#include<iostream>
#include<thread>
#include<mutex>
#include<ctime>
#include <semaphore.h>

using namespace std;
using namespace std::this_thread;
using namespace std::chrono;


int readcount=0, writecount=0; 
sem_t mutex1, mutex2, mutex3, w, r; 
//增加信号量r，初值是1：当至少有一个写进程准备访问数据区时，用于禁止所有的读进程。

// 增加一个记数器，即整型变量writecount，记录写者数，初值是0。 
// writecount为多个写者共享的变量，是临界资源。用互斥信号量mutex2控制, mutex2初值是1。

// 增加mutex3，初值是1：在r上不允许建造长队列，否则写进程将不能跳过这个队列，
// 因此，只允许一个读进程在r上排队，而所有其他读进程在等待r之前，在信号量mutex3上排队。

void read(int id){
    sem_wait(&mutex3);
    sem_wait(&r);
    sem_wait(&mutex1);

    readcount++;
    if (readcount == 1 )    sem_wait(&w);
    sem_post(&mutex1);
    sem_post(&r);
    sem_post(&mutex3);

    cout<<"reader"<<id<<"reading is performed"<<endl;
    sleep_for(seconds(2));
    
    sem_wait(&mutex1);
    readcount --;
    if (readcount == 0 )    sem_post(&w);
    sem_post(&mutex1);
 }


void write(int id){     
    sem_wait(&mutex2);
    writecount++;
    if(writecount == 1 )        sem_wait(&r);
    sem_post(&mutex2);
    sem_wait(&w);
    
    cout<<"writer"<<id<<"writing is performed"<<endl;
    sleep_for(seconds(4));

    sem_post(&w);
    sem_wait(&mutex2);
    writecount --;
    if (writecount == 0)    sem_post(&r);
    sem_post(&mutex2); 
}




int main(){
    sem_init(&mutex1,0,1);
	sem_init(&mutex2,0,1);
    sem_init(&mutex3,0,1);
    sem_init(&w,0,1);
    sem_init(&r,0,1);
	thread t1(read, 0);
    thread t2(write, 0);
	thread t3(read, 1);
    thread t4(write, 1);
	t1.join(); t2.join();t3.join();t4.join();
}