多线程同步之条件变量

对于多线程程序来说，同步是指在一定的时间内只允许某一个线程访问某个资源 。 而在
此时间内，不允许其他的线程访问该资源。同步资源的方式:互斥锁、条件变量、读写锁、
信号量。

条件变量的理念:当线程在等待满足某些条件时使线程进入睡眠状态，一旦条件满足，就唤
醒因等待满足特定条件而睡眠的线程
条件变量（条件锁）也可以解决线程同步和共享资源访问的问题，条件变量是对互斥锁的补充，它允许一个线程阻塞并等待另一个线程发送的信号，当收到信号时，阻塞的线程被唤醒并试图锁定与之相关的互斥锁。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <errno.h>
#include <iostream>
#include <pthread.h>
using namespace std;

/*提示出租车到达的条件变量*/
pthread_cond_t taxiCond = PTHREAD_COND_INITIALIZER; 
/*同步锁*/
pthread_mutex_t taxiMutex = PTHREAD_MUTEX_INITIALIZER;   

int travelerCound=0;

void * traveler_arrive(void * name)
{
    cout<<"Traveler: "<<(char *)name<<" needs a taxi now!"<<endl;
    pthread_mutex_lock(&taxiMutex);
    travelerCound++;
    pthread_cond_wait(&taxiCond,&taxiMutex);
    pthread_mutex_unlock(&taxiMutex);
    cout<<"Traveler: "<<(char *)name<<" now got a taxi!"<<endl;
    pthread_exit((void*)0);
}

void * taxi_arrive(void * name)
{
    cout<<"Taxi: "<<(char *)name<<" arrives."<<endl;
    sleep(3);
    while(1)
    {
        pthread_mutex_lock(&taxiMutex);
        if(travelerCound>0)
        {
            pthread_cond_signal(&taxiCond);
            pthread_mutex_unlock(&taxiMutex);   
            cout<<"Taxi: "<<(char *)name<<" Got a traveler."<<endl;
            travelerCound--;
            break;          
        }
        else
        {
            cout<<"Taxi: "<<(char *)name<<" Left."<<endl;
           pthread_exit((void*)0);
        }
        pthread_mutex_unlock(&taxiMutex);
    }
    pthread_exit((void*)0);
}

int main(){
    pthread_t tids[3];
    int iRet = pthread_create(&tids[0],NULL,taxi_arrive,(void*)(" Jack "));
    if(iRet)
    {
        printf("pthread_create error: iRet=%d\n",iRet);
        return iRet;
    }
    printf("Time passing by.\n");
    sleep(1);
    iRet = pthread_create(&tids[1],NULL,traveler_arrive,(void*)(" Susan "));
    if(iRet)
    {
        printf("pthread_create error: iRet=%d\n",iRet);
        return iRet;
    }
    printf("Time passing by.\n");
    sleep(1);   
    iRet = pthread_create(&tids[2],NULL,taxi_arrive,(void*)(" Mike "));
    if(iRet)
    {
        printf("pthread_create error: iRet=%d\n",iRet);
        return iRet;
    }   
    printf("Time passing by.\n");
    sleep(1);
    
    void *retval;   
    for(int i=0;i<3;i++)
    {
        iRet=pthread_join(tids[i],&retval);
        if (iRet)
        {
            printf("pthread_join error: iRet=%d\n",iRet);
            return iRet;
        }
        printf("retval=%ld\n",(long)retval);    
    }
    return 0;   
}