关于条件变量的使用网上有很多的,大部分是使用生产和消费的模型来说明的,或者是哲学家吃饭的问题。
本着学以致用,不断创新的思想,我也调了一个模型。
大概是描述是这样的。
1.创建多个线程;
2.每个线程都是消费者,可能是最后一个消费者,也可能不是最后一个消费者,总之我们关心最后一个消费者,因为它要做些擦屁股的工作;
3.如果不是最后一个消费者,那就该吃吃就行了;
这个场景我也不知道什么时候会用,但是我现在的项目中会用到,如记录一下,示例代码如下:
#include<stdio.h>
#include<pthread.h>
#include<stdlib.h>
#include<unistd.h>
#include<stdint.h>
typedef struct
{
pthread_mutex_t mutex;
pthread_cond_t cond;
uint32_t value;
}shareddata_t;
shareddata_t shareddata;
uint32_t prechannel=0;
uint32_t channel=7;
void *consume(void *ptr)
{
uint32_t *idx=(uint32_t *)ptr;
uint32_t cnt=0;
printf("Enter consume thread [%d] \n",*idx);
while(1)
{
pthread_mutex_lock(&shareddata.mutex);
//0.标记通道
channel &= ~(1<<(*idx));
//1.复制数据
printf("channel [%d] copying data.---cnt[%d]---->\n",*idx,cnt++);
//2.判断整体通道是否已经取完数据
if( channel == 0)
{
//2.1 已经取完数据,说明现在的这个通道是最后一个通道
//因为线程是不按顺序调度的。
//2.2 给其他通道发送信号,告诉它们可以继续执行了。
pthread_cond_broadcast(&shareddata.cond);
printf("[%d] send broadcast.\n",*idx);
prechannel |= (1<<(*idx));
}
else
{
//2.3进入到这里说明本线程不是最后一个获取数据的线程
//既然不是最后一个线程,那就进入睡眠状态,等待最后一个线程来发送信号叫醒。
printf("channel [%d] enter cond-wait\n",*idx);
while(0!=channel)
{
pthread_cond_wait(&shareddata.cond,&shareddata.mutex);
}
printf("channel [%d] exit cond-wait\n",*idx);
printf("enable channel [%d] again.\n",*idx);
prechannel |= (1<<(*idx));
if(prechannel==7)
{
channel=7;
prechannel=0;
}
printf("prechannel=[0x%x],channel=[0x%x]\n\n",prechannel,channel);
}
pthread_mutex_unlock(&shareddata.mutex);
sleep(1);
}
return NULL;
}
int main(void)
{
uint32_t i=0;
uint32_t idx[10];
pthread_t tid[10];
pthread_t protid;
void *retval=NULL;
pthread_mutex_init(&shareddata.mutex,NULL);
pthread_cond_init(&shareddata.cond,NULL);
for(i=0;i<3;i++)
{
idx[i]=i;
pthread_create(&tid[i],NULL,consume,&idx[i]);
}
for(i=0;i<3;i++)
{
pthread_join(tid[i],&retval);
}
return 0;
}
最后运行的结果:
zz@server5:testcond$ ./test
Enter consume thread [0]
channel [0] copying data.---cnt[0]---->
channel [0] enter cond-wait
Enter consume thread [2]
channel [2] copying data.---cnt[0]---->
Enter consume thread [1]
channel [2] enter cond-wait
channel [1] copying data.---cnt[0]---->
[1] send broadcast.
channel [0] exit cond-wait
enable channel [0] again.
prechannel=[0x3],channel=[0x0]
channel [2] exit cond-wait
enable channel [2] again.
prechannel=[0x0],channel=[0x7]
channel [2] copying data.---cnt[1]---->
channel [2] enter cond-wait
channel [1] copying data.---cnt[1]---->
channel [1] enter cond-wait
channel [0] copying data.---cnt[1]---->
[0] send broadcast.
channel [2] exit cond-wait
enable channel [2] again.
prechannel=[0x5],channel=[0x0]
channel [1] exit cond-wait
enable channel [1] again.
prechannel=[0x0],channel=[0x7]
channel [0] copying data.---cnt[2]---->
channel [0] enter cond-wait
channel [1] copying data.---cnt[2]---->
channel [1] enter cond-wait
channel [2] copying data.---cnt[2]---->
[2] send broadcast.
channel [0] exit cond-wait
enable channel [0] again.
prechannel=[0x5],channel=[0x0]
channel [1] exit cond-wait
enable channel [1] again.
prechannel=[0x0],channel=[0x7]
channel [1] copying data.---cnt[3]---->
channel [1] enter cond-wait
channel [0] copying data.---cnt[3]---->
channel [0] enter cond-wait
channel [2] copying data.---cnt[3]---->
[2] send broadcast.
channel [1] exit cond-wait
enable channel [1] again.
prechannel=[0x6],channel=[0x0]
channel [0] exit cond-wait
enable channel [0] again.
prechannel=[0x0],channel=[0x7]
channel [2] copying data.---cnt[4]---->
channel [2] enter cond-wait
channel [1] copying data.---cnt[4]---->
channel [1] enter cond-wait
channel [0] copying data.---cnt[4]---->
[0] send broadcast.
channel [2] exit cond-wait
enable channel [2] again.
prechannel=[0x5],channel=[0x0]
channel [1] exit cond-wait
enable channel [1] again.
prechannel=[0x0],channel=[0x7]
channel [2] copying data.---cnt[5]---->
channel [2] enter cond-wait
channel [0] copying data.---cnt[5]---->
channel [0] enter cond-wait
channel [1] copying data.---cnt[5]---->
[1] send broadcast.
channel [2] exit cond-wait
enable channel [2] again.
prechannel=[0x6],channel=[0x0]
channel [0] exit cond-wait
enable channel [0] again.
prechannel=[0x0],channel=[0x7]
channel [1] copying data.---cnt[6]---->
channel [1] enter cond-wait
channel [2] copying data.---cnt[6]---->
channel [2] enter cond-wait
channel [0] copying data.---cnt[6]---->
[0] send broadcast.
channel [1] exit cond-wait
enable channel [1] again.
prechannel=[0x3],channel=[0x0]
channel [2] exit cond-wait
enable channel [2] again.
prechannel=[0x0],channel=[0x7]
其他消费线程在消费完之后会进入条件变量的等待状态,最后一个消费的线程负责向它们发送同步信号,其他线程接收到同步信号后会继续运行。在本例中继续运行的操作就是重新初始化自己的状态。这里使用了channel和prechannel来标记他们。当channel对应的bit为1表示这个线程还没有进行消费,为0表示已经消费。prechannel是用来辅助实现channel的重新初始化的。