设置读观察点

#include <stdio.h>
#include <pthread.h>

int a = 0;

void *thread1_func(void *p_arg)
{
        while (1)
        {
                printf("%d\n", a);
                sleep(10);
        }
}

int main(void)
{
        pthread_t t1;

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

        sleep(1000);
        return;
}

技巧

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，参见 gdb手册.