pthread 条件变量

淩亂°似流年 2022-09-23 05:52 200阅读 0赞

# pthread条件变量 #

muduo/base/Condition.h中有对条件变量的封装，底层是pthread 条件变量。

## 条件变量 ##

条件变量是线程使用的一种同步机制。条件变量给多个线程提供了会合的场所。**条件变量和互斥量一起用的时候，允许线程以无竞争的方式等待特定的条件发生。**条件是受互斥量保护的。线程在改变条件状态时必须首先对互斥量加锁。

## 相关函数 ##

### 1.初始化与销毁 ###

`pthread_cond_t`数据类型表示条件变量，使用以下两个函数对条件变量进行初始化和销毁：

#include <pthread.h>
    int pthread_cond_init(pthread_cond_t *restrict cond,const pthread_condattr_t *restrict attr);
    int pthread_cond_destroy(pthread_cond_t *cond);

> 注：关键字restrict只用于限定指针；该关键字用于告知编译器，所有修改该指针所指向内容的操作全部都是基于(base on)该指针的，即不存在其它进行修改操作的途径；这样的后果是帮助编译器进行更好的代码优化，生成更有效率的汇编代码。

### 2.等待条件变量 ###

我们使用`pthread_cond_wait`等待条件变量变为真：

#include <pthread.h>
    int pthread_cond_wait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex);

当线程调用`pthread_cond_wait()`函数时，会将调用线程放到等待条件的线程列表上，并原子的对互斥量解锁（这样就不会死锁）。当`pthread_cond_wait()`返回时，互斥量再次锁住。

### 3.通知线程条件满足 ###

我们使用两个函数可以通知线程条件已经满足。`pthread_cond_signal`函数至少能唤醒一个等待该条件的线程。`pthread_cond_broadcast`函数能唤醒等待该条件的所有线程。

#include <pthread.h>
    int pthread_cond_signal(pthread_cond_t * cond);
    int pthread_cond_broadcast(pthread_cond_t * cond);

这两个函数都可以给条件等待线程发信号，不过需要注意的是，**一定要在改变条件状态以后再给线程发信号。**

## 示例 ##

#include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    
    #define NUM_THREADS  3
    #define TCOUNT 10
    #define COUNT_LIMIT 12
    
    int     count = 0;
    pthread_mutex_t count_mutex;
    pthread_cond_t count_threshold_cv;
    
    void *inc_count(void *t) //增加count
    {
        int i;
        long my_id = (long)t;
    
        for (i=0; i < TCOUNT; i++)
        {
            pthread_mutex_lock(&count_mutex);
            count++;
    
            if (count == COUNT_LIMIT) //满足条件后
            {
                printf("inc_count(): thread %ld, count = %d  Threshold reached. ",my_id, count);
                pthread_cond_signal(&count_threshold_cv);//通知
                printf("Just sent signal.\n");
            }
            printf("inc_count(): thread %ld, count = %d, unlocking mutex\n",my_id, count);
    
            //这里释放锁的同时 sleep 1 秒中可以保证线程2和线程3交替获得锁并执行
            pthread_mutex_unlock(&count_mutex);
            sleep(1);
        }//end for
        pthread_exit(NULL);
    }
    
    void *watch_count(void *t) //检查条件变量
    {
        long my_id = (long)t;
    
        printf("Starting watch_count(): thread %ld\n", my_id);
    
        pthread_mutex_lock(&count_mutex);
        while (count < COUNT_LIMIT)//这里用while防止虚假唤醒
        {
            printf("watch_count(): thread %ld Count= %d. Going into wait...\n", my_id,count);
            pthread_cond_wait(&count_threshold_cv, &count_mutex);//阻塞后自动释放锁
            printf("watch_count(): thread %ld Condition signal received. Count= %d\n", my_id,count);
            printf("watch_count(): thread %ld Updating the value of count...\n", my_id,count);
            count += 125;
            printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
        }
        printf("watch_count(): thread %ld Unlocking mutex.\n", my_id);
        pthread_mutex_unlock(&count_mutex);
        pthread_exit(NULL);
    }
    
    int main(int argc, char *argv[])
    {
        int i, rc; 
        long t1=1, t2=2, t3=3;
        pthread_t threads[3];//3个线程
        pthread_attr_t attr;//attr
    
        /* 初始化 mutex 和 condition variable  */
        pthread_mutex_init(&count_mutex, NULL); 
        pthread_cond_init (&count_threshold_cv, NULL);
    
        pthread_attr_init(&attr);
        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
        pthread_create(&threads[0], &attr, watch_count, (void *)t1);//线程1关注count值
        pthread_create(&threads[1], &attr, inc_count, (void *)t2);//线程2增加count值
        pthread_create(&threads[2], &attr, inc_count, (void *)t3);//线程3增加count值
    
        for (i = 0; i < NUM_THREADS; i++) 
        {
            pthread_join(threads[i], NULL);
        }//等待所有线程完成
        printf ("Main(): Waited and joined with %d threads. Final value of count = %d. Done.\n", NUM_THREADS, count);
    
        /* Clean up and exit */
        pthread_attr_destroy(&attr);
        pthread_mutex_destroy(&count_mutex);
        pthread_cond_destroy(&count_threshold_cv);
        pthread_exit (NULL);
    
    }