斷點的命令來控制。如 disable、enable、delete等

貢獻者

nanxiao