5 请问为什么yield不会造成线程切换

发布于 2021-06-09 14:31:56 浏览：1196 订阅该版

[FJ_rt_thread_mq.zip](https://oss-club.rt-thread.org/uploads/20210609/88d9a147e4b67adb29f448dded03277c.zip)
附件是代码我练习信号量，新建了2个线程
```c
static void sem_send_thread_entry(void* parameter)
{
	rt_err_t uwRet = RT_ERROR;
	while (1)
	{
		uwRet = rt_sem_take(test_sem,RT_WAITING_FOREVER);
		if(uwRet != RT_ERROR)
		{
			rt_kprintf("send得到信号量");
		}
		ucValue[0]++;
		ucValue[1]++;
		rt_sem_release(test_sem);
		rt_thread_mdelay(500);
//		rt_thread_yield();
	}
}
```

```c
static void sem_rev_thread_entry(void* parameter)
{
	rt_err_t uwRet = RT_ERROR;
	while (1)
	{
		uwRet = rt_sem_take(test_sem,RT_WAITING_FOREVER);
		if(uwRet != RT_ERROR)
		{
			rt_kprintf("rev得到信号量");
		}
		if(ucValue[0] == ucValue[1])
		{
			rt_kprintf("sem ok %d = %d\n",ucValue[0],ucValue[1]);
		}
		else
		{
			rt_kprintf("fail\n");
		}
		rt_sem_release(test_sem);
		rt_thread_mdelay(500);
	}
}
```
代码是在send线程里变量自增，在rev线程里判断是否相等。
在优先级相同的情况下，send线程中如果使用`rt_thread_yield();`  
则会一直显示send得到信号量

如果使用`rt_thread_mdelay(500);`
则会正常连续打印变量的值，并且send 和rev交替得到信号量。

请问为什么yield不会造成线程切换呢？谢谢

NOUSED 2021-06-09

这家伙很懒，什么也没写！

伊森亨特 2021-06-09

这家伙很懒，什么也没写！

为啥咱俩的不一样，我是nano3.1.3
```c
rt_err_t rt_thread_yield(void)
{
    register rt_base_t level;
    struct rt_thread *thread;

/* disable interrupt */
    level = rt_hw_interrupt_disable();

/* set to current thread */
    thread = rt_current_thread;

/* if the thread stat is READY and on ready queue list */
    if ((thread->stat & RT_THREAD_STAT_MASK) == RT_THREAD_READY &&
        thread->tlist.next != thread->tlist.prev)
    {
        /* remove thread from thread list */
        rt_list_remove(&(thread->tlist));

/* put thread to end of ready queue */
        rt_list_insert_before(&(rt_thread_priority_table[thread->current_priority]),
                              &(thread->tlist));

/* enable interrupt */
        rt_hw_interrupt_enable(level);

rt_schedule();

return RT_EOK;
    }

/* enable interrupt */
    rt_hw_interrupt_enable(level);

return RT_EOK;
}
```
除此之外我觉得不是这个原因，我让send线程yeild，然后把send线程优先级改成24，rev优先级改成3，这样yeild一下，rev优先级高，会执行rev线程吧，可是我看打印出来的还是 send得到信号量。
![image.png](https://oss-club.rt-thread.org/uploads/20210609/e687b1f2de3ef19523a0169082d31a3a.png)

NOUSED 2021-06-09

这家伙很懒，什么也没写！

@伊森亨特 
```
uint8_t ucValue[2];
rt_sem_t test_sem;

static void sem_send_thread_entry(void* parameter)
{
    rt_err_t uwRet = RT_ERROR;
    while (1)
    {
        uwRet = rt_sem_take(test_sem,RT_WAITING_FOREVER);
        if(uwRet != RT_ERROR)
        {
            rt_kprintf("send\n");
        }
        ucValue[0]++;
        ucValue[1]++;
        rt_sem_release(test_sem);
        rt_thread_yield();
    }
}

static void sem_rev_thread_entry(void* parameter)
{
    rt_err_t uwRet = RT_ERROR;
    while (1)
    {
        uwRet = rt_sem_take(test_sem,RT_WAITING_FOREVER);
        if(uwRet != RT_ERROR)
        {
            rt_kprintf("rev\n");
        }
        if(ucValue[0] == ucValue[1])
        {
            rt_kprintf("sem ok %d = %d\n",ucValue[0],ucValue[1]);
        }
        else
        {
            rt_kprintf("fail\n");
        }
        rt_sem_release(test_sem);
        rt_thread_mdelay(50);
    }
}

int main(void)
{
    printf("hello rt-thread\n");

test_sem = rt_sem_create("sem", 1, RT_IPC_FLAG_PRIO);

rt_thread_t send = rt_thread_create( "send",
                                        sem_send_thread_entry,
                                        RT_NULL,
                                        2048,
                                        24,
                                        100 );

rt_thread_t recv = rt_thread_create( "recv",
                                        sem_rev_thread_entry,
                                        RT_NULL,
                                        2048,
                                        24,
                                        100 );

rt_thread_startup(send);
    rt_thread_startup(recv);

return 0;
}
```

![image.png](https://oss-club.rt-thread.org/uploads/20210609/7938e14be47bae3c6c26980d778e9323.png)