设置读写观察点

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

int a = 0;

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

void *thread2_func(void *p_arg)
{
        while (1)
        {
                printf("%d\n", 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;
}

技巧

gdb可以使用“awatch”命令设置读写观察点，也就是当发生读取变量或改变变量值的行为时，程序就会暂停住。以上面程序为例:

(gdb) aw a
Hardware access (read/write) watchpoint 1: 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 16938)]
[Switching to Thread 0x7ffff782c700 (LWP 16938)]
Hardware access (read/write) watchpoint 1: a

Value = 0
0x00000000004005c6 in thread1_func (p_arg=0x40076c) at a.c:10
10                      a++;
(gdb) c
Continuing.
Hardware access (read/write) watchpoint 1: a

Old value = 0
New value = 1
thread1_func (p_arg=0x40076c) at a.c:11
11                      sleep(10);
(gdb) c
Continuing.
[New Thread 0x7ffff6e2b700 (LWP 16939)]
[Switching to Thread 0x7ffff6e2b700 (LWP 16939)]
Hardware access (read/write) watchpoint 1: a

Value = 1
0x00000000004005f2 in thread2_func (p_arg=0x400775) at a.c:19
19                      printf("%d\n", a);;
(gdb) c
Continuing.
1
[Switching to Thread 0x7ffff782c700 (LWP 16938)]
Hardware access (read/write) watchpoint 1: a

Value = 1
0x00000000004005c6 in thread1_func (p_arg=0x40076c) at a.c:10
10                      a++;

可以看到，使用“aw a”命令（aw是awatch命令的缩写）以后，每次读取或改变a的值都会让程序停下来。
需要注意的是awatch命令只对硬件观察点才生效，参见 gdb手册.