使用“$_thread”变量

#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从7.2版本引入了$_thread这个“convenience variable”，用来保存当前正在调试的线程号。这个变量在写断点命令或是命令脚本时会很有用。以上面程序为例:

(gdb) wa a
Hardware watchpoint 2: a
(gdb) command 2
Type commands for breakpoint(s) 2, one per line.
End with a line saying just "end".
>printf "thread id=%d\n", $_thread
>end

首先设置了观察点：“wa a”（wa是watch命令缩写），也就是当a的值发生变化时，程序会暂停，接下来在commands语句中打印线程号。
然后继续执行程序：

(gdb) c
Continuing.
[New Thread 0x7ffff782c700 (LWP 20928)]
[Switching to Thread 0x7ffff782c700 (LWP 20928)]
Hardware watchpoint 2: a

Old value = 0
New value = 1
thread1_func (p_arg=0x400718) at a.c:11
11                      sleep(10);
thread id=2
(gdb) c
Continuing.
[New Thread 0x7ffff6e2b700 (LWP 20929)]
[Switching to Thread 0x7ffff6e2b700 (LWP 20929)]
Hardware watchpoint 2: a

Old value = 1
New value = 2
thread2_func (p_arg=0x400721) at a.c:20
20                      sleep(10);
thread id=3

可以看到程序暂停时，会打印线程号：“thread id=2”或者“thread id=3”。
参见 gdb手册.