【gdb】设置读观察点

设置读观察点

1. 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);
  }
}

void *thread2_func(void *p_arg)
{
  while (1)
  {
    a++;
    sleep(10);
  }
}

int main(void)
{
  pthread_t t1, t2;

  pthread_create(&t1, NULL, thread1_func, "Thread 1");
  pthread_create(&t2, NULL, thread2_func, "Thread 2");

  sleep(1000);
  return 0;
}

gdb可以使用“rwatch”命令设置读观察点，也就是当发生读取变量行为时，程序就会暂停住。以上面程序为例:

(gdb) start
Temporary breakpoint 1 at 0x4005f3: 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) rw a
Hardware read watchpoint 2: a
(gdb) c
Continuing.
[New Thread 0x7ffff782c700 (LWP 5540)]
[Switching to Thread 0x7ffff782c700 (LWP 5540)]
Hardware read watchpoint 2: a

Value = 0
0x00000000004005c6 in thread1_func (p_arg=0x40071c) at a.c:10
10                      printf("%d\n", a);
(gdb) c
Continuing.
0
Hardware read watchpoint 2: a

Value = 0
0x00000000004005c6 in thread1_func (p_arg=0x40071c) at a.c:10
10                      printf("%d\n", a);
(gdb) c
Continuing.
0
Hardware read watchpoint 2: a

Value = 0
0x00000000004005c6 in thread1_func (p_arg=0x40071c) at a.c:10
10                      printf("%d\n", a);

可以看到，使用“rw a”命令（rw是rwatch命令的缩写）以后，每次访问a的值都会让程序停下来。
需要注意的是rwatch命令只对硬件观察点才生效

参考资料

1. 设置读观察点

2. gdb手册