浅谈生产者消费者模型(Linux系统下的两种实现方法)
<p>生产者消费者问题是同步问题中的一种常见情况，借用一下维基百科的话</p>
<p>
生产者消费者问题（英语：Producer-consumer problem），也称有限缓冲问题（英语：Bounded-buffer problem），是一个多线程同步问题的经典案例。该问题描述了两个共享固定大小缓冲区的线程——即所谓的“生产者”和“消费者”——在实际运行时会发生的问题。生产者的主要作用是生成一定量的数据放到缓冲区中，然后重复此过程。与此同时，消费者也在缓冲区消耗这些数据。该问题的关键就是要保证生产者不会在缓冲区满时加入数据，消费者也不会在缓冲区中空时消耗数据。</p>
<p>
<span><strong>第一种实现信号量配合互斥锁实现，这种方法很清晰简单</strong></span></p>
<p>
信号量：</p>
<p>
信号量的特性如下：信号量是一个非负整数（车位数），所有通过它的线程/进程（车辆）都会将该整数减一（通过它当然是为了使用资源），当该整数值为零时，所有试图通过它的线程都将处于等待状态。在信号量上我们定义两种操作： Wait（等待） 和 Release（释放）。当一个线程调用Wait操作时，它要么得到资源然后将信号量减一，要么一直等下去（指放入阻塞队列），直到信号量大于等于一时。Release（释放）实际上是在信号量上执行加操作，对应于车辆离开停车场，该操作之所以叫做“释放”是因为释放了由信号量守护的资源。</p>
<p>
<span><strong>wait, release在Linux下</strong></span></p>
<p>
int sem_wait(sem_t * sem);<br>
int sem_post(sem_t * sem);</p>
<p>
设定两个信号量，empty用来表示空槽的个数，full用来表示占有的个数</p>
<p>
生产者在向任务队列里放资源时，调用sem_wait(&full)来检查队列是否已满，如果满的话，就阻塞，直到有消费者从里面取资源再苏醒，如果不满，就放资源，并通知消费者来取。</p>
<p>
消费者在从任务队列里取资源时，调用sem_wait(&empty)来检查队列是否为空，如果空的话，就阻塞，直到有生产者向里面放资源再苏醒，如果不空，就取资源，并通知生产者来放。</p>
<p>
而互斥锁仅仅是为了防止多个线程同时对队列进行操作，造成未知的结果。</p>
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<code class="xhtml plain">#include <</code><code class="xhtml keyword">stdio.h</code><code class="xhtml plain">></code>
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<code class="xhtml plain">#include <</code><code class="xhtml keyword">pthread.h</code><code class="xhtml plain">></code>
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<code class="xhtml plain">#include <</code><code class="xhtml keyword">semaphore.h</code><code class="xhtml plain">></code>
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<code class="xhtml plain">#define MAX 5 //队列长度</code>
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<code class="xhtml plain">pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;</code>
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<code class="xhtml plain">sem_t full; //填充的个数</code>
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<code class="xhtml plain">sem_t empty; //空槽的个数</code>
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<code class="xhtml plain">int top = 0; //队尾</code>
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<code class="xhtml plain">int bottom = 0; //队头</code>
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<code class="xhtml plain">void* produce(void* arg)</code>
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<code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int i;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">for ( i = 0; i < MAX*2; i++)</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">printf("producer is preparing data\n");</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">sem_wait(&empty);//若空槽个数低于0阻塞</code>
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<code class="xhtml spaces"> </code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_lock(&mutex);</code>
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<code class="xhtml spaces"> </code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">top = (top+1) % MAX;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">printf("now top is %d\n", top);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_unlock(&mutex);</code>
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<code class="xhtml spaces"> </code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">sem_post(&full);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">}</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">return (void*)1;</code>
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<code class="xhtml plain">}</code>
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<code class="xhtml plain">void* consume(void* arg)</code>
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<code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int i;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">for ( i = 0; i < MAX*2; i++)</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">printf("consumer is preparing data\n");</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">sem_wait(&full);//若填充个数低于0阻塞</code>
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<code class="xhtml spaces"> </code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_lock(&mutex);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">bottom = (bottom+1) % MAX;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">printf("now bottom is %d\n", bottom);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_unlock(&mutex);</code>
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<code class="xhtml spaces"> </code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">sem_post(&empty);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">}</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">return (void*)2;</code>
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<code class="xhtml plain">}</code>
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<code class="xhtml plain">int main(int argc, char *argv[])</code>
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<code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_t thid1;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_t thid2;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_t thid3;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_t thid4;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int ret1;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int ret2;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int ret3;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int ret4;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">sem_init(&full, 0, 0);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">sem_init(&empty, 0, MAX);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_create(&thid1, NULL, produce, NULL);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_create(&thid2, NULL, consume, NULL);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_create(&thid3, NULL, produce, NULL);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_create(&thid4, NULL, consume, NULL);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_join(thid1, (void**)&ret1);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_join(thid2, (void**)&ret2);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_join(thid3, (void**)&ret3);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_join(thid4, (void**)&ret4);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">return 0;</code>
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<code class="xhtml plain">}</code>
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<p>
注：如果把sem_wait()和sem_post()放到pthread_mutex_lock()与pthread_mutex_unlock()之间会如何呢？</p>
<p>
答案是：死锁，因为我们不能预知线程进入共享区顺序，如果消费者线程先对mutex加锁，并进入，sem_wait()发现队列为空，阻塞，而生产者在对mutex加锁时，发现已上锁也阻塞，双方永远无法唤醒对方。</p>
<p>
<span><strong>第二种是条件变量配合互斥锁实现</strong></span></p>
<p>
条件变量的常见用法是在不满足某些条件时，阻塞自己，直到有线程通知自己醒来。</p>
<p>
而互斥量在这里的作用依然还是防止多线程对共享资源同时操作，造成未知结果。</p>
<p>
生产者消费者的行为与之前相同，只不过原来只调用sem_wait()可以完成两步，1是检查条件，2是阻塞，现在条件变量需要我们自己来设定条件(所以说条件变量配合互斥锁比信号量的功能更强大，因为它可以自定义休眠条件，但是这对使用者的要求也提高了，必须理清逻辑关系避免死锁)</p>
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<code class="xhtml plain">#include <</code><code class="xhtml keyword">stdio.h</code><code class="xhtml plain">></code>
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<code class="xhtml plain">#include <</code><code class="xhtml keyword">pthread.h</code><code class="xhtml plain">></code>
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<code class="xhtml plain">#define MAX 5</code>
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<code class="xhtml plain">pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;</code>
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<code class="xhtml plain">pthread_cond_t notfull = PTHREAD_COND_INITIALIZER; //是否队满</code>
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<code class="xhtml plain">pthread_cond_t notempty = PTHREAD_COND_INITIALIZER; //是否队空</code>
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<code class="xhtml plain">int top = 0;</code>
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<code class="xhtml plain">int bottom = 0;</code>
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<code class="xhtml plain">void* produce(void* arg)</code>
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<code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int i;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">for ( i = 0; i < MAX*2; i++)</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_lock(&mutex);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">while ((top+1)%MAX == bottom)</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">printf("full! producer is waiting\n");</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_cond_wait(¬full, &mutex);//等待队不满</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">}</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">top = (top+1) % MAX;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">printf("now top is %d\n", top);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_cond_signal(¬empty);//发出队非空的消息</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_unlock(&mutex);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">}</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">return (void*)1;</code>
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<code class="xhtml plain">}</code>
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<code class="xhtml plain">void* consume(void* arg)</code>
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<code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int i;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">for ( i = 0; i < MAX*2; i++)</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_lock(&mutex);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">while ( top%MAX == bottom)</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">printf("empty! consumer is waiting\n");</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_cond_wait(¬empty, &mutex);//等待队不空</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">}</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">bottom = (bottom+1) % MAX;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">printf("now bottom is %d\n", bottom);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_cond_signal(¬full);//发出队不满的消息</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_unlock(&mutex);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">}</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">return (void*)2;</code>
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<code class="xhtml plain">}</code>
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<code class="xhtml plain">int main(int argc, char *argv[])</code>
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<code class="xhtml plain">{</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_t thid1;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_t thid2;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_t thid3;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_t thid4;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int ret1;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int ret2;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int ret3;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">int ret4;</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_create(&thid1, NULL, produce, NULL);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_create(&thid2, NULL, consume, NULL);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_create(&thid3, NULL, produce, NULL);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_create(&thid4, NULL, consume, NULL);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_join(thid1, (void**)&ret1);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_join(thid2, (void**)&ret2);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_join(thid3, (void**)&ret3);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_join(thid4, (void**)&ret4);</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">return 0;</code>
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<code class="xhtml plain">}</code>
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<p>
<strong>注：</strong></p>
<p>
<span><strong>为什么信号量在互斥区外，而条件变量在互斥区内呢？</strong></span></p>
<p>
因为互斥锁本质上是二元信号量，和信号量互斥的原理相同，而且放在互斥区会死锁，而条件变量是和互斥锁协同配合的，</p>
<p>
我们从pthread_cond_wait()和pthread_cond_signal()的内部实现就可以看出</p>
<p>
pthread_cond_wait()是先将互斥锁解开，并陷入阻塞，直到pthread_signal()发出信号后pthread_cond_wait()再加上锁，然后退出，可以看到它们在设计时就是为了协同配合，而互斥锁和信号量都是由Linux下的futex机制实现的，这里就不展开说了</p>
<p>
这里贴出了pthread_wait()源码图</p>
<p>
<img title="浅谈生产者消费者模型(Linux系统下的两种实现方法)" alt="浅谈生产者消费者模型(Linux系统下的两种实现方法)" src="https://zhuji.jb51.net/uploads/img/202305/42459ab5e8da131f4f83909aaef36f96.jpg"></p>
<p>
以上就是小编为大家带来的浅谈生产者消费者模型(Linux系统下的两种实现方法)全部内容了，希望大家多多支持~</p>
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