互斥锁实现生产者消费者队列

在muduo里经常看到生产者消费者模型，所以用互斥锁和条件变量实现了一些。

一些tips:如果使用循环的话，生产者和消费者线程个数应该相等，这样才能做到供需平衡。

还有对于pthread_cond_wait这个函数的讲解，为什么要传递两个参数进去呢？比较好奇的是为什么传递互斥锁变量上去，因为在调用pthread_cond_wait的时候，会先对互斥锁变量mutex进行解锁，然后当前面的条件变量满足以后，会重新获得互斥锁。

还有就是pthread_create传递参数的用法，写个小的错误实例。

先上传一份有问题的代码：

#include<pthread.h>
#include<iostream>
#include<stdio.h>
using namespace std;

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t notEmpty = PTHREAD_COND_INITIALIZER;
pthread_cond_t notFull = PTHREAD_COND_INITIALIZER;

int top ,bottom;

const int MAX = 5;

void* produce(void* arg)
{
    int thread_number  = *(int*)arg;
    for(int i = 0; i< MAX*2; i++)
    {
        pthread_mutex_lock(&mutex);
        while((top+1)%MAX == bottom)
        {
            cout << "the area is full,please consume" <<endl;
            pthread_cond_wait(&notFull, &mutex);
        } 
        top = (top+1) % MAX;
        cout << "pthread " << thread_number << " produce, "<<"now top is " << top <<endl;
        pthread_cond_signal(&notEmpty);
        pthread_mutex_unlock(&mutex);
    }
}

void* consume(void* arg)
{
    int thread_number  = *(int*)arg;
    for(int i = 0; i < MAX*2; i++)
    {
        pthread_mutex_lock(&mutex);
        while(top % MAX == bottom)
        {
            cout << "the area is empty"<<endl;
            pthread_cond_wait(&notEmpty, &mutex);
        }
        bottom = (bottom+1)%MAX;
        cout << "pthread " << thread_number << " consume, " << "now bottom is " << bottom <<endl;
        pthread_cond_signal(&notFull);
        pthread_mutex_unlock(&mutex);
    }
}


int main()
{
    pthread_t pid[MAX-1];
    top = 0;
    bottom = 0;
    for(int i = 0; i < MAX-1; i++)
    {
        if(i%2 == 0) pthread_create(&pid[i], NULL, produce, &i);
        else pthread_create(&pid[i], NULL, consume, &i);
    }

    
    for(int i = 0 ; i < MAX-1; i++)
        pthread_join(pid[i], NULL);
    return 0;
}

看一下运行结果有点意外：

在这里插入图片描述
为什么所有的线程的号码都是4呢，

你看我们传递的是i的地址，但是运行的太快，所以最后i的地址的值变为4，然后初始化函数都没来得及执行

所以我们应该传递一个地址里的值不会变的值进去。

更新后的代码：

#include<pthread.h>
#include<iostream>
#include<stdio.h>
using namespace std;

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t notEmpty = PTHREAD_COND_INITIALIZER;
pthread_cond_t notFull = PTHREAD_COND_INITIALIZER;

int top ,bottom;

const int MAX = 5;

void* produce(void* arg)
{
    int thread_number  = *(int*)arg;
    for(int i = 0; i< MAX*2; i++)
    {
        pthread_mutex_lock(&mutex);
        while((top+1)%MAX == bottom)
        {
            cout << "the area is full,please consume" <<endl;
            pthread_cond_wait(&notFull, &mutex);
        } 
        top = (top+1) % MAX;
        cout << "pthread " << thread_number << " produce, "<<"now top is " << top <<endl;
        pthread_cond_signal(&notEmpty);
        pthread_mutex_unlock(&mutex);
    }
}

void* consume(void* arg)
{
    int thread_number  = *(int*)arg;
    for(int i = 0; i < MAX*2; i++)
    {
        pthread_mutex_lock(&mutex);
        while(top % MAX == bottom)
        {
            cout << "the area is empty"<<endl;
            pthread_cond_wait(&notEmpty, &mutex);
        }
        bottom = (bottom+1)%MAX;
        cout << "pthread " << thread_number << " consume, " << "now bottom is " << bottom <<endl;
        pthread_cond_signal(&notFull);
        pthread_mutex_unlock(&mutex);
    }
}


int main()
{
    pthread_t pid[MAX-1];
    top = 0;
    bottom = 0;
    int thread_number[MAX-1];
    for(int i = 0; i < MAX-1; i++) thread_number[i] = i;
    for(int i = 0; i < MAX-1; i++)
    {
        **int* t = thread_number + i;**
        if(i%2 == 0) pthread_create(&pid[i], NULL, produce, t);
        else pthread_create(&pid[i], NULL, consume, &i);
    }

    
    for(int i = 0 ; i < MAX-1; i++)
        pthread_join(pid[i], NULL);
    return 0;
}

在这里插入图片描述

这样就可以了！

总之，生产者消费者模型学到的东西不少！

互斥锁实现生产者消费者队列

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