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gdb 调试-观察点

1. 设置观察点

gdb 通过 watch 命令设置观察点，也就是当一个变量值发生变化时，程序就会停下来。

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// code
int a = 0;
void *thread1_func(void *p_arg) {
    while (1) {
        a++;
        sleep(10);
    }
}
int main(void) {
    pthread_t t1;
    pthread_create(&t1, NULL, thread1_func, (void*)"Thread 1");
    sleep(1000);
    return 0;
}

// gdb 调试
(gdb) watch a
Hardware watchpoint 1: a
(gdb) r
Starting program: /data/code/cpp/test/gdb_test/main 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff77c2700 (LWP 28025)]
[Switching to Thread 0x7ffff77c2700 (LWP 28025)]

Thread 2 "main" hit Hardware watchpoint 1: a

Old value = 0
New value = 1
thread1_func (p_arg=0x555555554834) at main_01.cpp:10
10              sleep(10);

如上代码和调试结果，当 a 的值发生变化时，程序就会停下来。也可以使用地址来监测：watch *(int*)0x555555755014

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(gdb) p &a
$1 = (int *) 0x555555755014 <a>
(gdb) watch *(int*)0x555555755014
Hardware watchpoint 2: *(int*)0x555555755014

2. 设置观察点只针对特定线程生效

使用 watch expr thread threadnum 命令设置观察点只针对特定线程生效，也就是只有编号为 threadnum 的线程改变了变量的值，程序才会停下来。其他编号线程改变变量的值不会让程序停下。 

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(gdb) i threads
  Id   Target Id         Frame 
* 1    Thread 0x7ffff7fcc740 (LWP 28904) "main" main () at main_01.cpp:22
  2    Thread 0x7ffff77c2700 (LWP 28973) "main" 0x00007ffff78a7680 in __GI___nanosleep (requested_time=requested_time@entry=0x7ffff77c1ea0, 
    remaining=remaining@entry=0x7ffff77c1ea0) at ../sysdeps/unix/sysv/linux/nanosleep.c:28
  3    Thread 0x7ffff6fc1700 (LWP 28974) "main" 0x00007ffff78a7680 in __GI___nanosleep (requested_time=requested_time@entry=0x7ffff6fc0ea0, 
    remaining=remaining@entry=0x7ffff6fc0ea0) at ../sysdeps/unix/sysv/linux/nanosleep.c:28
(gdb) watch a thread 3
Hardware watchpoint 3: a
(gdb) c
Continuing.
[Switching to Thread 0x7ffff6fc1700 (LWP 28974)]

Thread 3 "main" hit Hardware watchpoint 3: a

Old value = 2
New value = 4
thread2_func (p_arg=0x55555555488d) at main_01.cpp:15
15              sleep(10);

3. 设置读观察点

使用 rwatch 命令设置读观察点，也就是发生读取变量行为时，程序就会停住。

rwatch a 表示每次访问 a 的值就会让程序停下来。

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// code
int a = 0;
void *thread1_func(void *p_arg) {
    while (1) {
        a++;
        sleep(10);
    }
}
void *thread2_func(void *p_arg) {
    while (1) {
        a++;
        sleep(10);
    }
}
int main(void) {
    pthread_t t1, t2;
    pthread_create(&t1, NULL, thread1_func, (void*)"Thread 1");
    pthread_create(&t2, NULL, thread2_func, (void*)"Thread 2");
    sleep(1000);
    return 0;
}

// gdb 调试
(gdb) rwatch a
Hardware read watchpoint 2: a
(gdb) c
Continuing.
[Switching to Thread 0x7ffff6fc1700 (LWP 29502)]

Thread 3 "main" hit Hardware read watchpoint 2: a

Value = 2
0x0000555555554763 in thread2_func (p_arg=0x55555555488d) at main_01.cpp:14
14              a++;

4. 设置读写观察点

使用 awatch 命令设置读写观察点，也就是发生读取变量或改变变量值的行为，程序就会停住。