生产者消费者模型（条件变量）

三种关系：互斥，同步，互斥和同步
两类角色:生产者，消费者（线程）
一个交易场所：生产者消费者共享的区域

卖苹果的模型

• dish上面只有一个苹果
• 买家必须要等卖家把苹果放到dish上才可以去买苹果。
• 卖家必须等待买家把苹果买走才可以生产苹果
• pthread_mutex_lock(&mutex); 和pthread_mutex_unlock(&mutex); 成对出现，里面的操作为一个原子操作
• pthread_cond_wait(&empty,&mutex);内部有一个加锁解锁操作
• 过程详解
  
  • 苹果为0
  • 初始化时苹果为0，买家在pthread_cond_wait(&empty,&mutex); 开始等待，卖家不进入while循环，生产苹果后发出pthread_cond_signal(&empty);让正在因dish上苹果为0的买家停止等待pthread_cond_wait(&empty,&mutex);
  • 苹果为1
  • 当一个卖家进入pthread_mutex_lock(&mutex); 但是发现苹果为1时，就在while循环处等待pthread_cond_wait(&empty,&mutex);，直到买家发出信号 pthread_cond_signal(&full);卖家停止等待，此时苹果为0，跳出while循环，开始生产苹果。生产苹果后发出pthread_cond_signal(&empty);让正在因dish上苹果为0的买家停止等待pthread_cond_wait(&empty,&mutex); 这样这个过程就结束了
    *

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>

int dish = 0;
pthread_cond_t full;
pthread_cond_t empty;
pthread_mutex_t mutex;


void * consumer(void * arg)
{
    int id = (int)arg;
    while(1)
    {
        pthread_mutex_lock(&mutex);//上锁
        while(dish == 0)
        {
            //买家发现没有苹果开始等待
            pthread_cond_wait(&empty,&mutex);
        }
    usleep(100000);

    dish = 0;
    printf("consumer %d get an apple!!\n", id);
    //给卖家发送信号
    pthread_cond_signal(&full);
    pthread_mutex_unlock(&mutex);//解锁

    }
    return NULL;
}




void * product(void *arg)
{
    int id = (int)arg;
    while(1)
    {
        pthread_mutex_lock(&mutex);    
        while(dish == 1)
            {
                pthread_cond_wait(&full,&mutex);
            }
            dish =1;
             printf("producer %d put an apple!!\n", id);      
        pthread_cond_signal(&empty);
        pthread_mutex_unlock(&mutex);
    }
    return NULL;
}


int main()
{
    pthread_t tid;
    int set ;
    //初始化
    pthread_cond_init(&full,NULL);
    pthread_cond_init(&empty,NULL);
    pthread_mutex_init(&mutex,NULL);

    //四个卖家生产苹果
    for(int i = 0;i<4;i++)
    {
        pthread_create(&tid,NULL,product,(void *)i);
    }
    //四个买家买苹果
    for(int i = 0;i<4;i++)
    {
        pthread_create(&tid,NULL,consumer,(void *)i);
    }

    //等待thread标识的线程终止，防止僵尸进程产生
    pthread_join(tid,NULL);

    //销毁所有资源
    pthread_cond_destroy(&full);
    pthread_cond_destroy(&empty);
    pthread_mutex_destroy(&mutex);
}

如果对于互斥量含义尚不清楚，建议阅读Linux/UNIX系统编程手册

pthread_mutex_t
pthread_mutex_init
pthread_mutex_lock
pthread_mutex_unlock
pthread_mutex_destroy

模型采取Linux/UNIX系统编程手册
这个模型很好，也把概念讲清楚了，加了中文注释

生产者

s = pthread_mutex_lock(&mtx);
if (s != 0)
    errExitEN(s, "pthread_mutex_lock");

avail++;        /*这是一个原子操作 */

s = pthread_mutex_unlock(&mtx);
if (s != 0)
    errExitEN(s, "pthread_mutex_unlock");

s = pthread_cond_signal(&cond);         /* 唤醒消费者 */
if (s != 0)
    errExitEN(s, "pthread_cond_signal");

消费者

s = pthread_mutex_lock(&mtx);
if (s != 0)
    errExitEN(s, "pthread_mutex_lock");

while (avail == 0)
 {           
    s = pthread_cond_wait(&cond, &mtx);
    if (s != 0)
        errExitEN(s, "pthread_cond_wait");
}

完整代码

#include <time.h>
#include <pthread.h>
#include "tlpi_hdr.h"

static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

static int avail = 0;

static void *
threadFunc(void *arg)
{
    int cnt = atoi((char *) arg);
    int s, j;

    for (j = 0; j < cnt; j++) {
        sleep(1);

        /* Code to produce a unit omitted */

        s = pthread_mutex_lock(&mtx);
        if (s != 0)
            errExitEN(s, "pthread_mutex_lock");

        avail++;        /* Let consumer know another unit is available */

        s = pthread_mutex_unlock(&mtx);
        if (s != 0)
            errExitEN(s, "pthread_mutex_unlock");

        s = pthread_cond_signal(&cond);         /* Wake sleeping consumer */
        if (s != 0)
            errExitEN(s, "pthread_cond_signal");
    }

    return NULL;
}

int
main(int argc, char *argv[])
{
    pthread_t tid;
    int s, j;
    int totRequired;            /* Total number of units that all threads
                                   will produce */
    int numConsumed;            /* Total units so far consumed */
    Boolean done;
    time_t t;

    t = time(NULL);

    /* Create all threads */

    totRequired = 0;
    for (j = 1; j < argc; j++) {
        totRequired += atoi(argv[j]);

        s = pthread_create(&tid, NULL, threadFunc, argv[j]);
        if (s != 0)
            errExitEN(s, "pthread_create");
    }

    /* Loop to consume available units */

    numConsumed = 0;
    done = FALSE;

    for (;;) {
        s = pthread_mutex_lock(&mtx);
        if (s != 0)
            errExitEN(s, "pthread_mutex_lock");

        while (avail == 0) {            /* Wait for something to consume */
            s = pthread_cond_wait(&cond, &mtx);
            if (s != 0)
                errExitEN(s, "pthread_cond_wait");
        }

        /* At this point, 'mtx' is locked... */

        while (avail > 0) {             /* Consume all available units */

            /* Do something with produced unit */

            numConsumed ++;
            avail--;
            printf("T=%ld: numConsumed=%d\n", (long) (time(NULL) - t),
                    numConsumed);

            done = numConsumed >= totRequired;
        }

        s = pthread_mutex_unlock(&mtx);
        if (s != 0)
            errExitEN(s, "pthread_mutex_unlock");

        if (done)
            break;

        /* Perhaps do other work here that does not require mutex lock */

    }

    exit(EXIT_SUCCESS);
}