【Unix环境高级编程】条件变量

pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);

分析：

•  函数作用：阻塞等待一个条件变量
• 阻塞等待条件变量cond(参数1)满足
• 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}
• 当被唤醒，pthread_cond_wait函数返回，解除阻塞并重新获取互斥锁pthread_mutex_lock(&mutex);

 1. 示例

#include <pthread.h> 
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
 
struct msg
{
    struct msg *next;
    int num;
};
 
struct msg *head;
struct msg *mp;
 
pthread_cond_t has_product = PTHREAD_COND_INITIALIZER;  //静态初始化：一个条件变量和一个互斥量 
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
 
void *consumer(void *p)
{
    for (; ;)
    {
        pthread_mutex_lock(&lock);
        while (head == NULL)                           //头指针为空，说明没有节点 可以为if吗 
            pthread_cond_wait(&has_product, &lock);
        mp = head;
        head = mp->next;                               //模拟消费掉一个产品
        pthread_mutex_unlock(&lock);
        printf("-consume-----%d
", mp->num);
        free(mp);
        sleep(rand() % 5);
    }
}
 
void *producer(void *p)
{
    for (; ;) 
    {
        mp = malloc(sizeof(struct msg));
        mp->num = rand() % 1000 + 1;                  //模拟生产一个产品
        printf("-Produce----%d
", mp->num);
 
        pthread_mutex_lock(&lock);
        mp->next = head;
        head = mp;
        pthread_mutex_unlock(&lock);
 
        pthread_cond_signal(&has_product);          //将等待在该条件变量上的一个线程唤醒
        sleep(rand() % 5);
    }
}
 
int main()
{
    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;
}