设置观察点

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”命令设置观察点，也就是当一个变量值发生变化时，程序会停下来。以上面程序为例:

(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”命令以后，当a的值变化：由0变成1，由1变成2，程序都会停下来。
此外也可以使用“watch *(data type*)address”这样的命令，仍以上面程序为例:

(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，接着用“watch *(int*)0x6009c8”设置观察点，可以看到同“watch a”命令效果一样。
观察点可以通过软件或硬件的方式实现，取决于具体的系统。但是软件实现的观察点会导致程序运行很慢，使用时需注意。参见gdb手册.

如果系统支持硬件观测的话，当设置观测点是会打印如下信息： Hardware watchpoint num: expr

如果不想用硬件观测点的话可如下设置： set can-use-hw-watchpoints

查看断点

列出当前所设置了的所有观察点：
info watchpoints

watch 所设置的断点也可以用控制断点的命令来控制。如 disable、enable、delete等

贡献者

nanxiao