为什么输出语句中显示线程2发送5次后线程1就结束挂起状态实现一次接收呢？

在官方的消息队列例程中，用来发送消息的线程2每次循环延时5ms，接收消息的线程1延时50ms，为什么输出语句中显示线程2发送5次后线程1就结束挂起状态实现一次接收呢？

#include <rtthread.h>
/* 消息队列控制块 /
static struct rt_messagequeue mq;
/ 消息队列中用到的放置消息的内存池 /
static rt_uint8_t msg_pool[2048];
ALIGN(RT_ALIGN_SIZE)
static char thread1_stack[1024];
static struct rt_thread thread1;
/ 线程 1 入口函数 */
static void thread1_entry(void parameter)
{
char buf = 0;
rt_uint8_t cnt = 0;
while (1)
{
/ 从消息队列中接收消息 /
if (rt_mq_recv(&mq, &buf, sizeof(buf), RT_WAITING_FOREVER) == RT_EOK)
{
rt_kprintf("thread1: recv msg from msg queue, the content:%c\n", buf);
if (cnt == 19)
{
break;
}
}
/ 延时 50ms /
cnt++;
rt_thread_mdelay(50);
}
rt_kprintf("thread1: detach mq \n");
rt_mq_detach(&mq);
}
ALIGN(RT_ALIGN_SIZE)
static char thread2_stack[1024];
static struct rt_thread thread2;
/ 线程 2 入口 */
static void thread2_entry(void parameter)
{
int result;
char buf = 'A';
rt_uint8_t cnt = 0;
while (1)
{
if (cnt == 8)
{
/ 发送紧急消息到消息队列中 /
result = rt_mq_urgent(&mq, &buf, 1);
if (result != RT_EOK)
{
rt_kprintf("rt_mq_urgent ERR\n");
}
else
{
rt_kprintf("thread2: send urgent message - %c\n", buf);
}
}
else if (cnt>= 20)/ 发送 20 次消息之后退出 /
{
rt_kprintf("message queue stop send, thread2 quit\n");
break;
}
else
{
/ 发送消息到消息队列中 /
result = rt_mq_send(&mq, &buf, 1);
if (result != RT_EOK)
{
rt_kprintf("rt_mq_send ERR\n");
}
rt_kprintf("thread2: send message - %c\n", buf);
}
buf++;
cnt++;
/ 延时 5ms /
rt_thread_mdelay(5);
}
}
/ 消息队列示例的初始化 /
int msgq_sample(void)
{
rt_err_t result;
/ 初始化消息队列 /
result = rt_mq_init(&mq,
"mqt",
&msg_pool[0], / 内存池指向 msg_pool /
1, / 每个消息的大小是 1 字节 /
sizeof(msg_pool), / 内存池的大小是 msg_pool 的大小 /
RT_IPC_FLAG_PRIO); / 如果有多个线程等待，优先级大小的方法分配消息 /
if (result != RT_EOK)
{
rt_kprintf("init message queue failed.\n");
return -1;
}
rt_thread_init(&thread1,
"thread1",
thread1_entry,
RT_NULL,
&thread1_stack[0],
sizeof(thread1_stack), 25, 5);
rt_thread_startup(&thread1);
rt_thread_init(&thread2,
"thread2",
thread2_entry,
RT_NULL,
&thread2_stack[0],
sizeof(thread2_stack), 25, 5);
rt_thread_startup(&thread2);
return 0;
}
/ 导出到 msh 命令列表中 */
MSH_CMD_EXPORT(msgq_sample, msgq sample);
仿真结果：

\ | /

RT - Thread Operating System
/ | \ 3.1.0 build Aug 24 2018
2006 - 2018 Copyright by rt-thread team
msh > msgq_sample
msh >thread2: send message - A
thread1: recv msg from msg queue, the content:A
thread2: send message - B
thread2: send message - C
thread2: send message - D
thread2: send message - E
thread1: recv msg from msg queue, the content:B
thread2: send message - F
thread2: send message - G
thread2: send message - H
thread2: send urgent message - I
thread2: send message - J
thread1: recv msg from msg queue, the content:I
thread2: send message - K
thread2: send message - L
thread2: send message - M
thread2: send message - N
thread2: send message - O
thread1: recv msg from msg queue, the content:C
thread2: send message - P
thread2: send message - Q
thread2: send message - R
thread2: send message - S
thread2: send message - T
thread1: recv msg from msg queue, the content:D
message queue stop send, thread2 quit
thread1: recv msg from msg queue, the content:E
thread1: recv msg from msg queue, the content:F
thread1: recv msg from msg queue, the content:G
…
thread1: recv msg from msg queue, the content:T
thread1: detach mq