设置观察点只针对特定线程生效

例子

#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)
        {
                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“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，其它编号线程改变变量的值不会让程序停住。以上面程序为例:

(gdb) start
Temporary breakpoint 1 at 0x4005d4: file a.c, line 28.
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:28
28              pthread_create(&t1, NULL, thread1_func, "Thread 1");
(gdb) n
[New Thread 0x7ffff782c700 (LWP 25443)]
29              pthread_create(&t2, NULL, thread2_func, "Thread 2");
(gdb)
[New Thread 0x7ffff6e2b700 (LWP 25444)]
31              sleep(1000);
(gdb) i threads
  Id   Target Id         Frame
  3    Thread 0x7ffff6e2b700 (LWP 25444) 0x00007ffff7915911 in clone () from /lib64/libc.so.6
  2    Thread 0x7ffff782c700 (LWP 25443) 0x00007ffff78d9bcd in nanosleep () from /lib64/libc.so.6
* 1    Thread 0x7ffff7fe9700 (LWP 25413) main () at a.c:31
(gdb) wa a thread 2
Hardware watchpoint 2: a
(gdb) c
Continuing.
[Switching to Thread 0x7ffff782c700 (LWP 25443)]
Hardware watchpoint 2: a

Old value = 1
New value = 3
thread1_func (p_arg=0x400718) at a.c:11
11                      sleep(10);
(gdb) c
Continuing.
Hardware watchpoint 2: a

Old value = 3
New value = 5
thread1_func (p_arg=0x400718) at a.c:11
11                      sleep(10);
(gdb) c
Continuing.
Hardware watchpoint 2: a

Old value = 5
New value = 7
thread1_func (p_arg=0x400718) at a.c:11
11                      sleep(10);

可以看到，使用“wa a thread 2”命令（wa是watch命令的缩写）以后，只有thread1_func改变a的值才会让程序停下来。
需要注意的是这种针对特定线程设置观察点方式只对硬件观察点才生效，参见gdb手册.

贡献者

nanxiao