5.线程与并发同步-条件变量pthread_cond_t 实现生产者消费者

条件变量本身不是锁！但它也可以造成线程阻塞。通常与互斥锁配合使用。给多线程提供一个会合的场所。

使用STL标准库queue队列
入队—模仿生产
出队—模仿消费

pthread_cond_wait函数
阻塞等待一个条件变量
int pthread_cond_wait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex);
函数作用：
1.阻塞等待条件变量cond（参1）满足
2.释放已掌握的互斥锁（解锁互斥量）相当于pthread_mutex_unlock(&mutex);
1.2.两步为一个原子操作。
3.当被唤醒，pthread_cond_wait函数返回时，解除阻塞并重新申请获取互斥锁pthread_mutex_lock(&mutex);

pthread_cond_signal函数
唤醒至少一个阻塞在条件变量上的线程
int pthread_cond_signal(pthread_cond_t *cond);

pthread_cond_broadcast函数
唤醒全部阻塞在条件变量上的线程
int pthread_cond_broadcast(pthread_cond_t *cond);
一个生产者，一个消费者

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

using namespace std;

pthread_cond_t has_product = PTHREAD_COND_INITIALIZER;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

queue<int>my_Queue;  //产品队列

void *consumer(void *p)
{
    for (;;) {
        pthread_mutex_lock(&lock);
        while (my_Queue.empty()) {          
            pthread_cond_wait(&has_product, &lock);
        }
		//模拟消费掉一个产品
        int num=my_Queue.front(); //获取队头元素
		my_Queue.pop();//出队
        pthread_mutex_unlock(&lock);
        
        printf("-Consume ---%d, now total = %lu\n", num,my_Queue.size());
        
        sleep(rand() % 2+1);
    }
}
void *producer(void *p)
{
    while (1) {
	
	    int num= rand() % 1000 + 1;        //模拟生产一个产品
	    
        pthread_mutex_lock(&lock);
        my_Queue.push (num);	//产品入队
        pthread_mutex_unlock(&lock);
        
	   printf("-Produce ---%d, now total = %lu\n", num,my_Queue.size());
        pthread_cond_signal(&has_product);  //将等待在该条件变量上的一个线程唤醒
        
        sleep(rand() % 2);
    }
}
int main(int argc, char *argv[])
{
    pthread_t pid, cid;
    srand(time(NULL));

    pthread_create(&pid, NULL, producer, NULL);
    pthread_create(&cid, NULL, consumer, NULL);

    pthread_join(pid, NULL);
    pthread_join(cid, NULL);
    return 0;
}	

编译输出
g++ s.cpp -pthread
./a.out

-Produce —77, now total = 1
-Consume —77, now total = 0
-Produce —140, now total = 1
-Consume —140, now total = 0
-Produce —231, now total = 1
-Produce —343, now total = 2
-Consume —231, now total = 1
-Produce —426, now total = 2
-Produce —691, now total = 3
-Consume —343, now total = 2
-Produce —414, now total = 3
-Consume —426, now total = 2
-Produce —640, now total = 3
-Produce —357, now total = 4
-Consume —691, now total = 3
-Produce —905, now total = 4
^C

一个生产者，3个消费者

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

using namespace std;

pthread_cond_t has_product = PTHREAD_COND_INITIALIZER;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

queue<int>my_Queue;  //产品队列

void *consumer(void *p)
{
    for (;;) {
        pthread_mutex_lock(&lock);
        while (my_Queue.empty()) {          
            pthread_cond_wait(&has_product, &lock);
        }
		//模拟消费掉一个产品
        int num=my_Queue.front(); //获取队头元素
		my_Queue.pop();//出队
        pthread_mutex_unlock(&lock);
        
        printf("-Consume ---%d, thread id = %lu, now total num = %lu\n", num,pthread_self(),my_Queue.size());//打印消费的产品，消费者线程id，剩余产品数量
        
        sleep(rand() % 2+2);
    }
}
void *producer(void *p)
{
    while (1) {
	
	    int num= rand() % 1000 + 1;        //模拟生产一个产品
	    
        pthread_mutex_lock(&lock);
        my_Queue.push (num);	//产品入队
        pthread_mutex_unlock(&lock);
        
	    printf("-Produce ---%d, now total num = %lu\n", num,my_Queue.size());
        pthread_cond_broadcast(&has_product);  //将等待在该条件变量上的所有线程唤醒
        
        sleep(rand() % 2);
    }
}
int main(int argc, char *argv[])
{
    int i;
    pthread_t pid, cid[3];
    srand(time(NULL));

    pthread_create(&pid, NULL, producer, NULL);
	
	for(i=0;i<3;i++)
        pthread_create(&cid[i], NULL, consumer, NULL);
	

    pthread_join(pid, NULL);
	
	for(i=0;i<3;i++)
        pthread_join(cid[i], NULL);
		
    return 0;
}

g++ cond2.c -pthread
./a.out

-Produce —305, now total num = 1
-Consume —305, thread id = 140698680510208, now total num = 0
-Produce —219, now total num = 1
-Consume —219, thread id = 140698688902912, now total num = 0
-Produce —297, now total num = 1
-Consume —297, thread id = 140698697295616, now total num = 0
-Produce —544, now total num = 1
-Produce —307, now total num = 2
-Consume —544, thread id = 140698680510208, now total num = 1
-Produce —163, now total num = 2
-Produce —997, now total num = 3
-Produce —829, now total num = 4
-Produce —829, now total num = 5
-Consume —307, thread id = 140698688902912, now total num = 4
-Consume —163, thread id = 140698697295616, now total num = 3
-Produce —850, now total num = 4
-Consume —997, thread id = 140698680510208, now total num = 3
-Produce —95, now total num = 4
-Produce —802, now total num = 5
-Consume —829, thread id = 140698688902912, now total num = 4
-Consume —829, thread id = 140698697295616, now total num = 3
-Produce —110, now total num = 4
-Produce —452, now total num = 5
-Produce —827, now total num = 6
^C